See Figures 1 and 2
The following procedure applies to models with the 12 pin diagnostic connector only.
- Turn the ignition switch OFF . Locate the Assembly Line Diagnostic Link (ALDL), usually under the instrument panel. It may be within a plastic cover or housing labeled DIAGNOSTIC CONNECTOR. This link is used to communicate with the ECM.
- Use a small jumper wire to connect Terminal B of the ALDL to Terminal A. As the ALDL connector is viewed from the front, Terminal A is on the extreme right of the upper row; Terminal B is second from the right on the upper row.
- After the terminals are connected, turn the ignition switch to the ON position but do not start the engine. The dash warning lamp should begin to flash Code 12. The code will display as one flash, a pause and two flashes. Code 12 is not a fault code. It is used as a system acknowledgment or handshake code; its presence indicates that the ECM can communicate as requested. Code 12 is used to begin every diagnostic sequence.
- After Code 12 has been transmitted 3 times, the fault codes, if any, will each be flashed in the same manner three times. The codes are stored and transmitted in numeric order from lowest to highest.
The order of codes in the memory does not indicate the order of occurrence.
- If one or more codes are stored, record them. At the end of the procedure, perform a visual and physical check of each system.
- Switch the ignition OFF when finished with code retrieval.
A detailed examination of connectors, wiring and vacuum hoses can often lead to a repair without further diagnosis. This step relies on the skill of the technician performing it; a careful inspector will check the undersides of hoses as well as the integrity of hard-to-reach hoses blocked by the air cleaner or other component. Wiring should be checked carefully for any sign of strain, burning, crimping, or terminal pull-out from a connector. Checking connectors at components or in harnesses is required; usually, pushing them together will reveal a loose fit. It is important to note that the fault code indicates a fault or loss of signal within that system, not necessarily the specific component. Due to the intricacy of the systems and the special testing equipment required, it is recommended to have a qualified technician perform any further testing, if needed.
FIELD SERVICE MODE
On fuel injected models, if the ALDL terminal B is grounded to terminal A with the engine running, the system enters the Field Service Mode. In this mode, the MIL will indicate whether the system is operating in open loop or closed loop.
If working in open loop, the MIL will flash rapidly 2 1 / 2 times per second. In closed loop, the flash rate slows to once per second. Additionally, if the system is running lean in closed loop, the lamp will be off most of the cycle. A rich condition in closed loop will cause the lamp to remain lit for most of the one second cycle.
When operating in the Field Service Mode, additional codes cannot be stored by the ECM. The closed loop timer is bypassed in this mode.
Stored fault codes may be erased from memory at any time by removing power from the ECM for at least 30 seconds. It may be necessary to clear stored codes during diagnosis to check for any recurrence during a test drive, but the stored should be written down when retrieved. The codes may still be required for subsequent troubleshooting. Whenever a repair is complete, the stored codes must be erased and the vehicle test driven to confirm correct operation and repair.The ignition switch must be
OFF any time power is disconnected or restored to the ECM. Severe damage may result if this precaution is not observed.
Depending on the electric distribution of the particular vehicle, power to the ECM may be disconnected by removing the ECM fuse in the fusebox or disconnecting the inline ECM power lead at the positive battery terminal. Disconnecting the negative battery cable to clear codes will achieve the desired result, but this will also clear other memory data in the vehicle such as radio presets or seat memory.