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Buick Century Limited 1990 - G - TESTS W/CODES 1990 ENGINE PERFORMANCE General Motors 2.5L TBI Model Self-Diagnostics
Buick Century Limited 1990 - INTRODUCTION DESCRIPTION
Most engine control problems are the result of mechanical breakdowns, poor electrical connections or damaged vacuum hoses. Before considering the computer system as a possible cause of problems, perform checks and inspections covered in BASIC TESTING article. Failure to do so may result in lost diagnostic time.
If no faults were found while performing BASIC TESTING , proceed with DIAGNOSTIC PROCEDURE. If no fault codes or only a non-running Code 12 is present and driveability problems exist, proceed to H - TESTING W/O CODES article for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.). If only intermittent codes are present, see INTERMITTENTS in H - TESTING W/O CODES article in this section.
The ECM is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, ECM will illuminate the SERVICE ENGINE SOON light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in ECM memory. To retrieve stored codes, see RETRIEVING CODES (NON-SCAN) in this article. Malfunctions are recorded as HARD FAILURES or as INTERMITTENT FAILURES.
Buick Century Limited 1990 - HARD FAILURES
Hard failures cause SERVICE ENGINE SOON light to illuminate and remain on until the malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause of malfunction must be determined using diagnostic (code) charts. If a sensor fails, control unit will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but loss of good driveability will most likely be encountered.
Buick Century Limited 1990 - INTERMITTENT FAILURES
Intermittent failures cause SERVICE ENGINE SOON light to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in ECM memory. If related fault does not reoccur within 50 engine restarts, related trouble code will be erased from ECM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in H - TESTING W/O CODES article in this section.
Buick Century Limited 1990 - DIAGNOSTIC PROCEDURE
Diagnosis of the computerized engine control system should be performed in the following order:
- Make sure all engine systems not related to the computer system are operating properly. Do not proceed with testing unless all other problems have been repaired. Appropriate DIAGNOSTIC CIRCUIT CHECK for that system MUST be performed prior to utilizing trouble code charts. See BASIC TESTING article.
- If trouble codes were displayed (other than Code 12), decide whether codes are hard or intermittent trouble codes. Hard codes will cause the SERVICE ENGINE SOON light to illuminate continuously while engine is running. See HARD OR INTERMITTENT TROUBLE CODE DETERMINATION in this article. For diagnosing hard codes, proceed to appropriate trouble code chart in this article. For diagnosing intermittent codes, proceed to INTERMITTENTS in H - TESTING W/O CODES article. Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes.
- If no trouble codes were displayed and a driveability problem exists, refer to SYMPTOMS in H - TESTING W/O CODES article. The comments there will send you to the proper system or component to check in SYSTEM/COMP TESTS article.
- After any repairs are made, clear any trouble codes and perform SYSTEM PERFORMANCE CHECK or FIELD SERVICE MODE check in BASIC TESTING article.
Buick Century Limited 1990 - RETRIEVING CODES (NON-SCAN)
- Turn ignition on. Do not start engine. SERVICE ENGINE SOON light should glow. Locate Assembly Line Data Link (ALDL) connector attached to ECM wiring harness. Most ALDL connectors are located under dash on driver's side of vehicle. For exact location of ALDL, see appropriate COMPONENT LOCATIONS illustration in SYSTEM/COMP TESTS article. Turn ignition on. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of ALDL connector. See Fig. 1 .
NOTE: Inserting jumper wire into test and ground terminals of ALDL connector with engine running will cause fuel injected vehicles to enter field service mode. Codes will not flash if this is done. See FIELD SERVICE MODE in BASIC TESTING article. 
Fig. 1: Buick Century Limited 1990 - Component Locations - ALDL Connector Terminal Identification - SERVICE ENGINE SOON light should begin to flash codes. Each code will be repeated 3 times. If codes are not flashed, perform DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.
Buick Century Limited 1990 - READING TROUBLE CODES
The ECM stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Trouble codes may be read by counting flashes of the SERVICE ENGINE SOON light, or by reading the output of a diagnostic "Scan" tester connected to the ALDL connector. The tester is faster to use, more accurate, and capable of reading information which otherwise would necessitate testing individual ECM and sensor/solenoid connector terminals with a digital voltmeter. See SCAN DATA TABLES and SCAN TESTER USAGE in this article.
NOTE: When using a "Scan" tester, there is a time delay between serial data updates. For instantaneous response, a digital voltmeter must be used.
If "Scan" tester is not available, it is possible to read flashes of the SERVICE ENGINE SOON light by grounding the diagnostic terminal of the ALDL with ignition on and engine off. For example, FLASH, FLASH, pause, FLASH, longer pause, identifies Code 21. The first series of flashes are the first digit of trouble code. The second series of flashes are the second digit of trouble code. Trouble codes are displayed starting with the lowest numbered code. Each code is displayed 3 times. Codes will continue to repeat as long as ALDL test terminal is grounded.
