MatthewMiller

If you can, try to test for a/c voltage at the actual pinout from the PCM. If there is voltage there, then the PCM is good and the wire is grounding out or intermittently broken inside. If the there is no voltage there, then I think you've isolated your problem to the actual PCM.

Yes, they can go bad. You can certainly open the PCM case and look over the two boards inside it. I would not spend time trying to track down and resolder connections in there, though. I'd just replace it or send it to be repaired. You can get a new one programmed to your exact VIN and it will plug right in for a 10-minute fix. If your car is all stock (i.e., doesn't need to have the new PCM custom-tuned), then you're all done and you get to go enjoy the car.

KenMathisHD

Yea, she's all stock, probably will be for a while longer. For a new PCM, don't miles and system info have to be manually transferred from the old PCM though?

The programmed for your VIN ones seem to cost quite a pretty penny, is it possible to flash them ourselves on just a remanned PCM from ACDELCO or A1-Cardone or somewhere?

On a sad note, RockAuto just ran out of ECM's in their inventory

MatthewMiller

Nope, the miles are stored either in the actual odometer or in the CCM under/behind your dashboard (not sure which). The PCM just sends a speedo signal to the CCM, which then runs the odometer. Ditto any system info. I have swapped between to different PCMs a number of times with no problems at all.

I really don't know what the programming to the VIN actually accomplishes, other than to give the stock original performance calibrations that your car originally came with. That was the route I went because I didn't have a stock calibration file to start from. There is an eBay seller called "Flashmasters" (among others) that will sell you a good used PCM programmed to your VIN for under $200. Also, you could contact SIA Electronics in southwest IL to see if they can diagnose and repair your PCM or sell you a reman.

KenMathisHD

does the ecm have to be from a vette, or can it be from any of the other lt1 engine cars like the camaro or the roadmaster or a pontiac firebird?

MatthewMiller

No, it can be from any 94/95 LT1 vehicle for your 94 C4.

KenMathisHD

Yay!

Flashmaster didn't have any computers specifically for the C4's, but they've got a whole selection of Camaro, Roadmaster, and Firebird comps for the LT1 engine. Should probably go with a Camaro comp since it's branded Chevy, just cause I'm paranoid that the Vette might see a Buick comp and eject me or something

They should still be able to program the VIN to it, right?

MatthewMiller

I was just looking there, and for some reason they don't list Corvette under their compatibility table. I don't know why. I would suggest emailing them to verify that they can flash it for your car before purchasing. People use Camaro PCMs all the time in Y-bodies (Corvettes), though.

KenMathisHD

Ok. I popped them and the SIA electronics an email, hopefully one of them can get back with good news. We shall see

KenMathisHD

Ok, so, good news, bad news, other news time

So, good news, I know basically what it takes to flash the PCM and where to get the stuff

Other news, I'm getting a signal to the ICM connector now, but still no spark. I changed the coil out but no dice. I don;t know if maybe I was reading it wrong or had it set up there wrong but the signal is there clear as a bell now

Bad news, I don't know what it is anymore

Some more good news, I know it's not the coil

I'm still getting the correct reads on opti connector and ICM connector, signal at injectors, pump, and now ICM again (or perhaps the whole time)

I'm still getting not getting spark though. I checked for voltage at coil connector (the gray plastic connector) and got voltage there, though none on the other connector (the black plastic one). I assume the black connector there has something to do with collapsing the field to send enough voltage to make spark.....?

The AutoZone near me doesn't have the equipment to test an ICM so I tried swapping in an ACDELCO, but no dice. Idk if it was DOA or if it's not the ICM. Hooking up the connector to the module should be sufficient yes? Or does it need to be attached to its little metal plate in order to actually work?

Right now, I'm shmucksed. The opti is sending a signal, evident of the injectors and ICM getting a signal, and the PCM is working because it's actually sending the signals it's getting from the opti, and the coil is replaced but there's still no spark, so the only thing I can think of at this point would be the ICM. Maybe the part was just a bad part?

