ECM differences

[justacruzr2]

I'm probably asking a question that no one can answer but here goes. Does anyone know the difference between a 1983 Pontiac GP ECM and a 1984 Pontiac GP ECM. I have both and they do have different part numbers on them. They are both form vehicles that have the 305 4 bbl in them. The 84 ECM I think is bad so I am using the 83 ECM. I am getting a code 41 which is related to the ignition module. But the distributor is new and it made me think that the ECM might be why. Here is something I found on RepairPro.com

"GM OBD Trouble Code 41
Ignition Control Error

The ECM monitors the ignition module performance. Code 41 can set when the ignition system performance does not meet factory specifications.

Fault Code Description

41 - Ignition control error
Symptoms

Engine cranks but won't start
Engine stalling
Lack of power
Common Problems

Faulty Ignition Module
Faulty Crankshaft Position sensor
Faulty ECM or wiring issue between ECM and ignition module
MEM-CAL Error
If the ECM detects an internal fault with the MEM-CAL, code 41 will set. The MEM-CAL is a special "chip" that is installed in the ECM, based on the particular vehicle configuration.

Fault Code Description

41 - MEM-CAL error
Symptoms

Engine may not start
Engine stalling or erratic performance
Common Problems

Internal ECM or MEM-CAL fault"

I am not experiencing any of those problems (no start, stalling, lack of power, etc.) and obviously it doesn't have a crank or cam sensor but this is what peeked my interest: "The MEM-CAL is a special "chip" that is installed in the ECM, based on the particular vehicle configuration". Even though both vehicles are essentially the same, It made me wonder if something like EPA regulations might have changed between the 2 years causing a small difference in specs and calculations. I would be willing to remove the ROM chip from the 84 ECM and install it in the 83 ECM if I knew what chip that was. Even though the 84 ECM might be bad, it's pretty rare that the ROM chip in it would be bad.

 

[Clone TIE Pilot]

Only thing I can think of is that the 83 may be the lower compression version of the LG4 with no knock sensor while the 84 is the higher compression version of the LG4 with the knock sensor / ignition timing retard system.

 

[justacruzr2]

Only thing I can think of is that the 83 may be the lower compression version of the LG4 with no knock sensor while the 84 is the higher compression version of the LG4 with the knock sensor / ignition timing retard system.

The 84 has no knock sensor. It also occurred to me that fuel requirements may have changed causing a small difference in specs and calculations. Also found out that there is only one chip that is socketed in the ECM and it's the Rom/Prom chip. Think I'll try swapping them and see what happens.

 

[57 Handyman]

Justacruzr2,

When dealing with the ECM, there are a boatload of things that make each of them different and unique. The most identifiable are the Service Number (7-8 digits) and broadcast code (BCC) 4 letters that are on a label attached to the case. Both of these pieces of information will get you in the ballpark for getting your GP to run and operate. While you maybe able to get your vehicle running, that is because each of these ECMs has what is called a "limp home" mode. Unfortunately, there are many variations/nuances in and among the internal programming and software that will/may need tweaking which drive you nuts! Let me just say, I do not believe it is as simple as removing and swapping the ROM-read only memory/chip.

Here is my suggestion: Go to the site gearheads-efi.com and seek the assistance of the folks who contribute their knowledge and expertise. For starters, cut and paste what you initially posted above and ask for their help. I'm certain there will be someone willing to provide assistance and point you in the right direction. Keep us posted on your progress/successes as there are others who face the same situation you are in. Good luck!

 

[69hurstolds]

Did you install the PROM correctly into the ECM? Those puppies are fragile and so easy to bend pins if you don't do it right. Were you getting the same code with the other ECM? I'm not sure if it would cause a 41 code or not.

I don't know exactly what is different between the two different ECM's. One thing that MIGHT be different is that there are different components that may interface with the ECM and the ability for the ECM to accomodate, like the broad use of OD transmissions for 84 vs. 83, etc. I just don't know. And for that reason, I doubt if you can swap the chips between 83 and 84, either. But I don't know. I don't follow Pontiac or Chevrolet that much on the ECM side of things.

According to what I can ascertain, your 1984 box broadcast code should be BDY if 3 speed auto trans., and BCZ or DCS if OD trans. (CJR/DKZ for 3 speed auto in CALIFORNIA).

