Looking for a 1984 Corvette ECU

[EmTee]

I'm looking for a good used or remanufactured/rebuilt engine control unit (aka: "computer") for my father's 1984 Corvette.  It has the infamous Cross-Fire Injection and automatic transmission.  Any leads on a replacement or suggestions for what to look for on the one we have (i.e., are cracked solder joints or lifted traces typical) would be greatly appreciated.

[Bloo]

8 hours ago, EmTee said:

Any leads on a replacement or suggestions for what to look for on the one we have (i.e., are cracked solder joints or lifted traces typical) would be greatly appreciated.

 

How sure are you the control unit is bad?

 

I was a driveability tech in that era, so here's a bit of unasked for advice. Apologies if you are already miles ahead of me.

 

Bad control units (we called them ECMs back in those days) were pretty rare. The verified bad ones I saw could probably be counted on one hand, excluding a couple specific makes/models that had known design problems. I did see a lot of cars come through my service bay where the ECM had been replaced, but the car was still broken. The remanufactured ECM business was booming in those days. Great work if you can get it. 99% of the ones turned in for cores probably had nothing wrong with them, and of course there are no returns on electrical parts.

 

I imagine it is more common to see bad ECMs now, after all it is 1980s electronics, there's likely a bunch of electrolytic capacitors in there that have been deteriorating for decades.

 

Nevertheless, "it's never the ECM" was an unwritten rule in my trade back in those days. Often factory service manuals had flowcharts that would lead you down the wrong path, especially if there were multiple problems.

 

When you have come to the conclusion that the ECM is bad, in other words it is NOT doing something it is supposed to be doing. You have verified that, for instance, there is no injector pulse, or the unit is completely dead (wont flash code 12 in the case of GM), or there is one particular code that won't go away despite the circuit and sensor being checked, etc. The next thing to do is verify that all the power and ground leads are present and working.

 

Get out the wiring diagram, and find a pinout for the ECM. Pull the ECM, and test every power and ground pin on the wiring harness ECM plug. There will be Power, ignition power (key switched), and ground. There will be more than one pin for each of these. Make sure to check each and every power or ground pin.

 

Sensor circuits can be critical, and usually one or more sensor grounds from the ECM run all the way back through the wiring harness to the engine. Often one would break due to engine vibration, sometimes inside the insulation where you can't see. It will show up as a missing ground back at the ECM plug.

 

Only when you have verified that the ECM is misbehaving and all it's power and ground circuits are good should you assume the ECM is bad. If the trouble is with one specific sensor or actuator circuit, it's wires should be checked for open or shorts all the way back to the ECM plug as well.

 

 

Edited August 28, 2020 by Bloo (see edit history)

[EmTee]

Thanks Bloo, I concur with everything you said above.  Some history: my father bought the car from a guy he knew that had a used car lot and I believe the car was picked-up by him at an auction.  When my father bought it from him it did run & drive, however there was lamp cord and hardware store wire nuts and switches in multiple locations.  The most significant 'workaround' I saw then was the fuel pump, which was inoperative using the factory harness.  The pump and relay checked out, but wouldn't function without the lamp cord...  He drove it that way until it eventually refused to start.  I determined there was spark, but no fuel (it fired when gas was poured  into the throttle body).  Knowing the history, I checked the fuel supply and determined that fuel was being pumped to the throttle bodies, but the injectors weren't firing.  Fortunately he had a Chilton's manual with a wiring diagram and I traced the wiring to the ECM pin responsible for injector control.  I was able to activate the injectors by grounding the appropriate pin on the harness leading to the engine.  At that point, (now ~20 years ago) I concluded that the ECU was bad.  At that time, it was simply a matter of going to the auto part store and swapping the old one for a remanufactured unit.  As soon as the new ECU was installed, it fired right up.  Additionally, I determined that the fuel pump was now working as designed using the factory harness, so I ripped all of the hardware store crap out and the car ran like a bear!  Now, last year it did exhibit similar symptoms (no injector fire), but my father removed the ECU and found a couple of 'green' pins.  He cleaned up the connectors and reconnected the ECU and all was well again.  Today, there's no injector action again, but he checked the connectors at the ECU and the connections looked good.  So now the ECU is on my bench and I'm trying to decide how to proceed.  There is an area on one of the circuit boards that does look discolored, as if from heat.  I haven't inspected it closely for cracked solder joints, or other issues yet, though it does look to me as though traces are beginning to partially lift off the PCB.

 

I have not gone back to the car and verified that I can fire the injectors by grounding the injector pin on the ECU harness -- I need to do that (as well as the power & ground checks that you mentioned above) to rule-out a wiring issue.  Thanks again for the advice and I'll let you know what else (if anything) I find out...  -Tim

Edited August 29, 2020 by EmTee
add'l info (see edit history)

[Guest]

EmTee,

                    Good news and bad news.

The good news is, if you can locate an ECM for a 1982 Corvette it will work in an '84 Corvette, provided the '84 is an automatic. 

Bad news is, both year ECMs seem to be made of unobtanium.

I recently had a friend come to me looking for an ECM for his '82. The best deal I could find was the nearest Autozone store, they offer Blue Streak remans but have none in stock as there are no cores available. However, they would take his core, send it in to Blue Streak to be rebuilt for a cost of $200 which sounds pretty fair to me. 

Greg