NOTE: Trouble codes will be recorded at various operating times. Some codes require operation of that sensor or switch for 5 seconds; others may require operation for 5 minutes or longer at normal operating temperature, road speed and load. Therefore, some codes may not set in a service bay operational mode.
Buick Century Limited 1990 - ECM TROUBLE CODE DEFINITION TABLE
Buick Century Limited 1990 ECM TROUBLE CODE DEFINITION
Code No. Circuit Affected 12 (1) No RPM reference pulse 13 Open oxygen sensor circuit 14 CTS signal voltage low 15 CTS signal voltage high 16 System voltage high 21 TPS signal voltage high 22 TPS signal voltage low 23 MAT sensor signal voltage high, M/C solenoid voltage low 24 VSS circuit 25 MAT sensor signal voltage low 26 Quad-Driver error 27, 28 & 29 Gear switch problem 31 Park/Neutral switch 32 EGR system error 33 MAP sensor signal voltage high 34 MAP sensor signal voltage low 35 IAC idle speed error 38 Brake switch 39 TCC 41 C(3)I ignition - cam sensor loss, Cylinder select error (MEM-CAL) 42 EST circuit open or grounded 44 Lean exhaust indicated 45 Rich exhaust indicated 46 Anti-theft fault, Power steering pressure switch 48 Misfire diagnosis 51 Faulty PROM, MEM-CAL or ECM 52 Faulty/missing CALPAC or MEM-CAL 53 Faulty alternator, voltage high, EGR system malfunction, Anti-theft circuit fault 54 Fuel pump voltage low 55 Faulty ECM 61 Degraded O2 sensor 62 Gear switch error 63 EGR flow error 64 EGR flow error 65 EGR flow error, Fuel injector current low 66 A/C pressure sensor voltage, out of specification
(1) Display of a Code 12 is normal when no reference pulses are received by ECM (engine not running).
NOTE: Trouble code charts should only be used if SERVICE ENGINE SOON light is illuminated (indicating a current problem exists). Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes. Anytime Codes 51, 52, 54 or 55 are displayed with another code, start with 50-series code first, then proceed to low profile numbered codes.
Buick Century Limited 1990 - HARD OR INTERMITTENT TROUBLE CODE DETERMINATION
During any diagnostic procedure, it must be determined if codes are hard failure codes or intermittent failure codes. Diagnostic charts will not usually help analyze intermittent codes. To determine hard codes and intermittent codes, proceed as follows:
- MANUALLY enter diagnostic mode. Read and record all stored trouble codes. Exit diagnostic mode and clear trouble codes. See CLEARING TROUBLE CODES .
- Apply parking brake and place transmission in Neutral or Park. Block drive wheels and start engine. SERVICE ENGINE SOON light should go out. Run warm engine at specified curb idle for 2 minutes and note SERVICE ENGINE SOON light.
- If SERVICE ENGINE SOON light comes on, MANUALLY enter diagnostic mode. Read and record trouble codes. This will reveal hard failure codes. Codes 13, 15, 24, 44, 45 and 55 may require a road test to reset hard failure after trouble codes were cleared.
- If SERVICE ENGINE SOON light does not come on, all stored trouble codes were intermittent failures. Exceptions are noted under each DIAGNOSTIC PROCEDURE.
Buick Century Limited 1990 - CLEARING TROUBLE CODES
Turn ignition switch to ON position and ground diagnostic terminal lead at ALDL connector. Turn ignition switch to OFF position and remove ECM fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead.
Buick Century Limited 1990 - ECM LOCATION
On most vehicles the ECM is located behind the right side of the dash, or behind the right kick panel. On the Lumina, the ECM is located on the right side of the engine compartment.
Buick Century Limited 1990 - DIAGNOSTIC MATERIALS DIAGNOSTIC AIDS
Diagnostic aids (located in many trouble code charts) are additional tips used to help diagnose trouble codes when inspected circuit checks out okay. Diagnostic aids may help lead to a definitive solution to that trouble code problem.
Buick Century Limited 1990 - FIELD SERVICE MODE CHECK (FUEL INJECTED)
On fuel injected models, SERVICE ENGINE SOON light will indicate operational mode of engine if ALDL is grounded while engine is running. This test confirms proper operation of fuel system and verifies closed loop operation. Clear codes and perform this test after any repair is completed. Field service mode check can be found by proceeding to FIELD SERVICE MODE CHECK (FUEL INJECTED) in BASIC TESTING article.
Buick Century Limited 1990 - SPECIAL TOOLS (DIAGNOSTIC)
NOTE: Special "Scan" testers plugged into the ALDL may be used to read trouble codes and check voltages in the system on the serial data line (terminal "E" on EFI and terminal "M" on EFI with P-4 systems). These testers can save a great deal of time. For additional information, see SCAN TESTER USAGE and SCAN TESTER DATA tables in this article.
