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1994 GMC Sonoma 4.3L VIN W
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Discussion Starter #1 (Edited)
At the request of the moderators, I am posting a new topic.

Here is the issue happening on my '94 Sonoma 4.3 Vin W. I've been having horrible acceleration ever since my timing was fixed. Prior to this, I wasn't able to pass smog and they told me my timing was off. It was. It was somewhere around 20 BTDC and should be 0. Ran great at 20, felt like pulling an anchor at 0. It's odd because just two years earlier the truck passed with flying colors. The only repair done since then has been a replaced distributor. I took it to a shop to checkout and they get the code 42. The checked the distributor and it looks like the guys who replaced my distributor didn't replace the ICM (Ignition Control Module). They replaced it, still got 42. After spending several hours they gave up and I took home to fix myself.

340396


I've got the actual shop manuals from GM. I check my Code 42 using the shop manual it tells me the ICM isn't grounded. I pulled the cap and it may look like the distributor was replaced, but they reused the old ICM (I think after replacing the ICM and it didn't work, they put the old one back on). Instead of playing around, I bought a new one (Cardone) from Rock Auto. Installed and set to 0 and still running like garbage (now before someone tries telling me the proper way to check timing, I already know). I'm at a loss.I'm following the steps from the shop manual and this is what is says to do:
  1. Ignition off.
  2. Disconnect ECM connectors.
  3. Ignition on.
  4. Ohmmeter selector switch in the 1000 to 2000 Ohms range.
  5. Probe ECM harness connector CKT 423 with an Ohmmeter to ground.
  6. It should read less than 500 Ohms.
I get ZERO Ohms. I should at least get some resistance, less than 500 but certainly not 0. I've checked continuity for all four wires from the ECM plug to the ICM plug and each wire is good. I've checked that I have ground at the distributor using a probe light and it's good. I can't believe I've now had 3 bad ICM's.

If I continue on with the troubleshooting, my next step is: Probe ECM harness connector CKT 424 with a test light to 12 Volts and note light. I get a light.. Next step is: Disconnect ignition module 4-way connector. The light goes off as expected. The next step is also impossible since step one had zero Ohms, but it says:
With Ohmmeter still connected to ECM harness CKT 423 and ground. Again probe ECM harness CKT 424 with the test light connected to 12 Volts. (As test light contacts CKT 424, resistance should switch from under 500 to over 5000 Ohms). And of course, nothing happens. So, there really is no point going on since step 1 was no good and basically means Open CKT 423, faulty connection or faulty ignition module. Now, since I know the connection isn't open, at least the wire plug-to-plug is good, that can only mean a bad ICM. I'm very stubborn and refuse to think 3 bad ICM's now. UGH!!!!! I wish I lived back in Detroit where I grew up. There was an automotive electronics shop that would figure this out. This guy was amazing.

For reference, here are some of the components replaced over the past year or so.
Coil, plugs, wires, MAP sensor, and the spider fuel injector and fuel filter. Most were a precaution due to age. The MAP sensor because the connector was broken and unusable. Replaced the plug and the sensor. Spider because I suspected it based on gas fumes. It was bad, fuel all over inside the intake manifold. Either way, lots of part replaced and the only thing that makes this truck perform properly is 20 BTDC and it won't pass smog like this.

I'm interested if anyone has some other suggestions. I'm going to play around some more at the distributor to see if I get any ohms testing right from the prong on the ICM, rather than the plug to the ECM. It is possible that even though I got continuity that the connector is still bad. I can't be 100% certain that when plugged in to the ICM that it's making the same contact I am by shoving a probe into the connector plug. The shop manual is pointless now because it's just an endless loop of replace ICM and try again. One thing I really hate is how this damn distributor is jammed up right at the firewall. I practically have to lay on top of the engine to work on it. And, that plug coming from the ECM has no slack. If I do have to replace it, I will be laying on top and trying to solder a new plug on.

If anyone wants to see the pinouts for the ECM, this is a good reference:
Part 1 -1994 PCM Pin Out Chart (GM 4.3L, 5.0L, 5.7L GAS with A/T)

Here's a good diagnostic site noted by the other post and it follows exactly what the shop manual from GM says:
Code 42 - Electronic Spark Timing Circuit (EST)

I give props back to the OP who at least got me off in the right direction:
92 4.3 VIN Z Code 42
 

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Quick question. When the timing was checked was the est wire unplugged? 0 degrees before top dead center would only work if, the computer advance is disabled. I know my 94 will typically pull 17 degrees of timing at idle with the est hooked up but when unhooked it rests at 0. That being said is the wire still unplugged? its under the dash on the passenger side next to the ecm separate from the harness but next to it. It's a brown wire with a black stripe i believe.
Edit: I'll get a pic of it for tomorrow for you i have to plug it back in on my 95 later it's the same set up.
 

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1994 GMC Sonoma 4.3L VIN W
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Discussion Starter #3
I know I typed a lot of data...
(now before someone tries telling me the proper way to check timing, I already know)
 

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1994 GMC Sonoma 4.3L VIN W
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Discussion Starter #5
Very lucky. From all my readings it seems to be one of the more elusive.
 

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1994 GMC Sonoma 4.3L VIN W
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Discussion Starter #7
That’s a thought but when you’re testing these connectors aren’t even plugged into the ECM. Just the ICM.
 

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1994 GMC Sonoma 4.3L VIN W
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Discussion Starter #8
After finally getting the correct pin out I was able to successfully complete the steps in the shop manual. It was the ECM. Replaced with remanufactured and now it’s running right. Still needs smog but at least it’s timed right and got acceleration again.

Follow the wire color but the shop manual has the wrong pin out location. This is what my shop manual shows and it’s definitely for my make/model. Follow the image from my first post. Also, this website is great with connector colors and locations.



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