Diagnostic Circuit Check
Diagnostic circuit check is an organized approach in identifying a problem created by an electronic engine system malfunction. It must be the starting point for any driveability complaint diagnosis, because it directs the service technician to the next logical step in diagnosing a complaint. Understanding the chart and using it correctly will reduce diagnostic time and prevent unnecessary replacement of good parts.
NOTE:
Test numbers refer to test numbers on diagnostic chart.
- Checks Malfunction Indicator Light (MIL) operation. MIL should be on steady.
- No MIL at this time indicates a problem with MIL circuit or ECM's control of that circuit.
- Checks ability of ECM to control MIL. With Data Link Connector (DLC) grounded, MIL should flash Code 12 three times, followed by any codes stored in memory. Depending on type of ECM, ECM error may result in inability to flash Code 12.
- Most self-diagnostic procedures use a scan tester to aid in diagnosis; therefore, ensure serial data is available. If ECM or EEPROM error is present, ECM may have been able to flash Code 12 or 51, but not enable serial data.
- Although ECM is powered up, a cranks but will not run symptom could still exist because of ECM or system problem Go to appropriate CHART A-3 in BASIC TESTING article in this section.
- Checks if customer complaint is a MIL or a driveability problem with no MIL. See FAULT CODE IDENTIFICATION table under TROUBLE CODE DEFINITIONS for valid codes. An invalid code may be the result of a faulty scan tester or ECM.
- Comparison of actual control system data with the typical values is a quick check to determine if any parameter is not within limits. A base engine problem, (i.e. advanced cam timing), may substantially alter sensor values.
- Installation of a scan tester will provide a good ground path for ECM and may hide a driveability complaint due to poor ECM grounds.
- If actual data is not within typical values established, see I - SYS/COMP TESTS article in this section for functional check of suspect component or system.