The computerized engine control system is most easily diagnosed using a "Scan" tester; however, other tools may aid in diagnosing problems if a "Scan" tester is unavailable. These tools are a tachometer, dwell meter, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights (TBI and PFI) and 6 jumper wires 6" long (one wire with female connectors at both ends, one wire with male connector at both ends and 4 wires with male and female connectors at opposite ends). A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart.
Before engine reaches operating temperature, dwell should be fixed between 10-50 degrees, indicating open loop operation. With engine at operating temperature and idling, dwell meter needle should be varying between 10-50 degrees. This indicates closed loop operation. If (after reaching normal operating temperature) dwell is fixed between 10-50 degrees, less than 10 degrees or more than 50 degrees, refer to SYSTEM PERFORMANCE CHECK in BASIC TESTING article.
Buick Century Limited 1990 - SCAN TESTER USAGE DESCRIPTION
NOTE: Prior to connection of "Scan" tester to vehicle, diagnostic system should be checked to determine if system is operating properly and if information received by "Scan" tester will be accurate. This is done by performing appropriate DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article. If vehicle does not pass diagnostic circuit check, information received by "Scan" tester may be invalid.
The "Scan" tester is a specialized tester which, when plugged into ALDL, can be used to diagnose on-board computer control systems by providing instant access to circuit voltage information without need to crawl under dash or hood to back-probe sensors and connectors. "Scan" testers cut down diagnostic time dramatically by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN DATA tables in this article. They may also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication that output signals have been sent to devices by the ECM. It does not indicate if devices have responded properly to that signal. This will need to be verified at output device using a voltmeter or test light.
NOTE: Code 12 should always exist when ALDL is grounded with key on and engine not running, but may not be indicated by all makes of "Scan" testers.
If trouble codes are not present, this is not an indication that there is not a problem. Driveability related problems with codes displayed occur about 20 percent of the time, while driveability problems without codes occur about 80 percent of the time. Sensors that are out of specification WILL NOT set a trouble code but WILL cause driveability problems. Using a "Scan" tester is easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See SCAN DATA tables in this article.
NOTE: Information obtained by "Scan" tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, it will be necessary to verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off, remove tester, turn ignition on and ground ALDL test terminal. If same codes are not flashed by SERVICE ENGINE SOON light that were indicated by "Scan" tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate.
Buick Century Limited 1990 - SCAN DATA
NOTE: Information contained in the following tables is typical of readings taken on vehicle with engine idling, upper radiator hose hot, closed throttle, transmission in Park or Neutral, closed loop status achieved and all accessories off (except as noted in tables). Data parameters are updated every 1 1/4 seconds. On systems using P-4 computers, parameter updates are more often. Not all devices & systems are used on all models.
Buick Century Limited 1990 THROTTLE BODY INJECTION
Tester Position Units Measured Nominal Value A/C Clutch On/Off Off (On with A/C) A/C Request Yes/No No/Yes (with request) AIR Divert Sol. On/Off On (air to switching sol.); Off (air to atmosphere) AIR Switching Sol. On/Off On (to exhaust manifold); Off (to catalytic converter) BARO Volts 3-4.5 Battery Voltage Volts 13.5-14.5 Block Learn Counts 118-138 (128 normal) Brake Switch On/Off On when engaged Canister Purge Sol. On/Off On/engine cold (idle some) Clear Flood On/Off ***See Tester Manual*** Coolant Fan On/Off Off below 216?F (102?C) Coolant Temp. ?C 85-105? (norm. temperature) Crank RPM RPM 100-900 Cross Counts Counts 0-255 Cruise Cont. Sw. On/Off On when engaged EGR Solenoid On/Off On when energized EGR Duty Cycle 0-100% 0/closed-100/fully open Fan Relay On/Off On when energized Fan Request On/Off On with request Fuel Back-Up Yes/No Yes when engaged IAC Counts 0-50 Ignition/Crank On/Off On with ignition/crank Injector Pulse Width Mil./Sec .8-3.0 INT (Integrator) Counts 110-145 (128 normal) Knock Signal Yes/No Yes when knock exists MAT ?C 10-90? MAP Volts 1 (idle) to 4.5 (WOT) Open/Closed Loop Status OL/CL Closed/Open during, extended idle O2 Sensor Millivolts 100 (lean) to 999 (rich) P/N Switch P/N/RDL Park/Neutral P/S Switch Norm/Hi Normal PROM I.D. PROM # Original factory number RPM RPM Spec. +/-25 RPM Drive (A/T); Spec. +/-50 RPM Neut. (M/T) Spark Advance No. Deg Varies TCC On/Off Off (On with command) TPS Volts 1.25 (idle) to 5.0 (WOT) Throttle Angle 0-100% 0 (idle) to 110 (WOT) Trouble Codes Code # No codes Turbo Boost On/Off On when activated Upshift Light (M/T) On/Off Off VSS or MPH MPH 0-actual 3rd Gear Switch On/Off On/3rd & 4th gear 4th Gear Switch On/Off On/4th gear
Buick Century Limited 1990 - SUMMARY
If no hard fault codes are present, driveability symptoms exist or intermittent codes exist, proceed to H - TESTING W/O CODES article for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.), or intermittent diagnostic procedures.
