I have had this H42 code for years. No ICM problems But I have this history code of H42 saying ignition circuit grounded. I was looking at the electric diagrams and I see the black is grounded and the white wire goes to the PCM.. So wouldn't the White wire out of the ICM pigtail be the one being grounded somewhere? I would think the wire going to the PCM would be the only one being able to be read seeing if it is grounded or not.

I know of a couple other guys who have this problem. I might tear into my wiring harness, I have a couple other AC/fan issues going on. I might as well find out whats going on.


Note: I have reset the computer a tun of times, No ICM issues, just the SES light.

 

It's that not kind of short you would be thinking of. The code is set because the PCM is counting crankshaft revolutions via the resolution pulses from the opti. For some reason if it detects 84 revolutions and no activity on the white wire (B at ICM), it will set the code.

The PCM must think the white wire is shorted to ground. But if the engine is running, how could it be. I don’t know if it is detecting a low level voltage on the white wire triggering the code. It could be ICM related but only substitution might prove that point.

 

buying another ICM wont be a problem. I am just tired of looking at the SES light.. thank u so much for the info bud

 

If I test the wire by giving it voltage and using a test light, the light should eluminate showing it being grounded correct? The FSM says to do something like that. If no light pops on, on the test light, its not being grounded. That should rule the ICM in our out right?

 

I am ordering a new ac-delco ICM from rockauto tomorrow and I hope it fixes it. I like to take chances

 

Wrong. And don’t put voltage into anything or you will do some damage. The stuff you are reading in the FSM is probably targeted to finding a hard short. You do not have one because the engine is running. So you can throw all that out the window because it will not show anything up.

And that stuff about a test light is really only relevant when testing for 12 volts DC on terminals A & D of the ICM which comes from the ignition coil. If the 12 volts was not there, the engine would not run. Being in the electronics business, I never used a test light only meters for a very long time.

And you can't use test light on the white wire (ICM drive) because it is not a DC voltage. It is an AC voltage which is a varying pulse train only when the engine is turning. The faster the RPM, the higher the frequency of the signal.

In the FSM it tell you the test this AC voltage by connecting a meter the pin B of the ICM and cranking the engine. It won't start because the connector is off but the measurement is between 1 and 4 volts AC. This can be read on a scope also but the measurement is somewhat different.

I might consider inspecting, cleaning and re-seating the connections for the ICM and coil. Cleaning the connections on the black connector of the PCM which is the ICM output would be a nice idea, but with the age of the car, I hate touching those connectors if fear of breakage.