Diagnostic Circuit Check

Diagnostic Circuit Chart:

Wiring Diagram For Diagnostic Circuit:

ALDL Connector:

CIRCUIT DESCRIPTION:

The diagnostic circuit check is an organized approach to identifying a problem created by an electronic engine control or Computerized Engine Control (CEC) system malfunction. This is the starting point for any driveability complaint diagnosis because it directs the service technician to the next logical diagnostic step. Understanding the chart and using it correctly will reduce diagnostic time and prevent the unnecessary replacement of good parts.
CAUTION: Not all SCAN testers are compatible with every system. A SCAN tool that produces faulty readings should NOT be used, and the problem should be reported to the tool manufacturer. Use of a faulty SCAN tool can result in improper diagnosis and unnecessary component replacement. Follow the proper instructions, located in the owners manual, for the specific SCAN tool being used.

TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.
1. This step checks for the proper operation of the "SERVICE ENGINE SOON" ("SES") light. The "SES" light should be "ON" steady.
2. No "SES" light at this point indicates that there is a problem with the "SES" light circuit.
3. This checks the ability of the ECM to control the "SES" light. With the diagnostic terminal grounded, the "SES" light should flash a Code 12 three times, followed by any trouble Codes stored in memory.
NOTE: An optional diagnostic code retrieval method can be used in TESTING PROCEDURES/ON-BOARD DIAGNOSTICS using the Electronic Climate Control (ECC) head.
4. Attach the "Scan" tool to read serial data. If a PROM error is present, the ECM may have been able to flash a Code 12/51, but not enable serial data.
5. Although the ECM is powered up, a "Cranks But Will Not Run" symptom could exist because of an ECM or system problem.
6. This step will isolate either of the following problems; an "SES" light complaint or a driveability problem with no "SES" light. An invalid code may be the result of a faulty "Scan" tool.
7. A comparison of actual control system data with typical "Scan" tool data (see TYPICAL SCAN TOOL VALUES under SCAN TESTER DIAGNOSIS) is a quick way to check if any parameter is not within limits. Keep in mind that a basic engine problem (i.e., advanced timing) may substantially alter sensor values.
8. Installation of a "Scan" tool will provide a good ground path for the ECM and may hide a driveability complaint due to poor ECM grounds.
9. If the actual data is not within the typical "Scan" tool data values (see step 7), see DIAGNOSTIC CHARTS/COMPONENT TESTING (C-CHARTS) for a functional check of the suspected component or system.