Diagnostic Circuit Check
- The diagnostic circuit check makes sure that self-diagnostic system works, determines that trouble codes will display properly, and guides diagnosis to other problem areas.
- When engine is running and a problem develops in system which self diagnosis can evaluate, "Check Engine" light will come on and a trouble code will be stored in ECM trouble code memory.
- Light will remain on with engine running as long as there is a problem. If problem is intermittent "Check Engine" light will go out, but trouble codes will be stored in ECM trouble code memory.
- With ignition turned on and engine stopped, "Check Engine" light should be on. This is a bulb check to indicate that light is working. A 3-terminal connector, located near ECM connector, is used to actuate trouble code system in ECM.
- This connector is also known as Assembly Line Diagnostic Link (ALDL) or Assembly Line Communications Link (ALCL). Two terminals of this connector (A and C) are used to activate trouble code system in ECM. With ignition on, a jumper wire between terminal "A" and "C" will activate trouble code system. See Figure .
- The "Check Engine" light will begin to flash a trouble code 12. Code 12 consists of one flash, a short pause, then 2 flashes. There will be a longer pause and code 12 will repeat 2 more times. This check indicates that self-diagnostic system is working.
- Cycle will then repeat itself until engine is started or ignition is off. Codes should be checked with engine running since no codes other than code 12 or 51 will be present on initial key on.NOTE: Remove ground from test terminal before starting engine.
- A trouble code indicates a problem in a given circuit (i.e., trouble code 15 indicates a problem in coolant sensor circuit). This includes coolant sensor, connector, harness, and ECM. The procedure for finding problem can be found in TROUBLE CODES table.
- When engine is started, "Check Engine" light is off. With engine running, trouble may be intermittent or may only occur at certain operating conditions. Faults indicated by trouble codes 13, 44 and 45 require engine operation at part throttle for up to 5 minutes before light will come on.
- Fault indicated by trouble code 15 takes 5 minutes of engine operation before it will display. Diagnostic charts for trouble codes 13, 15, 44 and 45 should be used if any of these trouble codes can be obtained. Remove ground from test terminal before starting engine.
TROUBLE CODES
| Application | Trouble Code |
|---|---|
| No distributor reference | 12 |
| Oxygen Sensor Circuit | 13 |
| Shorted Coolant Sensor Circuit | 14 |
| Open Coolant Sensor Circuit | 15 |
| Idle Switch Circuit | 21 |
| Open Fuel Cut-Off Relay Circuit | 22 |
| Open or Grounded M/C Solenoid Circuit | 23 |
| Vacuum Switching Valve Circuit | 25 |
| Shorted Fuel Cut-Off Relay Circuit | 42 |
| Lean Oxygen Sensor Condition | 44 |
| Rich Oxygen Sensor Condition | 45 |
| Faulty Calibration Unit (PROM) | 51 |
| Shorted Switching Valve or Faulty ECM | 53 |
| Shorted M/C Solenoid Circuit or Faulty ECM | 54 |
| Faulty ECM | 55 |