Code 42, Electronic Spark Timing - Est

Code 42 indicates that the ECM has seen an open or grounded bypass, or EST circuit, with engine running.

1. Grounding the diagnostic terminal should cause the ECM to set a fixed timing value. The timing should change compared to the timing before grounding the terminal. This indicates the EST system is working.
2. Jumpering "B" to "B" and "D" to "D" completes the reference circuit so the car will run. Measuring "A" voltage checks the EST signal from the ECM.
3. Jumpering "A" to "A" completes the EST circuit with the bypass circuit still open. Now the engine is running on module timing. In this condition, the EST circuit is grounded by the HEI module so the EST voltage should now be less than .4 volt.
4. The test light serves as a jumper to apply voltage to the bypass terminal. This will switch the module to allow EST. Because the EST signal will no longer be grounded, there should be a voltage reading on the EST circuit.
5. This test checks the bypass circuit in the ECM. Normal reading is 4-5 volts with engine running.

Fig 1: Code 42, Electronic Spark Timing - EST