MatthewMiller

FWIW, I got a brand new ICM once that was DOA from the get-go. So that happens, probably a lot more these days than it used to. I don't know if the ICM has to ground through that plate or not. I assume not, because terminal C on the ICM goes to ground. But I do know it needs to be mounted to its heat sink with heat sink paste or it will fry pretty quickly if the car actually starts and runs for any length of time. I would try it fully installed before I gave up on that ICM.

When you say the ICM is getting a signal, you mean on terminal B from the PCM, right? But is it sending out a signal on D to the coil? If so, then the ICM is good, at least when it's cold. If that output signal from the ICM is getting to the coil, then the problem has to be at the coil or distributor cap (not distributor optical sensor). I just ran across this site that tells you how to test your own ICM. I am specifically referring to Test 3 in that writeup, on page 3 of it. If all the signals are present up to the ICM, but there is no signal output on terminal D, then I think the ICM has to be the problem.

KenMathisHD

That's both reassuring and troubling to hear. It sucks that these things are DOA so often! Though I'm hoping that might save my butt lol

Yea, I tested terminal B to ground and got the 1-4v range. I know I'm getting a constant 12v at terminals A & D at the ICM connector. I'm gonna use the test on the site you shared to make sure that the coil is or isn't receiving a signal. I called AutoZone earlier and had them order in another ICM just in case I found that to be the problem. I just hope that's it!

Renfield

ICM can be mounted in cooler, non-metal locations. So I don't think that mounting it to the block will make any difference.

Check out this thread where it was mounted to the radiator cover.

https://www.corvetteforum.com/forums...their-icm.html

MatthewMiller

Okay, but a constant voltage on D isn't what you should see when the engine is cranking. That should be a signal switching on and off four times for every crank rotation while cranking. As that site says, it could either be sending a constant voltage or no voltage with the key on but engine not turning - it just depends on the position of the crankshaft. So try the test light method while cranking. Things would definitely be so easy if this were the problem!

auggy

I had the same question on the ICM on did it have to be attached (grounded) to work. Was told no because it has its own ground. This is true because I tried it with a new ICM and it worked unattached. This is a good reason to carry a spare since it takes less than a minute to hook up another one. Their are many on ebay to buy including used ones. It is not easy because I tried to make sure to get the right one. Some sights said it was correct where others said the same one would not work. Dah?? Finally I just tried it. Sorry it's not the coil. Keep us informed---really want to know what's going on with your car. I hear their are a lot of grounds near the oil filter--- might want to check those out.

KenMathisHD

Ok, so I tried the method. Before trying I again made sure that the ICM connector was getting a signal at B while cranking. Last time I noticed my multimeter wasn't changing voltage a whole lot when cranking, so I used a logic probe this time in hopes it would be faster. I connected it to the battery, and probed terminal B while cranking, with the connector not attached to the ICM, where I got a little green light that flashed a whole bunch while it cranked.

And I got a solid green light with key on but not cranking at terminal B.

I thought that was weird because green is for ground and red for voltage, so I pulled the connector for the ICM from the PCM and checked to see if maybe the wire was grounding out, but no light came on.

I probed the connector terminal at D with key on to make sure the probe wasn't being a dufus and got a solid red light, so I know that the probe is fine and that voltage is getting to that terminal with key on.

I reconnected the connector to the ICM and probed wire D and cranked it, didn't seem to get anything but a solid red signal. It dimmed when cranked, but never flashed. Just to be super duper certain, I took my multimeter, hooked up one lead to battery positive and one to the wire connecting to terminal D, and cranked it. It went from reading 12.5 volts to 10 volts while cranking, but never below that.

I figured this meant that the ICM wasn't making the signal like it should, so I went up to AutoZone, got the ICM I ordered (different brand from the last one), popped it in, cranked her and she still didn't start

Cooling the original didn't help to start, the ACDELCO ICM didn't change the start condition, and the Duralast ICM didn't start her either. I'm hoping I'm not just THAT unlucky, but at this point I feel that the probability of the ICM being the problem got a lot lower. The coil is replaced but that's not making a difference either, though if it's just my unlucky month then maybe that's DOA
Or maybe the problem is in the wiring to the coil?