NOTE: The following diagnostic flow charts and mini-schematics are supplied courtesy of General Motors Corp.
Buick Century Limited 1990 - DIAGNOSTIC CODES CODE 13, OPEN OXYGEN SENSOR CIRCUIT
The O2 sensor is like an open circuit and produces no voltage when its temperature is less than about 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 13 will set at normal operating temperature if at least 2 minutes have passed since engine start, O2 signal voltage is steady between .35-.55 volt and throttle position sensor signal is greater than 7 percent (1200 RPM). All conditions must be met for a precalibrated period of time.
- This test determines if the O2 sensor, ECM or wiring is at fault.
- Use only a high-impedance DVOM for this test. This checks for continuity in signal and ground circuits. If ground circuit is open, voltage on signal circuit will be greater than .6 volt.
Buick Century Limited 1990 - Diagnostic Aids
Verify a clean, tight connection for ground circuit No. 413. An open circuit at sensor signal terminal or ground terminal will result in a Code 13.
Fig. 2: Buick Century Limited 1990 - Component Locations - Code 13, Open O2 Sensor Ckt Diag (W Body)
Fig. 4: Buick Century Limited 1990 - Component Locations - Code 13, Open O2 Sensor Flow Chart (All)
Buick Century Limited 1990 - CODE 14, COOLANT SENSOR SIGNAL VOLTAGE LOW
Coolant temperature is one of the inputs used by the ECM in determining control of fuel delivery, engine timing (EST), idle (IAC) and converter clutch (TCC). As the engine warms, the sensor resistance reduces. At normal operating temperature, voltage signal will be about 1.5-2.0 volts at ECM coolant sensor signal terminal.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This checks if code was set because of a hard failure or intermittent condition. Code 14 will set if signal voltage indicates a coolant temperature above 275?F (135?C) for more than 3 seconds.
- This simulates conditions for a Code 15. If the ECM recognizes the open circuit by displaying a low temperature, the ECM and wiring are not at fault.
Buick Century Limited 1990 - Diagnostic Aids
After the engine is started, the temperature should rise steadily to about 194?F (90?C), then stabilize when thermostat opens. If the engine is allowed to cool overnight, the coolant temperature sensor and MAT sensor should read close to each other, when measured with a "Scan" tester.
When Code 14 is set, the ECM will turn on the cooling fan. A Code 14 will also set if sensor signal circuit is shorted to ground.
Buick Century Limited 1990 - CODE 15, COOLANT SENSOR SIGNAL VOLTAGE HIGH
As the engine warms, the sensor resistance reduces and voltage drops. At normal engine operating temperature, voltage will be about 1.5-2.0 volts at ECM coolant sensor signal terminal. If sensor signal circuit opens, ECM will see -40?F (-40?C) and deliver fuel for this temperature.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This checks if code was set as a result of a hard failure or intermittent condition. Code 15 will set if time since engine start is more than 2 minutes and signal voltage indicates a coolant temperature less than -22?F (-30?C) for more than 60 seconds.
- This simulates conditions for a Code 14. If the ECM recognizes the grounded circuit, and displays a high temperature, the ECM and wiring are not at fault.
- This determines if there is a problem with ECM or wiring. If ground circuit is open, there may also be a Code 21 stored.
Buick Century Limited 1990 - Diagnostic Aids
After the engine is started, the temperature should rise steadily to about 194?F (90?C), then stabilize when thermostat opens. If the engine is allowed to cool overnight, the coolant temperature sensor and MAT sensor should read close to each other, when measured with a "Scan" tester. When Code 15 is set, the ECM will turn on the cooling fan. Code 15 will also set if sensor signal and ground circuits are open.
Buick Century Limited 1990 - CODE 21, TPS SIGNAL VOLTAGE HIGH
The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to throttle valve angle. Signal voltage will vary from less than about 1.25 volts at idle to about 4.5 volts at wide open throttle.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test confirms Code 21 and checks if fault is because of a hard failure or an intermittent condition. Code 21 will set if TPS voltage is greater than 2.5 volts for 2-8 seconds, engine speed is less than 1800 RPM or MAP sensor signal is equal to a no-load condition.
- This test simulates conditions for Code 22. If the ECM recognizes the low voltage signal and sets Code 22, the ECM and power and signal circuits are not at fault.
- This step isolates a faulty sensor, ECM or an open ground circuit. If ground circuit is open, there may also be a Code 15 stored.
