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New member here. I have an 94 C4 with under 50k miles. The car ran fine when I put it away for winter storage, but now it wont start. I have no spark at the spark plug, so I then tried the coil and also no spark.

I been reading the forums and watched some of the videos on YouTube. I followed a test I found online and if I did this correctly, my B terminal (White IGN control) for the ICM harness doesn’t do anything when I crank the motor over… I read to switch your meter to ac scale and it should read 1 - 4 vac? Well, I get nothing… I have 12v at A and D on the ICM. For the opti I have 4.99V at A and B, I have 12V on C, but it’s the B terminal on the ICM harness… So, what does this mean? Is my computer bad? Or Opti?

Where i'm stumped, is how do I figure out if its my Opti or computer? If it was easier to change I would just throw an opti on and see, but obviously this is a little job and hate to do it for no reason. I did hook up my OTC Encore and I didn't get any codes. I was able to get connection with my Encore and the computer, so that should tell me there my PC isn't fried?

Any ideas?

Thanks.
 

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Welcome to the forum.

The 1st thing you need to do is hook an obd 1 scanner up and read the codes. This is because the 4 big ignition codes don't light the CEL.

These codes include dtc 16- opti low resolution pulse missing. Dtc 36- opti high res pulse missing. Dtc 41 & 42 which are icm codes. All these codes except dtc 36 will prevent engine from running.

I'm not real familiar with corvettes, but I've read that vetts can display codes on instrument panel.

In order to have the 1 to 4 volts ac on the white ign control wire, there must be voltage at power input terminal of coil. I'm using f-body wiring diagrams so I can't say for sure if vette wires are same color.

But the pink wire needs to have battery voltage when ign sw is turned on. Also the pink/black wire that goes from coil to icm terminal A, which is ign input, has battery voltage when ign sw is on.

Also black ground wire from ICM needs to have a good clean ground.

Lastly, the opti must send out the proper 5 volt square wave signal to the PCM. There are tests shown on internet to test low & high res signals, but these test do not show if proper square wave signal is present. Only 100% way of testing low & high res signals from opti is with an oscilloscope.

You can buy a PC based oscilloscope for around $70 on ebay, amazon, etc.
 

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Discussion Starter #3
Thanks for the quick reply.

I have power to the coil and to the ICM with the ignition on. i'll see if I can get the codes.

I been wanting to buy a scope... Maybe now is the time.

Thanks
 
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