Scanner shows code "42" ??

[tuhy979]

Hey I've got a 94 Z28 LT1. I've had a problem with a bad missfire and stutter all over the RPM range cold and hot. WOT isn't so bad but there is a power loss. Replaced coil, ICM, Fuel filter, TPS, before I finally had it scanned. Snap-on scanner shows a code 42, ECS ground. The guy scanning it for me said a code 42 means bad ICM or if that was already replaced and condition still exists, it is my ECM (computer). I'm wondering if anybody has any thoughts and if he knows what he's talking about. Also I've never had a problem starting but this morning I went to the doghnut shop and when Icame back out it would turn but not start. Turned key all the way off and tried again and it started.???????? Any help appreciated.

[Trans Am]

I d check the opticrap

[TraceZ]

Do you know anyone with a PCM (Powertrain Control Module) you could swap in and test with? That would be easiest. I'd agree though, that it is either that or the optispark. I bet you can find a forum member with a spare PCM you could buy. It never hurts to have a few spare parts laying around for just these occasions.

In the future, I'd recommend buying an inexpensive OBD1 / 2 scanner like the AutoXray. Or if you have a laptop you can get a OBD1 cable and use freescan. Either way you are far batter off to own the scanner than to waste money on replacing parts randomly every time the check engine light comes on. Think of it as a wise investment.

Also, you call it a ECM. I think that GM stopped calling it a Engine Control Module at the end of 1993 and began calling it a PCM because integrated the function of the entire powertrain including the shifting of the automatic transmission. I'm pretty sure that it was still called a PCM even if your car was equipped with a manual transmission.

[Joe 1320]

just for kicks, check the condition of the ICM harness connection at the ICM itself. I have seen loose connections and mis-aligned pins cause this intermittant problem.

[fastTa]

Anytime I've ever seen a Code 42 on a OBDII, it has been a shorted Terminal B(white wire) on the ICM. The code 42 is actually listed as a grounded EST circuit (even in Alldata), but technically speaking, on the LT1 f-body, it is referred to as the Ignition Control Circuit. A code 42 is set when the PCM sees that the signal voltage was unusually low and likely grounded.

Check the white wire on the your ICM next to the coil on the front of your driver's side head. Look for any damage to the wire or see if is making contact with the bracket or something else close by that would allow it to ground.

It can also be a faulty ICM, or even a faulty PCM, but not likely.

[tuhy979]

Hey I checked the white wire terminal B on ICM. Looks OK all the way up through the harness, I played with all the wires and looked at all of them and they look alright. Also just noticed today the fans are always on. As soon as I start the car they are both on untill car shuts off. Don't know if that means anything. Seems like stutter is getting worse. I don't know but the ICM is only one day old. Has a knew coil also and I did the washer trick. In reply to the ECM PCM thoughts, I don't know what they call it but if don't you have to match it up with VIN's for it to work right? I looked into buying a used on and they had to know the #'s from the old. New ones had to be flashed before theyed work.

[fastTa]

Hey I checked the white wire terminal B on ICM. Looks OK all the way up through the harness, I played with all the wires and looked at all of them and they look alright. Also just noticed today the fans are always on. As soon as I start the car they are both on untill car shuts off. Don't know if that means anything. Seems like stutter is getting worse. I don't know but the ICM is only one day old. Has a knew coil also and I did the washer trick. In reply to the ECM PCM thoughts, I don't know what they call it but if don't you have to match it up with VIN's for it to work right? I looked into buying a used on and they had to know the #'s from the old. New ones had to be flashed before theyed work.

The fans are coming on full time because the PCM is recognizing a significant ignition problem.

You should clear the codes with a scanner first. Then start the motor and let it run for a minute and then turn off the motor. Look to see if DTC 42 has been set. If DTC 42 is set, pull the ICM connector. Using a multimeter, ground the negative lead and then measure the voltage at terminal 'B' (white wire) while someone is cranking the motor. Ideally you want over 4.5 volts. If you get a reading less than 1 volt then you likely have a faulty ground.

Straight from Alldata:

IF YOU DO NOT GET BETWEEN 1 AND 4 VOLTS:
Turn the ignition off, and disconnect the PCM harness connector B. Using a test light to positive on the battery, probe PCM harness terminal B5. If it lights up, then the problem is a bad ground between the PCM and the ICM. Inspect wiring as necessary. If the light DOESN't turn on, re-probe terminal B5 with the test light to ground. If the light comes on, then there is a short in the ignition control circuit to a (+) power source between the PCM and the ICM.

IF YOU GET BETWEEN 1 AND 4 VOLTS:
Turn the ignition off. Then using a test light to the positive battery terminal, probe the ignition coil module harness connector terminal 'C' (black wire). If it lights up, then the connection to the ICM is faulty or the ICM itself is the culprit. If it doesn't light, then the ground for the ignition control circuit is open. Check for shorts, wire breaks, rubbed-through insulation, etc. If the light does NOT come on, then the PCM is at fault.

[Injuneer]

There's a detailed logic tree in the GM manual for diagnosing DTC 42 in an LT1. Shoebox might have it on his website. I could fax you a copy.

[tuhy979]

OK I'm gonna do some tests with my meter, I can do the test light tests with my meter also, and I'll see what I find. Thanksalot for the info and I'll post results tomorrow.