Diagnosis of a driveability and/or emissions problems requires attention to detail and following the diagnostic procedures in the correct order. Resist the temptation to perform any repairs before performing the preliminary diagnostic steps. In many cases this will shorten diagnostic time and often cure the problem without electronic TESTING.
The proper troubleshooting procedure for these vehicles is as follows:
This is possibly the most critical step of diagnosis and should be performed immediately after retrieving any codes. A detailed examination of connectors, wiring and vacuum hoses can often lead to a repair without further diagnosis. Performance of 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.
If a fault occurs intermittently, such as a loose connector pin breaking contact as the vehicle hits a bump, the ECM will note the fault as it occurs and energize the dash warning lamp. If the problem self-corrects, as with the terminal pin again making contact, the dash lamp will extinguish after 10 seconds but a code will remain stored in the computer control module's memory.
When an unexpected code appears during diagnostics, it may have been set during an intermittent failure that self-corrected; the codes are still useful in diagnosis and should not be discounted.
The fault codes and the scan tool data will lead to diagnosis and checking of a particular circuit. It is important to note that the fault code indicates a fault or loss of signal in an ECM-controlled system, not necessarily in the specific component.
Refer to the appropriate Diagnostic Code chart to determine the codes meaning. The component may then be tested following the appropriate component test procedures found in this section. If the component is OK, check the wiring for shorts or opens. Further diagnoses should be left to an experienced driveability technician.
If a code indicates the ECM to be faulty and the ECM is replaced, but does not correct the problem, one of the following may be the reason:
- There is a problem with the ECM terminal connections: The terminals may have to be removed from the connector in order to check them properly.
- The ECM or PROM is not correct for the application: The incorrect ECM or PROM may cause a malfunction and may or may not set a code.
- The problem is intermittent: This means that the problem is not present at the time the system is being checked. In this case, make a careful physical inspection of all portions of the system involved.
- Shorted solenoid, relay coil or harness: Solenoids and relays are turned on and off by the ECM using internal electronic switches called drivers. Each driver is part of a group of four called Quad-Drivers. A shorted solenoid, relay coil or harness may cause an ECM to fail, and a replacement ECM to fail when it is installed. Use a short tester, J34696, BT 8405, or equivalent, as a fast, accurate means of checking for a short circuit.
- The Programmable Read Only Memory (PROM) or MEM-CAL may be faulty: Although the PROM rarely fails, it operates as part of the ECM. Therefore, it could be the cause of the problem. Substitute a known good PROM/MEM-CAL.
- The replacement ECM may be faulty: After the ECM is replaced, the system should be rechecked for proper operation. If the diagnostic code again indicates the ECM is the problem, substitute a known good ECM. Although this is a very rare condition, it could happen.