Codes came back after 2 weeks on 1991 Olds 88 Royale!
 

Hows everyone doing!
Well since I last posted, the dreaded codes 24,27,28,29,31 and even code 39 showed up after 2 weeks of no issues since replacing the ECM with a donor. I have been able to get some other things done on my car with the weather being so nice this month, I did the Delco ICM/Coil swap, and I can tell that it has helped my gas mileage some and I have not had any starting issues since. Second thing was that I went thru and cleaned up every single ground on the car that I could find. I have investigated the wiring under the hood and in the dash and tested all the wires connected to the transmission circuit and it all checked out. I was able to try a few other ECM units from the junk yard for a dirt cheap price along with even swapping mem-cal units and the codes still come back. However sometimes code 24 will show up at a stop light with the SES light then go off after I get moving again. Codes 27,28,29 and 31 will come back but not all together or at the same time. And the code 39 only showed up once. I have replaced the PNP switch, the stop light switch, the tcc/abs switch as well. I checked the speed sensor and even replaced the connector which was in bad shape, but for some reason the codes keep coming back. But I have no problem tracking this down even if it does cost some money because its such a great car and I plan on keeping it for as long as possible, since getting a used car means starting all over with god knows what kind of problems. Do you guys think its possible that all 4 ECMs are faulty? I mean 2 of them are remanufactured units and the other 2 are out of cars that only had 150000 miles or less on them.

This is one of those, rock and a hard place situations.....

Unfortunately, late 80's and early 90's, were not good years for ECM's.....

Even had problems with reman's right out of the box.......

So getting used one's, or reman's, might not solve the problem.....

In your case, you have the infamous, Red, White, and Blue ECM(colors of the connectors)....

So whether they are good or not, comes into question......

Back in the day, before you replaced an ECM, they wanted you to check every circuit that created the code in in the ECM......NOBODY DID IT......they would just take a known good working ECM, out of another vehicle, and try it.....Why? It was labor intensive......you had to power each circuit, and ground it with a Fluke meter, for a min of 10 minutes.......and then you would read the high and low amp readings of the circuit.....you needed a Fluke, because it had the ability to record high and low readings.....if you found a spike, then you had to replace the solenoid, relay, etc., retest, AND THEN, install the new ECM.....

I do know that these ECM of the late 80s and early 90s were very primitive at best. I have spent countless hours fixing damaged connectors and wiring since November, hoping to figure out these codes. I think its probably time to take it to a good transmission shop who can actually test the gear switches and tcc solenoid. I dont mind getting it fixed, I have become obsessed with finding out what is going on, but in the end it has been a very good learning expirence for me.