Code 42 - Est Ignition Circuit: Notes

When the engine is cranking, the ignition module sends a reference signal to the ECM. While the engine speed is less than 400 RPM, the ignition module controls the ignition timing. When the engine speed exceeds 400 RPM, the ECM sends a 5-volt signal on the by-pass circuit No. 424 to switch the timing to ECM control circuit No. 423. An open or ground in the EST circuit stalls the engine and sets a Code 42. The engine can be re-started but will not run on module timing.

To set a Code 42 the following conditions must occur:

• Engine speed greater than 600 RPM with no EST pulse for 200 milliseconds (open or grounded circuit No. 423).

Or

• ECM commanding by-pass mode (open or grounded circuit No. 424).

1. Checks to see if ECM recognizes a problem. If it doesn't set Code 42 at this point, it is an intermittent problem and could be due to a loose connection.
2. With the ECM disconnected, the ohmmeter should be reading less than 200 ohms, which is the normal resistance of the EST circuit through the ignition module. A higher resistance would indicate a fault in circuit No. 423, a poor ignition module connection, or a faulty ignition module.
3. If test light was on when connected from 12 volts to ECM harness terminal "D5", either circuit No. 423 is shorted to ground, or the ignition module is faulty.
4. Checks to see if ignition module switches when the by-pass circuit is energized by 12 volts through the test light. If the ignition module actually switches, the ohmmeter reading should shift to greater than 8000 ohms.
5. Disconnecting the ignition module should make the ohmmeter read as if it were monitoring an open circuit (infinite reading). Otherwise, circuit No. 423 is shorted to ground.