Arm-LT1

I have a 93 LT1. I gave it a "complete" tune-up which includes changing the Optispark, a month ago. It's been running great. Yesterday, I ran around to a few stores, no problem. Later that day I drove the car to my neighbors a few houses away. When I tryed starting it, it would only crank but would not start. :mad I tryed disconnecting the battery for a few moments. It started for about 30seconds then quit. :confused: This morning I was able to do more testing. I discovered there is no spark coming out of the coil wire but I do have 12V to the coil. Now, from reading the old messages I see I have 3 choices. The coil, the coil module, or the ECM. I can not believe its the Opti sine I replaced it a month ago. How can I narrow it down to one?

PS: I think my Vette got pissed-off because I was talking to my neighbor about trading it in for a newer Vette.

Thanks everyone

jfb

The opti provides pulses to the ECM which provides pulses to the spark module. No opti pulses, no spark! I believe you should have an error code stored if you are missing any opti timing pulses, check for them. Some auto parts places can test your spark module. Disconnect your coil and with a .05 to .1 mfd capacitor across the coil primary, jump 12v to the primary in momentary pulses and each time you disconnect 12v from the primary, you should get a fat spark out of the coil to either of the primary wires (hold a jumper wire from a primary wire to within a half inch of the secondary of the coil).

Goody

You can Ohm out the primary and secondary sections of the coil to see if they are good. Mine ended up being the ICM.

Arm-LT1

Just to make sure we are talking about the same thing, when you say ICM is that the same as the ECM? Basically the Computer.

Arm-LT1

Can someone tell me how to check for codes? I remember see a web site that shows how to check for codes and what they mean.

Nathan Plemons

Something you might want to check out is the connector that plugs into your opti from the rest of the wiring harness. I had one get all corroded and not make up. It acted exactly like a bad opti because the computer couldn't get any pulses from it.

Arm-LT1

Well I pulled the codes off the computer (grounded A&B) came up with code 41. I'm not sure how accurate this code is. According to this web site: http://www.c4vettes.com/ecm.htm code 41 is for C4s from 85-91(I've got a 93). Plus the code referrs to an incorrectly seated PROM. Before I checked the codes I had pulled the EMC(with the battery disconnected) and reseated the EPROM. So the question is: Did this code happen before or after I pulled the EMC? But it is all starting to point to a faulty ECM. Don't you think?

Bruce

Since you say your optic just been replaced month ago. I would not be to fast about bad optic there. I would get a new coil. Bad coil = NO spark) You can buy a cheap one for $39 at auto stores.

Bruce

Tom Piper

If you are missing the low-resolution pulse from the Opti-Spark, you won't have spark or the injectors firing.

Put a noid light on the injectors to see if it flashes. If the injectors aren't being pulsed by the ECM/PCM, you can probably rule out the ignition control module and coil -- in this case, the likely problems are the Opti low-res pulse or the ECM.

Tom Piper


[Modified by Tom Piper, 10:05 PM 9/6/2003]

Goody

No, ECM is the ECM. The ICM is the Ignition Control Module. Sorry for the confusion. Should have typed it out I guess.

Word of caution about Modules from parts stores. I got one and it didn't work, nor did the other three that where tried by the Corvette only shop I had it towed to after a week of troubleshooting. Had to get an ACDelco one installed before car would run correctly. Go figure.

Do as Tom suggested first off. Will save much trouble. Don't assume your Opti is good just because it is only a month old. If you bought a refurb unit, then it is very very possible for it to die. They reuse all useable parts when they are turned in as cores.

Find out if the injectors are firing. As Tom stated, the ECM must get the pulses from the Opti or it will not allow the coil to fire.

Arm-LT1

Thanks for clearing that up. The Opti was bought brand new from a dealership. Nobody around town sells or can even order them. As far as the injectors firing, if I check one with a test light should I get a pulse? Will it be safe to use a test light? I don't want to burnout the ECM.


[Modified by Arm-LT1, 7:56 AM 9/7/2003]

Arm-LT1

OK new test results:
I put a test light to the injectors. I have a constant 12V to both wires with the ignition on and a constant 12V while cranking.
I also have a constant 12V to one coil wire both with the ignition on and while cranking, none of the other wires give out anything.
At the Ign Mod, I have no power neither with the ignition on or cranking. Any suggestions.

Tom Piper

If you use a noid light that was designed to test the injector, it won't hurt the ECM.

The noid light should flash while the engine is cranking if the ECM is trying to pulse the injector.

I have my own oscilliscopes and electronics test equipment. If you know someone that has an electronics background and an oscilloscope, you can look for the low-res and high-res pulses from the Opti-Spark.
The shop manual says the engine should run at reduced performance when only the high-res pulse is missing. But, it won't run at all if the low-res pulse is missing. The low-res pulse is used for both the injector timing and the ignition timing.

Tom Piper

Arm-LT1

Rule out the coil. I put a new coil on only because it was the only thing the auto parts store has and it was only $16.00. I was hoping to get away cheap but no luck.

Arm-LT1

Has anybody had any issues with rebuilt computers from the parts stores? Or any issues with new Optisparks from the dealer.

Tom Piper

My guess is you are missing the low-res pulse from the Opti-Spark.

My theory is the only way the ECM/PCM can tell if the engine is turning is from the Opti-Spark pulses -- there are no injector pulses or tach signal without the Opti-Spark pulses.
So, if there is never a pulse from the Opti-Spark while cranking, no trouble-codes will set -- why should the ECM set a trouble-code for missing Opti-Spark pulses when it doesn't know the engine is turning.
However, if the Opti-Spark is missing the pulses intermittently, the ECM, seeing some pulses from the Opti-Spark, is aware the engine is turning but the Opti-Spark pulses are not being generated correctly and sets a trouble-code.

The first thing I would check is the connectors for the Opti-Spark to see if they are seated correctly.

Tom Piper

Arm-LT1

I unplugged and replugged everything at least 3 times. I also shot some air into the connector to make sure they are clean. Now one question I do have, if the ECM was bad would it still be giving out error codes?

jfb

The ECM can still provide error codes and have other problems. You need to use an oscilloscope and prove that you are getting both low and high resolution pulses from the optispark.

Arm-LT1

Is there any documentation or website to show me how to do that?

QuikZilver

I had a similar problem on my 93. I had it towed into the dealership because it was acting like there was no fuel getting to the engine. Turns out, the coil shorted out and took out the Ignition Control Module. I think that is going to be the source of your problem. As for rebuilt ECM's, I would go with one built by A-1 Cardone. They are great remanufacturers, that offer a lifetime warranty on their stuff. When I worked for Discount Auto Parts (now Advance Auto Parts), I never had an ECM come back in 4 years working there. Good luck!

:cheers: