81’ car with 87’ ECM ?

[PBGBodyFan]

Wondering if anyone has an earlier computer car, 81’-82’ or maybe 83’ and for whatever reason converted to a later year ECM and drivetrain. Specifically going from a Buick 3.8 to a 305. The computer interchanges and I assumed everything would function, I realized my 81’ ALDL is slightly different and was able to compare wiring schematics between the two, a couple minor differences but mostly the same. However after making those changes, when hooking up my ScanTool it isn’t reading when using the Chevrolet 305 87’-88’ ECM as my selection. The car runs fairly decent, no codes but I have seen the light on faintly at times, the light has been on before when it was stock so I know it should light up fully. I just want to make sure all sensors are reading as they should, to confirm lockup(doesn’t seem it’s locking up but rarely have gotten it up high in speed) and generally the ECM should think it’s in a 87’-88’ Monte.

I suppose I could go nuclear and swap in a later year Monte dash harness and then underhood wiring harness/headlight harness to match but rather not go that far and don’t think I should have to.

 

[maxi426]

If the numbers on the two ECMs interchange, did you remove the PROM from the original, and install it in the replacement ECM?

 

[PBGBodyFan]

The PROM is a “ZZ4” slightly altered copy, I run a similar setup on my SS using the CCC Q-Jet. The goal is that the computer/carb thinks it’s a 87’-88’ SBC

 

[69hurstolds]

The ECM already thinks that it (the ECM) is an 87/88 SS if you're using an 87 ECM and PROM. But does the car believe it is? More importantly, does the ECM think the car itself is an 87/88 SS?

If you've compared all the wiring and the inputs and outputs match up to respective connections and the setpoints/ranges are the same, there should be no reason you don't have a way to read it. Can't speak for MC ECMs, but Olds has a few differences on the inputs and outputs on the ECMs between the years, and control different things, some that can be ignored, and some that cannot. Will a paper clip method read any codes with the flashing bulb method or is that what you meant about being dim on the dash?

I'm the last guy you want to ask about wiring, but it's like any other communication- line them up to be the same and there shouldn't be any issue. The M/C solenoid and O2 sensor, etc., doesn't know or care what engine they're on. I'm sure there's an electronic spark timing system that it controls to advance/retard the ignition system. I think that didn't exist the same manner in 81, but I could be incorrect there. Some V6s use a knock sensor where I don't believe was used in V8 applications?

 

[ck80]

The PROM is a “ZZ4” slightly altered copy, I run a similar setup on my SS using the CCC Q-Jet. The goal is that the computer/carb thinks it’s a 87’-88’ SBC

Just curious, other than the prom being a copy and the issues that brings along, why not pick an ecm from an earlier LG4 of a similar year to the chassis/body? Can you not use your knockoff prom in an earlier ecm?

 

[PBGBodyFan]

Just curious, other than the prom being a copy and the issues that brings along, why not pick an ecm from an earlier LG4 of a similar year to the chassis/body? Can you not use your knockoff prom in an earlier ecm?

Correct, the ZZ4 PROM only works in the 87-88 ECM. My 86’ has an 87’-88’ ECM and it reads/works perfectly fine. I could put in a 85-86 ECM I have from a SS but don’t think it will change anything, there is obviously some minor differences between 81’ and 83’-84’ from glancing at an extra GP dash harness I have. I also assume the engine option doesn’t affect the dash harness at all, should just depend on year.

I plan to trace wires from extra dash harnesses I have to find the difference, I just assumed there are a few guys out there who have done this.

 

[ck80]

Correct, the ZZ4 PROM only works in the 87-88 ECM. My 86’ has an 87’-88’ ECM and it reads/works perfectly fine. I could put in a 85-86 ECM I have from a SS but don’t think it will change anything, there is obviously some minor differences between 81’ and 83’-84’ from glancing at an extra GP dash harness I have. I also assume the engine option doesn’t affect the dash harness at all, should just depend on year.

I plan to trace wires from extra dash harnesses I have to find the difference, I just assumed there are a few guys out there who have done this.

Here's what Hollander had to say on the ECMs minus prom as far as interchangability:

So, according to it/them.. . Ignoring the prom occasionally kept other differences???

Not saying close-enough can't apply, you see the same thing with carburetors. There were, somehow, non prom differences. I'm happy to pull other gbodies for you too if you think that could help.

 

[PVDave]

Comparing wiring diagrams, I see the following differences (Blank equals no wire at the terminal):

22 wire connector:
Pin 6 is a blank on the 3.8, it powers the Early Fuel Evaporation (EFE) Heater relay on the 5.0
Pin 15 is the "Cold Start Program Mod Input" on the 3.8, It's a blank on the 5.0
Pin 17 is a High gear input on the 3.8, EGR input on the 5.0
Pin 19 sends 5 volts to the VSS, it's a blank on the 5.0 (but probably still carries 5 volts)

18 wire connector:
Pin E is the Canister Purge Solenoid control on the 3.8, blank on the 5.0
Pin L is blank on the 3.8, Spark Control IND on the 5.0

Beyond that, the terminals appear to match up one to one on both engines.

D/S