1993 40th Anniversary with ECM Code 15

[Steve Barber]

I have a 40th Anniversary convertible. For almost a year now I have been searching for a trained and reliable technician to replace the optispark. So about a month ago I found a local automotive repair shop that does work on Corvettes but is somewhat limited in their knowledge of the C4.

I purchased the optispark kit from Petris Enterprises and I am very satified with the purchase. The Petris optispark is an upgraded model that is vented like the later C4 optisparks.

The problem now is a Service Engine Soon light with a code 15 in the ECM. The digital coolant temp displays 142 degrees all the time while the analog gauge appears to display correctly. Also, the electric fans run full time even when the car is started cold. The code 15 is coolant low temp issue.

I think I have completed enough diagnosis that points to the ECM as an issue, not the coolant temp sensor or coolant temp sensor plug or wiring issue.

I thought I read a post a couple of days ago on this forum, before I joined, where someone else was having the same issue. But until I joined, I could not see the follow-up posts. And now i can't find the same post. Has anyone else had similar issues and what was the solution?

 

[Robertwav1]

Did you replace the CTS? I know I had to replace it on my 93. This was a while ago. I believe the analog gauge uses a different sensor then the digital but I am not sure about this it's been so long ago. The analog is controlled by a resistor. Again, from what I remember....

Actually looked it up for you in the FSM. The engine coolant sensor (Overheat) is NOT hooked up to the ECM (Analog gauge), they are seperate so this is why it's misleading you...

 

[Steve Barber]

Did you replace the CTS? I know I had to replace it on my 93. This was a while ago. I believe the analog gauge uses a different sensor then the digital but I am not sure about this it's been so long ago. The analog is controlled by a resistor. Again, from what I remember....

Yes. The optispark died last May on our way to Orange Beach Alabama from just north of Nashville. We werew about 35+ miles south of Birmingham. It took me a long time to fiond someone to fix the car.

I purchased Petris Enterprises optispark kit. It came with everyhing needed to include optispark, water pump, CTS, spark plugs, wires, gaskets and seals. The tech did a good job of installing the parts but is not high;ly skilled with the C4 ECMs.

Chris Pertis has helped me rund several tests to eliminate all the possibilities so it appears the ECM is the problem. I have the ECM at the Cardone facility for repair. It appears Cardone does most of the GM repairs on electronics. I hope that will be the final piece.

 

[Robertwav1]

I've used Cardone in the past on different cars, never had a problem. I wish you the best.

 

[Steve40th]

Code 15 is Coolant temp sensor in the front of the water pump. Your digital cluster may read only 142 coolant temp.. But 15, which I just did 2 days ago on my 93 is this.
I have used several different ones over the years, the OEM is shorter and the resistance is always spot on. There is a chart as to what the sensor resistance across the posts should be. But this oem part works.
Only other issue is wiring of the sensor. Make sure its good from one end to the other.

https://www.amazon.com/dp/B000C9I038?psc=1&ref=ppx_yo2ov_dt_b_product_details

 

[Steve Barber]

I had a new CTS installed but got the code 15. I went thru several tests to eliminate a bad plug, failed ground, broken CTS plug wire, etc. I even sent the ECM off to be tested and it came bad OK.

So, I went and got another new CTS and that fixed the problem. Had to have been a bad CTS originally.

 

[Steve Barber]

Update:

Sorry for such a late update on getting my 40th annioversary fixed and back on the road again.

The final solution and piece the fixed the last bit of issues was to install and upgraded chip in the ECM for EGR delete and reprogram cooling fan temp sensors.

I have driven the car on a short trip and it did amazingly well. The car had about 32,000 miles when I bought it and I don't think it has run as good since. My fuel mileage increased a little with better horse power.

I just completed installing new Bilstein shocks, and what a major difference it has made. The car has 71,312 miles on the original factory shocks.

My desire to drive and continue improvements have been renewed. My next project is to find a dark ruby Stafast top. I am told the dark ruby color has been discontinued. Does anyone know where an original dark ruby top can be purchased?

 

[Robertwav1]

Nice getting that thing back in shape. Thanks for the update as well. Just a thought on the top if you can't find one. Think about vinyl, the same color as your car. I had an American flag done on my car about 10 years now, looks as good as the day I had it done. It was a quality vinyl shop. Good Luck!

 

[Steve Barber]

Nice getting that thing back in shape. Thanks for the update as well. Just a thought on the top if you can't find one. Think about vinyl, the same color as your car. I had an American flag done on my car about 10 years now, looks as good as the day I had it done. It was a quality vinyl shop. Good Luck!

Thanks.