Code 42 - Electronic Spark Timing: Notes

Code 42 indicates that the ECM has seen an open or grounded by-pass circuit (terminal No. 11) or an open or grounded EST circuit (terminal No. 12). A grounded EST circuit may not set a code unless cranked 10 seconds or longer with circuit grounded.

1. This test checks operation of EST. Grounding ALDL "test" terminal causes timing to go to a fixed value which is normally different from that obtained with EST operating, therefore, timing should change. Usually the change can be heard in engine RPM. If timing varies with increase in engine RPM (module advance), a problem is indicated.
2. This step eliminates the ECM and ECM connections from module input. By jumping terminals "A" and "B", the distributor reference signal is fed directly into the EST line of module. By putting voltage through the test light on harness terminal "C", the module is switched to EST mode and vehicle should run. If engine stops, there is no EST signal reaching the module due to open or poor connections, or module is faulty.
3. By removing the jumper, you are opening the EST signal, and engine should stop.
4. Since the engine ran when the module was jumpered, the problem is not in the distributor (if correct HEI module is installed). The wrong HEI module can set a Code 42.