DTC 42

Code 42 - Electronic Spark Timing (EST):

Wiring Diagram for Code 42 - Electronic Spark Timing (EST):

CODE 42 - ELECTRONIC SPARK TIMING (EST)

*** THIS PROCEDURE HAS BEEN UPDATED BY TSB #84-128, DATED 11/84

A false Code 42, electronic spark timing (EST) may be stored in the ECM. This condition does not cause a typical (EST) type condition such as "no start", "starts/will not run" or a change in RPM.

A false Code 42 can be detected in one of two ways, either the check engine light is on continuously or this is no check engine light but a Code 42 is stored in the memory of the ECM.

This condition can be caused by electrical feedback from the alternator light circuit back to the ECM. This condition can be corrected by installing a diode jumper assembly P/N 12047479 between the the alternator and harness.

Before the diode jumper assembly is installed it should be determined that a false Code 42 is causing the problem and not a legitimate EST problem.

Circuit Description:

Code 42 says that the ECM has seen:


^ Open or grounded By-pass Circuit (Term. "11")

^ Open or grounded EST Circuit (Term. "12").

A grounded EST may sometimes not set a code unless cranked 10 seconds or longer with circuit grounded.


Test Description:

1. This checks operation of EST. Grounding the "test" terminal causes timing to go to a fixed value which is normally different from that obtained with EST operating. Therefore, the timing should change. Usually the change can be heard in engine RPM. If so, the timing change does not have to be checked.

2. This step eliminates the ECM and ECM connections from the module in put. By jumpering Terminals "A" and "B", the distributor ref. signal is fed directly into the EST line of the module. By putting voltage through the test light on Terminal "C" of the harness, the module is switched to the EST mode and the vehicle should run. If the engine stops, there is no EST signal reaching the module due to open or poor connections, or the module is faulty.


3. By removing the jumper, you are opening the EST signal, and the engine should stop.


4. Since the engine ran when the module was jumpered, it says the problem is not in the distributor (if the correct HEI module is installed). The wrong HEI module can set a Code 42.