Go by the service part number on the ECM box. The same box part number was used for many different cars/trucks regardless of the broadcast code or original OEM factory ECM part number. For example, a BBP ECM box with the same service part number as an 84 GP 305 V8 is THE SAME BOX. Calibrators/PROM chips are different, obviously.

Code 41 is no distributor reference pulses, from what I'm used to. Do you have an 84 GP Chassis Service manual for your car?

Assuming yours is an 84, ECM p/n is 1226455 superseded to 88999114. If you can find a new (all are refurbished) 1226455 (100 and change from GMpartsgiant.com), which are out there (saw used ones on ebay for 15 bucks with free ship), then you can rule out a bad ECM box, anyway.

Not saying they don't ever go bad, but the ECM box HARDLY EVER goes bad. But regardless of what the broadcast code is (that's for the calibrator, i.e., prom chip, anyway), the box part number is what you want to go by. The chip is a whole nuther story. The box is just a vessel. The calibrator chip is the brains. The smaller number above the service number box appears only on the original car's ECM, and if there was a service number associated with the box, the service box number would be listed, in this case 1226455.

 

[justacruzr2]

Thanks. I'll check out that website.

 

[justacruzr2]

Did you install the PROM correctly into the ECM? Those puppies are fragile and so easy to bend pins if you don't do it right. Were you getting the same code with the other ECM? I'm not sure if it would cause a 41 code or not.

I don't know exactly what is different between the two different ECM's. One thing that MIGHT be different is that there are different components that may interface with the ECM and the ability for the ECM to accomodate, like the broad use of OD transmissions for 84 vs. 83, etc. I just don't know. And for that reason, I doubt if you can swap the chips between 83 and 84, either. But I don't know. I don't follow Pontiac or Chevrolet that much on the ECM side of things.

According to what I can ascertain, your 1984 box broadcast code should be BDY if 3 speed auto trans., and BCZ or DCS if OD trans. (CJR/DKZ for 3 speed auto in CALIFORNIA).

Go by the service part number on the ECM box. The same box part number was used for many different cars/trucks regardless of the broadcast code or original OEM factory ECM part number. For example, a BBP ECM box with the same service part number as an 84 GP 305 V8 is THE SAME BOX. Calibrators/PROM chips are different, obviously.

Code 41 is no distributor reference pulses, from what I'm used to. Do you have an 84 GP Chassis Service manual for your car?

Assuming yours is an 84, ECM p/n is 1226455 superseded to 88999114. If you can find a new (all are refurbished) 1226455 (100 and change from GMpartsgiant.com), which are out there (saw used ones on ebay for 15 bucks with free ship), then you can rule out a bad ECM box, anyway.

Not saying they don't ever go bad, but the ECM box HARDLY EVER goes bad. But regardless of what the broadcast code is (that's for the calibrator, i.e., prom chip, anyway), the box part number is what you want to go by. The chip is a whole nuther story. The box is just a vessel. The calibrator chip is the brains. The smaller number above the service number box appears only on the original car's ECM, and if there was a service number associated with the box, the service box number would be listed, in this case 1226455.