Buick Century Limited 1990 - Diagnostic Aids
A "Scan" tester displays throttle position in volts. Closed throttle voltage should be less than 1.25 volts. TPS voltage should increase at a steady rate to 4.5 volts as throttle angle increases. Code 21 will also result if ground circuit is open or TPS signal circuit is shorted to voltage.
Buick Century Limited 1990 - CODE 22, TPS SIGNAL LOW
The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to throttle valve angle. Signal voltage will vary from less than about 1.25 volts at idle to about 4.5 volts at wide open throttle.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test confirms Code 22 and tests if the fault is because of a hard failure or an intermittent condition. Code 22 will set if engine is running, TPS voltage is less than .2 volt for 2 seconds.
- This simulates Code 21. If the ECM recognizes a high voltage signal and sets Code 21, the ECM and wiring are not at fault.
- This checks for voltage to the ECM. If the ECM recognizes the voltage as over 4 volts, the ECM and TPS signal circuit are not at fault.
- If 5-volt reference circuit is open or shorted to ground, there may also be a Code 34 stored.
Buick Century Limited 1990 - Diagnostic Aids
A "Scan" tester displays throttle position in volts. Closed throttle voltage should be less than 1.25 volts. TPS voltage should increase at a steady rate to 4.5 volts as throttle angle increases. Code 22 will also set if TPS signal or ground circuits are open or grounded.
Buick Century Limited 1990 - CODE 23: MAT SENSOR TEMP LOW
The ECM supplies and monitors a voltage signal (4-6 volts) to the sensor. When temperatures are low, sensor resistance is high and the ECM will see a high-monitored voltage signal. As temperature increases, sensor resistance decreases and voltage sensed by the ECM drops.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This checks if Code 23 is because of a hard failure or an intermittent condition. Code 23 will set if engine has been running for a precalibrated period of time and signal voltage indicates a MAT temperature less than -22?F (-30?C).
- This simulates conditions for a Code 25. If the "Scan" tester displays a high temperature, the ECM and wiring are not at fault.
- This checks for continuity of sensor signal and ground circuits. If ground circuit is open there may also be a Code 33 or Code 21 stored.
Buick Century Limited 1990 - Diagnostic Aids
If the engine is allowed to cool overnight, the coolant and MAT sensors should read close to each other, when measured with a "Scan" tester. When Code 15 is set, the ECM will turn on the cooling fan. A Code 23 will also result if signal and ground circuits become open.
Fig. 18: Buick Century Limited 1990 - Component Locations - Code 23, MAT Sensor Temp Lo Flow Chart
Buick Century Limited 1990 - CODE 24: VSS
The speed sensor, which is a Permanent Magnet (PM) generator, provides the ECM with vehicle speed information. The PM generator, mounted in the transmission, produces a pulsing voltage signal whenever the vehicle speed is more than 3 MPH. The voltage level and pulses increase with vehicle speed. The ECM converts the pulsing voltage to MPH, which is used by the ECM in calculations to determine vehicle adjustments.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- A Code 24 will set when MPH reads zero, transmission is not in Park or Neutral, engine speed is between 1800-4400 RPM, TPS is at zero percent (closed throttle) and a high manifold vacuum is sensed by the MAP sensor. All of these conditions must be met for 3-5 seconds. The PM generator only produces a voltage signal if drive wheels are turning greater than 3 MPH.
- Before replacing the ECM, PROM should be checked for correct application.
Buick Century Limited 1990 - Diagnostic Aids
A faulty or misadjusted Park/Neutral switch may set a false Code 24. Use "Scan" tester and check for proper signal in Drive, while wiggling shifter.
Fig. 19: Buick Century Limited 1990 - Component Locations - Code 24, VSS Ckt Diag (A Body)
Fig. 20: Buick Century Limited 1990 - Component Locations - Code 24, VSS Ckt Diag (N Body)
Fig. 21: Buick Century Limited 1990 - Component Locations - Code 24, VSS Ckt Diag (W Body)
Fig. 22: Buick Century Limited 1990 - Component Locations - Code 24, VSS Flow Chart (All)
Buick Century Limited 1990 - CODE 25: MAT SENSOR TEMP HIGH
The ECM applies and monitors a voltage signal (4-6 volts) to MAT sensor. When manifold air is cold, the sensor resistance is high and the ECM will see a high signal voltage. As air warms, resistance decreases and voltage sensed by the ECM drops. Sensor resistance can be measured at sensor terminals with harness disconnected.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This checks if the code is a hard failure or an intermittent condition. Code 25 will set if a MAT temperature greater than 275?F (135?C) is sensed for more than 2 seconds.
Buick Century Limited 1990 - Diagnostic Aids
If the engine is allowed to cool overnight, the coolant temperature sensor and MAT sensor should read close to each other, when measured with a "Scan" tester. A Code 25 will also result if sensor signal circuit is shorted to ground.
