Lessons & recommendations after C4 ECM failure

[Mark E.]

A few days after my 1970 suffered a sudden roadside failure (see Silly mistake causes highway breakdown and tow back home) the same happened with my 28k mile 1993. After the tow-of-shame home with my neighbors watching the drop-off, I broke open the service manual and Tech 2 to diagnose the problem. First issue: the Tech 2 would not connect; second issue: both cooling fans turn on with the ignition even though the engine is cold. The manual’s “crank-no start” diagnostic chart A-3 concluded a bad distributor (RPM indicated during cranking [NO] à terminal C grounded? [YES] à terminal D grounded [YES] à Faulty Distributor).

Since I replaced the infamous Opti-spark 5,000 miles ago with an upgraded Petris vented unit and it’s bone dry, I was skeptical with this diagnosis. So I scheduled a call with the owner and founder of Petris Enterprises, Chris Petris (you too can schedule a free consultation through their website). His conclusion: 99% sure it’s the ECM, not the distributor. And the likely cause is the all-to-common broken solder joints on the circuit board. Why? Apparently not being able to connect the Tech 2 and continuously running cooling fans are two big tells of ECM failure. Although Chris was generous enough to spend 40 minutes on a phone, it actually only took him 5 minutes to reach this conclusion, but we spent the rest of the time chatting about his company’s rebranding as Throwback Automotive and keeping the company family owned.

His recommendation: Ship the ECM to SIA Electronics for testing and repair. 10 days later my ’93 was back on the road and running great. Sure enough the cause was several failed solder joints.

Lessons:

• Refer to the manual for diagnosis but rely on a trusted resource. If you have a C4 LT1, Petris and SIA are great resources to keep you on the road.
• A clean, low mileage C4 is a bargain. Their low market price may be due in part to their sullied reputation for ECM and distributor failure. But with the above resources (and a Tech 2) they can be a fun, attainable, easy to repair sports car.
• As insurance during road trips, today I purchased a spare ECM on eBay and will have it tested by SIA.
• Experiencing two road side failures a few days apart in separate Corvettes makes a guy rethink his choice of pass times.

Cheers,

Mark

[Gary B.]

Mark,

Thanks a great story and a great result. Good job and a good recommendation.

Gary

[Josh B.]

Unfortunately 1992-93 cars had that infamous ECM failure. Great job diagnosing and fixing the issue. Love our C4s even when they give us random electrical problems.

[Thomas N.]

Great writeup. The Petris distributor is a work of art. The caps are much heavier duty than any of the FLAPS distributors. Their company provides a great service to the Corvette Community.

[Edward J.]

Mark, GM had a lot of computer issues in the 80’s and early 90’s generally due to soldered connections, vibrations are what kill them most of the time. And create havoc for a tech to find intermittent problems. I think for the amount of driving they get you ECM will likely last you for many years, most of the places the do repair them do a reflow of the solder in the circuit board.keep your spare computer in a cool dry area of your house, and when you take a trip put in car. Heat car also be a killer of computers.

[Mark E.]

...Heat car also be a killer of computers.

Thanks Edward. In Chevy's wisdom the C4 ECM is mounted in the engine compartment. Easy to get to but a harsh environment.

Years ago I owned a 1981 Z-28, the first year for "Computer Command Control" (CCC). Even though the ECM was in the more hospitable passenger compartment, I had nothing but problems with the whole system. To this day I will not consider a computer controlled carburetor car, and for this reason do not recommend getting a 1981 Corvette unless CCC is entirely bypassed or driving it is optional.

[Terry M.]

I once was able to share a candid conversation with a retired GM Electrical Engineer who decried the need to reduce lead in soldered connections in the electrical control modules. She predicted endemic failures of those modules due to the low lead content in the electrical boards.

[Mark E.]

I once was able to share a candid conversation with a retired GM Electrical Engineer who decried the need to reduce lead in soldered connections in the electrical control modules. She predicted endemic failures of those modules due to the low lead content in the electrical boards.

As Paul Harvey used to say, Terry just told "the rest of the story."

[Edward J.]

Thanks Edward. In Chevy's wisdom the C4 ECM is mounted in the engine compartment. Easy to get to but a harsh environment.

Years ago I owned a 1981 Z-28, the first year for "Computer Command Control" (CCC). Even though the ECM was in the more hospitable passenger compartment, I had nothing but problems with the whole system. To this day I will not consider a computer controlled carburetor car, and for this reason do not recommend getting a 1981 Corvette unless CCC is entirely bypassed or driving it is optional.

Mark, CCC carb. We’re a little hard to understand in 81 ,but after some training at the Gm training center it gave me some knowledge to understand how they worked, it was a short lived as throttle body Z28 cross fire came in 82, another short lived component, but I did not mind working on them.