LannyL81

My '95 has a very intermittent problem. The engine will begin to run very rough, has no power...and the tack needle will go a bit crazy during this. The first time this happend, I stopped and had it towed home..only for it to be fine for 1.5 weeks. Then it did it again and I was able to just keep driving it....was able to keep it running and it cleared-up and was fine again.

I have checked the obvious....loose battery connections, frame grounds...etc...but is this a sign of the ECM going out?

Suggestions???

Thanks,
LannyL81

aminnich

Are you getting any codes? Or have you checked for codes? Sounds like there should be some, either active or historic, or both.

byedan

opti on its way to the grave yard?

Tom Piper

On the '95, the tach signal is generated in the ECM from the Opti-Spark signals.
I would check the ECM for Opti-Spark trouble-codes.


Earlier LT1s generated the tach signal from the ignition coil through a tach filter instead.


Tom Piper

LannyL81

I replaced the Opti Spark unit in Dec '06, along with plugs, wires, radiator, coolant, water pump.

I tried using my code reader to pull any codes (no Service Engine Soon light on)...but could not get the ECM to respond at all. I know the code reader is working as it works on my '95 Blazer.

Probably time to just pull the ECM and send it out for repair/rebuild.

Anything else I should check into before doing so????????????

aminnich

ECM should throw a C12 code, meaning "Hi, I am working" if it is interfacing with your reader.

Daddy J

I'm probably saying something that you already are aware of. The 95 has an OBD 2 plug but is mostly OBD1, so depending on your code reader, your reader may not be functioning with the 95 properly. You may have to put the codes manually (You can use a paperclip)

LouisvilleLT4

Absolutely use the paperclip method for your '95 to read your PCM codes. It is very cool. Refer to this for an introduction:

http://forums.corvetteforum.com/show...perclip+method

With your symptoms it seems the only things that could possibly be bad are your Opti's optical sensor, the PCM, or the wiring in between the two. If you car stops running altogether, I'll bet a quick check with a multimeter would reveal no AC signal going from the PCM to the ignition coil module, meaning the computer found the signal coming from the Opti to be unusable and was unable to forward it to the coil module, the injectors, and the tachometer. If the wiring checks out and the PCM hasn't shown signs of total failure your OptiSpark's optical sensor has probably failed.

In reality it's very hard to prove a PCM is 100% good since any number of little glitches could develop that only occur intermittently. If everything else about it seems to be working (digital gauges, communication to the CCM when using the paperclip method, etc.) you should probably replace the Opti. The only sure way to know it's the Opti is to hook up an oscilloscope to the optical sensor's output circuits and verify the signal from the opti is messed up.

My car did exactly what yours did and replacing the Opti fixed it. The wiring is easy enough to check out with a multimeter and the FSM's wiring diagrams. The FSM also has a very useful Optispark diagnosis chart.

LannyL81

Thanks so much for the info on getting the codes out. I will try this one afternoon this week and see what I get. Think I will also try this on my '96, just to see what I get.


Thanks again.

Lannyl81

byedan

did you ever find out what the problem was?

lt4red

Paperclip will give you trouble codes for other computers onboard on a 96, but will not give you the engine codes. You have to use a code reader that reads OBD II to get these.

LannyL81

Nope...not yet. Have not had the time to even begin to look. Just not enough hours in the day anymore. Hope to get to it this weekend though.

Later

REDC4CORVETTE

I would also check the plug at the alt .These cars when they get old
eat the connectors.