Really need some help
Mar 13, 2006 at 2:51 AM
1942 GMC Engine Code 42
1942 GMC
GUITARMAN5
- MEMBER
- 6 POSTS
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Trouble Code 42 indicates that there may be a malfunction in the Electronic Spark Timing (EST) system. During cranking, the timing is controlled by the ignition module and the ECM grounds the EST line. It expects to see no activity on this line at this time. When the ECM enters EST mode, it applies +5 volts to the BYPASS line and expects to see voltage variations on the EST line.
Reasons for the ECM to set Code 42:
- System in BYPASS mode (i.e. EST line supposedly grounded) but activity sensed on EST line
- System in EST mode (i.e. BYPASS line driven with +5 volts) but no activity seen on the EST line
Possible causes include:
1) BYPASS line is open or grounded
2) EST line is open or grounded
3) Poor connections between ignition module and ECM
4) Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise
5) Faulty or incorrect ignition module
6) Faulty ECM
Reasons for the ECM to set Code 42:
- System in BYPASS mode (i.e. EST line supposedly grounded) but activity sensed on EST line
- System in EST mode (i.e. BYPASS line driven with +5 volts) but no activity seen on the EST line
Possible causes include:
1) BYPASS line is open or grounded
2) EST line is open or grounded
3) Poor connections between ignition module and ECM
4) Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise
5) Faulty or incorrect ignition module
6) Faulty ECM
Mar 13, 2006 at 9:55 AM
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Suggested Repair
Diagnose Electronic Spark Timing code 42 by checking the BYPASS and EST lines for opens or grounds, inspecting ignition-module-to-ECM connections and EST harness routing, and testing ignition wire quality, the ignition module, and the ECM.
Conversation Summary
The owner of a 1942 GMC asked for help with engine code 42. The expert explained that code 42 indicates an Electronic Spark Timing fault: the ECM sees unexpected activity on the EST line in bypass mode, or no activity after applying 5 volts to the BYPASS line in EST mode. Suggested checks include the BYPASS and EST circuits, module-to-ECM connections, harness routing and ignition-wire noise, the ignition module, and the ECM.