Buick Century Limited 1990 - CODE 32: EGR SYSTEM ERROR
A properly operating EGR will directly affect the air/fuel mixture requirements of the engine. Since the exhaust gas introduced into the airflow is an inert gas (contains little or no oxygen), less fuel is required to maintain a correct air/fuel mixture. If the EGR system were to become inoperative, the inert exhaust gas would be replaced with oxygen and the air/fuel mixture would become leaner. This would be reflected in an increased block learn value. The engine control system operates within 2 block learn cells: a closed throttle cell and an open throttle cell. The block learn value of these 2 cells should be close to the same. The open throttle cell is affected by the operation of a functional EGR system. If the EGR system malfunctions, the open throttle cell block learn would increase, but the block learn for the closed throttle cell would remain unchanged. When the change becomes too great, the ECM will set a Code 32.
Buick Century Limited 1990 - Diagnostic Aids
The Code 32 chart is a functional check of the EGR system. If the EGR system works properly and a Code 32 is set, check for other causes for an increased open throttle block learn value.
- Check for a blocked/restricted EGR passage.
- Check the MAP sensor function. A MAP sensor may shift enough in calibration to affect the air/fuel mixture without setting a MAP-related code.
Fig. 26: Buick Century Limited 1990 - Component Locations - Code 32, EGR System Schematic
Fig. 27: Buick Century Limited 1990 - Component Locations - Code 32, EGR System Error Flow Chart
Buick Century Limited 1990 - CODE 33: MAP SENSOR SIGNAL VOLTAGE HIGH
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If the MAP sensor fails, the ECM will substitute a fixed MAP value and use the TPS input to control fuel delivery.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test confirms Code 33 and determines if it is the result of a hard failure or an intermittent condition. Code 33 will set when voltage signal reading is too high for a time greater than a precalibrated period of time and if the TPS voltage indicates throttle is closed.
- This step simulates conditions for a Code 34. If the ECM recognizes and sets Code 34, low MAP signal, the ECM and 5-volt reference and MAP signal circuits are not at fault. If ground circuit is open, there may also be a Code 23 stored.
Buick Century Limited 1990 - Diagnostic Aids
With the ignition switch in the "ON" position and the engine stopped, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Comparison of the BARO readings from a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. Code 33 will also result if ground circuit is open or MAP signal circuit is shorted to voltage or to 5-volt reference circuit.
Buick Century Limited 1990 - CODE 34: MAP SENSOR SIGNAL VOLTAGE LOW
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If the MAP sensor fails, the ECM will substitute a fixed MAP value and use the TPS input to control fuel delivery.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This confirms Code 34 and determines if code was because of a hard failure or an intermittent condition. Code 34 will set when ignition is on and MAP signal voltage is low. On some systems, engine must be running to set code.
- Jumpering harness terminals "B" to "C" will determine if sensor is at fault or if there is a problem with the ECM or wiring.
- "Scan" tester may not display 12 volts. The important thing is the ECM recognizes the voltage as greater than 4 volts (high MAP voltage signal), indicating the ECM and MAP signal circuit are not at fault.
Buick Century Limited 1990 - Diagnostic Aids
With the ignition switch in the ON position and the engine stopped, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Compare BARO readings with a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. A Code 34 will also result if 5-volt reference and MAP signal circuits are open or shorted to ground. If 5-volt reference circuit is not shorted to ground and a stored Code 22 is stored, check MAP signal circuit for short to ground.
Buick Century Limited 1990 - CODE 35: IDLE SPEED ERROR (A & N BODY)
Code 35 will set when closed throttle engine speed is 150 RPM greater or less than correct idle speed for 20 seconds.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- IAC tester is used to extend and retract IAC valve. Movement is verified by an engine speed change. If no change in speed occurs, valve can be retested when removed from throttle body.
- Checks IAC movement quality from step 1). Between 700-1500 RPM, engine speed should change smoothly with each flash of the tester light in both extend and retract. If IAC valve is retracted beyond the control range (about 1500 RPM), it may take many flashes in the extend position before engine speed begins to drop. This is normal on certain engines, fully extending the IAC may cause engine to stall. This may be normal.
- Steps 1) and 2) verified proper IAC valve operation while this step checks the IAC circuits. Each light on the node light should flash Red and Green while the IAC valve is cycled. While the sequence of color is not important if either light is off or does not flash Red and Green , check the circuits for faults beginning with poor terminal contacts.
Buick Century Limited 1990 - Diagnostic Aids
A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. "Scan" counts will be greater than 60 if too low, and zero counts, if too high. If idle is too high, stop engine. With ignition on, ground ALDL "test" terminal. Wait 45 seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is greater than 800 RPM, look for possible vacuum leaks.