The other thing I'm considering is that maybe the ground for the ICM connector is bad, or the ground for the coil, except that when I hook terminal A of the ICM connector up to terminal C, which is the ground for the connector, I'm getting a 12.5v reading. I also can't find a diagram indicating that the coil has it's own ground, but on that note I can't find a diagram that shows 4 wires connecting to the coil aside from the one I found on AutoZones site, except the colors of the wires going into the coil on my car and the labeled colors on the diagram don't match up, so I'm not entirely sure which is which and what I should be getting where at the connector coil

I know that with key on I'm getting 12.5v to one of the terminals on the coil connector, and that if I attatch it to the other terminal in it's connector I can read 12.5v, but I can't read that when connecting the power supplying terminal to any of the two terminals on the other connector, so maybe there's a bad ground there.......?

I feel like there's something way, way obvious that I'm missing, but for the life of me I'm not sure what it could be.

KenMathisHD

I went back and mucked around in the wiring more after I and she cooled off. I still can't find anything different between the ICM and the coil
However, and I don't know how I didn't notice this before, I did find this waiting for me under the little plastic corvette cover on the passenger side. There are exposed wires and some that actually have their insides jutting out of the wires. I wonder if it might be the culprit. Does anyone know what the part is? It connects to the other end of the harness that connects directly to the opti

Renfield

Now you're talking.

http://petrisenterprises.com/store/#...tegory=5760095

MatthewMiller

I think that is fine, because the PCM creates the signal by taking a constant voltage and periodically grounding it to create a binary on/off signal.

Okay, that signal coming out of the ICM on terminal D is probably best seen with a test light that just flashes on and off with the voltage. I'm honestly not sure what your voltage readings mean. If you have it set to read a/c voltage, then I am guessing that if it goes from 12.5v to 10v while cranking, then it is indeed creating a functioning signal to the coil. But that's just a guess.

Definitely make sure the signal wire going into the coil from the ICM is transmitting the signal. On my 96 wiring diagram, that is the dark green wire that goes into terminal B on the coil.

The coil isn't grounded. If you look at the diagram I posted, you see that the harness going into the coil only has two wires, at A and B. Some GM harnesses here apparently had a third wire that sends a signal to the tach, but ours gets a tach signal from the opti instead. So we just have the two wires.

I'm confused about this paragraph. On terminal A of the coil you should be getting constant 12.5v. B is the signal from the ICM, and with a test light in series it should flash when you crank the car.

If this is under the decorative plastic cover between the intake and the valve cover, then that is the other end of the four-wire harness from the opti sensor. If you look at the diagram I posted, look at the opti in the diagram at the bottom, and follow terminals A-D. As you follow them up toward the PCM, you'll see a dashed line with "C162". C162 is that connector. You definitely need to address these wires in the picture! However, I am not convinced that's the problem since you are getting the signals from the opti to your ICM and tach.

PS - The replacement harness that Renfield posted is the easy fix for those frayed wires.

Tom400CFI

I saw the pic and thought "THAT'S IT!"....but then recalled that he is (or says he is) getting a signal, so...

KenMathisHD

On AC I didn't get much of anything in a reading aside from a slight difference. When I didn't get anything AC, I checked DC and got 12.5 key on, 10v while cranking, which makes me think that maybe that's just the system drain, like how you would see a battery constant read lower voltage while cranking. The probe didn't flash, it just dimmed

Which one is terminal B? What's really throwing me off here with the coil is that there are four terminals at the coil and I can't make sense of where they come from, what they're going to or what they're for. What are the two other terminals for?

yea, that paragraph was worded pretty poorly on my part, sorry about that. What I was trying to say is there are 2 connectors, both with two terminals, that all plug into the coil. On one of the connectors, I can take the 12.5v signal and see it if I attach a lead to both terminals in that connector. But, I can't see that voltage if I attach a lead to the power-supplying terminal, and attach a lead to any of the other two terminals on the other connector. Let me grab a pic from the Vette here in a bit because I'm not certain if I'm accurately saying what I'm trying to say.

It is the one under the decorative plastic cover. That was a bad pic so I'll need to grab a different one, that was the only one I had in my phone when I posted. To me it looked like the one that connects the opti's connector all the way back to the PCM, but the connector c162 also looks like it has a deeper connector that fits into there. More pictures on the way!