Diagnostic Circuit Check
Flow Of Diagnostics Chart.:
NOTE:
a. Numbers below refer to circled numbers in flow chart.
b. Since this is the starting point for all diagnostic procedures or finding the cause of an emissions test failure, always begin here and always follow the flow chart as directed.
c. For trouble code diagnosis and Charts A-1 and A-2, Computers and Control Systems / Diagnosis and Testing Procedures.
1. Get an accurate description of the complaint and verify. Include filling out the DRIVEABILITY WORKSHEET and VEHICLE REPAIR HISTORY. Refer to INFO TYPE/TECHNICAL MANAGEMENT SUPPORT for forms.
Perform Underhood Inspection:
a. Check vacuum hoses for splits, kinks, and proper routing.
b. Check ignition wires for cracking, hardness, and proper connections at both the distributor and spark plugs.
c. Check all wiring for proper connections, pinches, and cuts.
d. Check wiring harness for proper routing.
e. Check for missing components.
2. Check TSBs for manual updates, testing procedures, warranty and recall information.
3. Check for latest PROM updates and replacements. Refer to COMPUTERIZED ENGINE CONTROLS/PROM I.D.
4. Perform the system check. The system check is performed through the twelve terminal assembly line data link (ALDL) connector under the instrument panel in the passenger compartment.
The Computer Command Control System Check is a procedure that determines the following:
a. Bulb Check - to check Service Engine Soon (SES) light circuit and that the ECM can complete the circuit.
b. Diagnostic Mode - this indicates if the diagnostic code system is working.
c. Scan Data - this determines if the ECM is supplying input and output visual data.
d. Engine Start - this step is done after it has been determined that the ECM will display codes and data.
e. Other Codes - Proceed to the applicable chart if a code is displayed. Scanning the data for typical values may indicate if they are incorrect.
5. At this point it would be advantageous to know if the problem is fuel related or other. Choose the path that best suits the individual situation.
a. For fuel related problems, Diagnosis By Symptom. Symptom Related Diagnostic Procedures
b. For all other systems, Computers and Control Systems / Diagnosis and Testing Procedures.
ALDL Connector:
EXTRACTING CODES:
Ground the diagnostic terminal (ALDL terminal A to B). The "SERVICE ENGINE SOON" ("CHECK ENGINE") light should flash a code 12. This is displayed, on the check engine light, by 1 flash followed by a short pause then 2 flashes. This should repeat 3 times. A code 12 should always flash when an on-board diagnostic check is performed. Any other codes, stored in the computer (ECM), will follow a code 12. If any other codes are stored, or code 12 does not flash, or the "CHECK ENGINE" light is on steady, or not at all, refer to Daignostic Charts.
CLEARING TROUBLE CODES:
CAUTION: To prevent major damage to the ECM, the key must be in the OFF position any time ECM power is disconnected or reconnected. This applies to not only trouble code clearing, but also when removing a battery cable for component repair or replacement and testing procedures.
To clear the trouble codes from the memory of the ECM, either to determine if the malfunction will occur again or because a repair has been completed, the ECM power feed must be disconnected for at least thirty (30) seconds. Depending on how the vehicle is equipped, the ECM power feed can be disconnected at the positive battery terminal pigtail, the in-line fuse holder that originates at the positive connection at the battery, or the ECM fuse in the fuse block. The negative battery terminal may be disconnected, but other on-board memory data, such as pre-set radio tuning, will also be lost.
When using a hand held diagnostic scanner, or SCAN tool to read codes, clearing the diagnostic trouble codes is done in the same manner as in the above procedure.
After the computer memory has been cleared, a change may be noted in vehicle performance. To restore normal vehicle performance, drive with engine at normal operating temperature, with the accelerator pedal going through idle, part throttle, and moderate acceleration modes until normal performance is restored.