Buick Century Limited 1990 - System Too Lean
If air/fuel ratio is too lean, the idle speed may be either too high (check for vacuum leaks) or too low. Engine speed may vary up and down and disconnecting the IAC may not help. "Scan" and/or digital voltmeter (10 megohm) will read an oxygen sensor output less than 300 mv (.3 volt). Check for low fuel pressure or water in the fuel. A contaminated O2 sensor (caused by silicone) will produce lean air/fuel mixtures with an oxygen sensor output fixed greater than 800 mv (.8 volt). This may also set Code 45.
Buick Century Limited 1990 - System Too Rich
If air/fuel ratio is too rich, idle speed will be too low and "Scan" tester counts will usually be greater than 80. The system may be obviously rich, with Black smoke from the tailpipe. "Scan" tester and/or voltmeter will read an oxygen sensor voltage signal fixed greater than 800 mv (.8 volt). Look for high fuel pressure or injectors leaking or sticking. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging the bore.
Buick Century Limited 1990 - Throttle Body
Remove IAC and inspect bore for evidence of IAC valve dragging.
Buick Century Limited 1990 - IAC Valve Connections
Inspect carefully for loose or corroded connections.
Buick Century Limited 1990 - PCV Valve
An incorrect PCV valve may cause incorrect idle speed.
Buick Century Limited 1990 - CODE 35: IDLE SPEED ERROR (W BODY)
Code 35 will set when closed throttle engine speed is 150 RPM greater or less than correct idle speed for 20 seconds.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Continue with test even if engine will not idle properly. If idle is too low, "Scan" tester will display 80 or more counts or steps. If idle is high it will display zero counts. Occasionally, an erratic or unstable idle may occur. Engine speed may vary 200 RPM or more up and down. Disconnect IAC. If condition is unchanged, IAC is not at fault.
- When engine was stopped, IAC valve retracted (more air) to a fixed position to provide increased airflow during next engine start. During next engine start, a "Scan" should display 100 or more counts.
- Disconnect IAC valve prior to this test. The test light will confirm the ECM signals by a steady or flashing light on all circuits.
- There is a remote possibility one of the circuits is shorted to voltage, which would have been indicated by a steady light. Disconnect ECM, turn ignition on and probe terminals with a DVOM to check for this condition.
Buick Century Limited 1990 - Diagnostic Aids
A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. "Scan" counts will be greater than 60 if too low, and zero counts, if too high. If idle is too high, stop engine. With ignition on, ground ALDL "test" terminal "B". Wait 45 seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is greater than 800 RPM, look for possible vacuum leaks.
Buick Century Limited 1990 - System Too Lean
If air/fuel ratio is too lean, the idle speed may be either too high or too low. Engine speed may vary up and down and disconnecting the IAC may not help. "Scan" and/or digital voltmeter (10 megohm) will read an oxygen sensor output less than 300 mv (.3 volt). Check for low fuel pressure or water in the fuel. A contaminated O2 sensor (usually caused by silicone) will produce lean air/fuel mixtures with an oxygen sensor output fixed greater than 800 mv (.8 volt). This may also set Code 45.
Buick Century Limited 1990 - System Too Rich
If air/fuel ratio is too rich, idle speed will be too low and "Scan" tester counts will usually be greater than 80. The system may be obviously rich, with Black smoke from the tailpipe. "Scan" tester and/or voltmeter will read an oxygen sensor voltage signal fixed greater than 800 mv (.8 volt). Look for high fuel pressure or injectors leaking or sticking. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging the bore.
Buick Century Limited 1990 - Throttle Body
Remove IAC and inspect bore for evidence of IAC valve dragging.
Buick Century Limited 1990 - IAC Valve Connections
Inspect carefully for loose or corroded connections.
Buick Century Limited 1990 - PCV Valve
An incorrect PCV valve may cause incorrect idle speed.
Buick Century Limited 1990 - CODE 42, ELECT SPARK TIMING W/DIS
Code 42 indicates the ECM has seen an open or short to ground in EST or by-pass circuits (DIS ignitions).
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test confirms Code 42 and determines if fault is hard or intermittent.
- This test checks for a normal EST ground path through the ignition module. If circuit No. 423 is shorted to ground, reading will be less than 500 ohms.
- As test light voltage touches circuit No. 424, the module should switch. This will cause the ohmmeter to "over-range" if meter is in the 1000-2000 ohm position. A higher ohm range will indicate over 5000 ohms. This test assures that the module "switched".
- If module did not switch, this step will test for a short in circuit No. 423, an open in circuit No. 424, and a faulty ignition module connection or module.
- This step confirms that Code 42 is a faulty ECM and not an intermittent problem in circuits No. 423 and 424.
Buick Century Limited 1990 - Diagnostic Aids
The "Scan" tester does not have the ability to help diagnose a Code 42 problem. See INTERMITTENTS in H - TESTING W/O CODES article.