=========================

Thanks for the valuable info. I haven't done anything yet with the PROM. Don't have a service manual either. My ECM's have all their original tags. A little bit of history first that should help you understand better. The 83 was a car I bought for parts back in 2007. Stripped it of everything that was useful which included the ECM. That car's long gone now. The 84 is a car that I've owned since 1995. I just retired it and am now using the 86 I just finished restoring. The 86 had the 231 in it and although I saved the ECM I knew it was useless for the 86 now that it has the 84's 305 in it. I originally put the 84's ECM in the 86 since the engine and all the wiring was transferred over from the 84. Seemed like the logical thing to do. But when I turned the ignition switch to the ON postion for the first time, the EGR solenoid and the anti-dieseling solenoid started clicking rapidly. Knew that wasn't right so I swapped in the 83 ECM and everything seemed OK. I do now remember that the 84 ECM was showing code 41 when it was in the 84 too. Thought it was probabbly the ignition control module and assumed that the new distributor (with new ignition moulde and pickup) would solve the problem. Most of the 84's sensors were bad but for the 86 I gave it all new sensors. Didn't know the trans was involved in this either. Reason is the 84 had a TH250C in it (that's strange). Thought it was the TH350C since on the outside they look exactly the same.....until you take them apart for a rebuild. That's when we found that out. It seems that it shouldn't make a difference to the reference signal though. I could be wrong. Those 3 letter codes you mention above I believe are for the guys on the assembly line. You find them on a number of parts such as master cylinder, air cleaner, instrument cluster. guage cluster, etc. It's easier for them to remember than an 8 digit part number. Especially when you are assembling thousands of cars. You can see these same codes on the build sheet for just about everything on the build sheet. Anyway, it seems that maybe this is a wiring problem since the 84 had the same code and now so does the 86 which now has the 84 wiring in it. The wiring that was transferred was the instrument panel wiring harness and the 2 wiring harnesses in the engine compartment. The only wiring that wasn't transfewrred was the rear harness from the fuse box to the rear of the car since it is only for the rear lighting and the fuel tank sender. Before I check the wiring I am going to install the new TPS that just came today. Last weekend I rebuilt the carb and the TPS tested bad. It passed the > 20K ohm test, but when depressing the plunger and watching the reading, It was very erratic...not smooth. It jumped all over the place. Didn't have another one on hand so I had to use that one until the new one came. I suppose that could have an effect on this too. The car does run OK but pings upon hard acceleration. TPS might have something to do with that since the lowest voltage reading/setting I could get on it was 1.08 volts....way above the 0.48 called for at curb idle and causing the ECM to think I'm accelerating. That wiil of course change the ignition timing too. A lot of interplay to think about when diagnosing this problem. If you have any more comments or suggestions after reading this please reply. It's appreciated.

 

[justacruzr2]

Well, here's the wind-up. I read a lot of articles on the web. And most of them talked about the normal symptoms of a true code 41. A few others also added checking the vacuum signal to the MAP sensor. I did. It was OK. But a couple others talked about code 41 being set when you turn on the ignition key without the engine running. Because the engine isn't running, of course there's no engine speed reference. So I tested this. I disconnected the battery for a minute, reconnected it and turned the ignition key to the ON position. Had the scanner hooked up for this test and it read a code 41....already! Another proof could be the fact that, other than the ignition ON self test, the check engine light does not come on like it does when there's a real problem. So in this case it looks as if I can ignore this. Had the scanner on and checked and set every adjustment. Also adjusted the curb idle (this time the correct way with the headlights on. Brother...can't believe I forgot that), reset the timing and set the electronic choke and the pull-off. Mostly carb adjustments. Purrs like a kitten now. So it seems that Code 41 doesn't necessarily mean there's a problem unless you sense a difference in engine performance.

Also, I'm running the 83 ECM in the 84. So it seems that even though it's a different part number, it's compatible with the 84 ECM. I don't feel any difference with the way the car runs.

 

[69hurstolds]

Sounds to me like you might've lucked up and are able to run the car with the basics at least. If you're getting a code 41, then SOMETHING is setting the code, although for some reason it's not puting you in limp home mode. IIRC, a code 41 by itself will go pig rich with locked base timing if you don't also have a Code 12 set. A Code 12 will not set. But the 41 will. Your reference signal is coming from the "R" terminal of the ignition module, through the "B" terminal of the distributor harness to ECM. A white/purple wire going to pinout #10 on the back of the ECM.

There may be peripheal equipment that isn't sending or getting any signals to the ECM. This MIGHT make it do weird things.

The 83 ECM is NOT 100% compatible with the 84. It just isn't.

A code 41 setting with key on and engine not running may mean your MAP or differential pressure sensor (VAC, aka D/P sensor) to the manifold is showing a running voltage when engine is off. Meaning the sensor could be hosed.

This is all based on an 84 307 5.0L troubleshooting procedures. GP SHOULD have the same, but the CSM would have the right info in it.

But if you like it, then go with it.

 

[Clone TIE Pilot]

What scanner arevyou using? The ECMs are setup to go into a weird diagnostic mode with limited timing advance anytime a scanner is plugged into ALDL port. Could just be something in your scanner causing a false code 41. Bitflipper's modified ZZ4 chip has this odd function edited out.