Fig. 39: Buick Century Limited 1990 - Component Locations - Code 42, EST W/DIS Ckt Diag (A, N Body)
Fig. 40: Buick Century Limited 1990 - Component Locations - Code 42, EST W/DIS Ckt Diag (W Body)
Fig. 41: Buick Century Limited 1990 - Component Locations - Code 42, EST W/DIS Flow Chart (All)
Buick Century Limited 1990 - CODE 44, LEAN EXHAUST INDICATION
Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 44 is set when O2 sensor signal at ECM is less than .2 volt for 50 seconds or more and system is operating in "closed loop".
Buick Century Limited 1990 - Diagnostic Aids
Using the "Scan" tester, observe the Block Learn Memory (BLM) value at different RPMs. If conditions for a Code 44 exist, the block learn value will be around 150.
Buick Century Limited 1990 - O2 Sensor Wire
O2 sensor wire may be mispositioned and laying against the exhaust manifold. Check for ground between sensor and wire connector.
Buick Century Limited 1990 - Fuel Contamination
Water, even small amounts, near the in-tank fuel pump inlet can be delivered to the injector. The water may cause a lean exhaust and set Code 44.
Buick Century Limited 1990 - Fuel Pressure
System will be lean if fuel pressure is low. It may be necessary to monitor fuel pressure while driving vehicle. For fuel pressure checking procedure, see BASIC TESTING article.
Buick Century Limited 1990 - Exhaust Leaks
If the exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into the system and past the O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.
If Code 44 is intermittent, see INTERMITTENTS in H - TESTING W/O CODES article.
Fig. 43: Buick Century Limited 1990 - Component Locations - Code 44, Lean Exhaust Ckt Diag (W Body)
Fig. 44: Buick Century Limited 1990 - Component Locations - Code 44, Lean Exhaust Flow Chart (All)
Buick Century Limited 1990 - CODE 45: RICH EXHAUST INDICATION
Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation.
Code 45 indicates a rich exhaust and diagnosis should begin with these items: fuel pressure, leaking injector, HEI shielding, canister purge saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Test checks to see if O2 sensor is registering a rich condition. Code 45 is set when vehicle is at operating temperature (in "closed loop"), throttle angle is between 3 and 45 percent, O2 sensor signal at ECM is greater than .7 volt for 30 seconds or more and time since engine start is one minute or more.
Buick Century Limited 1990 - Diagnostic Aids
Code 45, rich exhaust, is most likely caused by one of the following:
Buick Century Limited 1990 - Fuel Pressure High
If fuel pressure is too high, air/fuel ratios will be rich. For fuel pressure checking procedure, see BASIC TESTING article. The ECM can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.
Buick Century Limited 1990 - Ignition Ground
If an open occurs at circuit No. 453, HEI induced electrical "noise" may result, causing simulated reference pulses to be picked up by ECM on reference line of EST harness. Additional pulses result in a higher than actual engine speed signal. The ECM will increase injector pulse width ("on" time) to match the increased RPM signal. "Scan" tester will show higher than actual RPM, which can help in diagnosing this problem.
Buick Century Limited 1990 - Fuel Canister
Fuel saturation of the charcoal canister will cause a rich air/fuel ratio. If full of fuel, check canister control and hoses.
Buick Century Limited 1990 - MAP Sensor
An output that causes the ECM to sense a higher than normal manifold pressure (low vacuum) can cause the system to go rich. Disconnecting the MAP sensor will allow the ECM to substitute a fixed value for the MAP sensor. If the condition disappears, substitute a different MAP sensor and continue testing.
Buick Century Limited 1990 - TPS
An intermittent TPS output will cause the system to operate rich due to a false indication of engine acceleration.
Buick Century Limited 1990 - O2 Sensor Contamination
O2 sensor contamination, caused by silicone in certain fuels or use of improper RTV sealant, may cause a White powdery coating to cover the exterior of the O2 sensor. The false high signal voltage (low oxygen content sensed) produced is interpreted by the ECM as a rich mixture, causing the ECM to set Code 45.
Buick Century Limited 1990 - EGR Problem
EGR valve sticking open at idle is usually accompanied by a rough idle and/or stalling. If Code 45 is intermittent, see INTERMITTENTS in H - TESTING W/O CODES article.
Fig. 46: Buick Century Limited 1990 - Component Locations - Code 45, Rich Exhaust Ckt Diag (W Body)
Fig. 47: Buick Century Limited 1990 - Component Locations - Code 45, Rich Exhaust Flow Chart (All)
Buick Century Limited 1990 - CODE 51, FAULTY PROM/MEM-CAL
Check that all pins are fully inserted in socket. If okay, replace PROM/MEM-CAL, clear memory and recheck. If Code 51 reappears, replace ECM.
Buick Century Limited 1990 - CODE 53, SYSTEM OVERVOLTAGE
This code indicates a basic charging system problem. Code 53 will set when voltage at ECM terminal is greater than 16.9 volts for 50 seconds. Check and repair charging system.