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Old 11-05-2011, 11:30 AM   #1
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ECM, ECU, PCM ? what???

i just recieved my chilton manual and i was reading that diffrent year model of yjs have computers that refer to either as the ecm, ecu, pcm. i have 91, and so the manual says the computer is refered to as the ecm? the book says 91 and 92 have reference to ecm, and 93 and above refer to pcm, and 90 and below use ecu? im confuseddddd

i just posted a thread before this that probably didnt even make sense. i got confused about PCM (performance control module) and PCM (powertrain control module)? are they types of computers for diffrent year models? or do all jeeps have a ecu, ecm , and a pcm? i admit, im new to all this, ive never even known cars had computers... dontbash on me haha please...

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Old 11-05-2011, 11:48 AM   #2
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Old 11-05-2011, 01:15 PM   #3
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theyre not specified to which specific models? like i wanted to look for a new ecu for the jeep but i cant find a new one, but i can finda new ECM.
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Old 11-05-2011, 01:53 PM   #4
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All different names for the same thing.
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Old 11-05-2011, 03:31 PM   #5
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Originally Posted by tyewilly
all different names for the same thing.
x2
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Old 11-05-2011, 04:13 PM   #6
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soo if i buy a ecm from jc whitney, that fits for a 91 wrangler, theres a possibilty it could fix my cold start problem?
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Old 11-05-2011, 06:18 PM   #7
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Now that's a whole other animal. Lol. Cold start problem could be a couple things
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Old 11-05-2011, 11:09 PM   #8
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Now that's a whole other animal. Lol. Cold start problem could be a couple things
what do you mean by a whole other animal? lol i was researching and i stumbled across someone posting a cold start problem, where it couldnt be the batt, starter or alternator because my radio, lights, and volt gauge turned on, where someone started talking about ecms, ecus, pcms, saying how it could be causing the car not to start at cold temps. i feel so confident that its gotta be that i was experiencing the same exact thing that one dude was talking bout on this forum on how his car wasnt starting on a cold morning.
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Old 11-06-2011, 11:50 AM   #9
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Well what I mean is it's one possibility. But there's other things. For example I've heard of people having cold start problems and it was that during the night the fuel lines would not maintain fuel up top so when they were cranking it in the morning it took a second till the lines would fill again and the car or jeep in this instance would start
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Old 11-06-2011, 11:52 AM   #10
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Not saing that your problem. But just saying get some one to help you diagnose further before you spend money on something you don't need that's all.
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Old 11-06-2011, 03:57 PM   #11
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You were the closest to the correct answer early in your original post when you said: "different model years have different electronic modules called ECM, PCM, U812, etc. Your YJ does NOT have all of the electronic pieces you're asking about.

YJ's were going thru many changes every model year from '87-'90 then '91 and later. As the models changed (ex. 4.2L > 4.0L) the electronic management systems changed with them as did the acrynoms ECM, PCU and so on. So the 4.0L w/ FI (which was computer controlled) had different requirements electronically speaking. Ya still wit me?

As you read the Chilton manual, especially the wiring schematics, only pay attention to your 1991 model year. And above all remember this: if you can't fix it with a hammer or duc tape, you got an electrical problem!
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Old 11-06-2011, 08:21 PM   #12
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Well what I mean is it's one possibility. But there's other things. For example I've heard of people having cold start problems and it was that during the night the fuel lines would not maintain fuel up top so when they were cranking it in the morning it took a second till the lines would fill again and the car or jeep in this instance would start
hmmmm damm i keep changing my diagnoses, this am was cold and it started right away, diffrence is i filled that tank. ahhh dam i love learning. i love this forum
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Old 11-06-2011, 08:25 PM   #13
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You were the closest to the correct answer early in your original post when you said: "different model years have different electronic modules called ECM, PCM, U812, etc. Your YJ does NOT have all of the electronic pieces you're asking about.

YJ's were going thru many changes every model year from '87-'90 then '91 and later. As the models changed (ex. 4.2L > 4.0L) the electronic management systems changed with them as did the acrynoms ECM, PCU and so on. So the 4.0L w/ FI (which was computer controlled) had different requirements electronically speaking. Ya still wit me?

As you read the Chilton manual, especially the wiring schematics, only pay attention to your 1991 model year. And above all remember this: if you can't fix it with a hammer or duc tape, you got an electrical problem!
haha alright, i plan on restoring this rig to the closest i can to 100 percent, so sometime if ever i might buy that ecm i found on jc whitneyss
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Old 11-07-2011, 06:35 AM   #14
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Originally Posted by MrRaldood View Post
i just recieved my chilton manual and i was reading that diffrent year model of yjs have computers that refer to either as the ecm, ecu, pcm. i have 91, and so the manual says the computer is refered to as the ecm? the book says 91 and 92 have reference to ecm, and 93 and above refer to pcm, and 90 and below use ecu? im confuseddddd
Anymore, the names are used for any of the computers as people are too confused on what they are. But the difference between a ECM (Engine Control Module) and a PCM (Powertrain Controm Module), is that the ECM was only the engine and transmission's computer. The ABS, air bags and other accessories had a computer of their own to control them. So like for GM, and OBDI computer is technically an ECM. (Sorry, I don't know what diagnostics Jeep used). Then when OBDII came out, the computer controlled everything, therefore it was the PCM. As for ECU, it's the same concept as ECM, i'm assuming it was just a different diagnostic language for the computer.
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Old 11-07-2011, 08:47 AM   #15
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If my memory serves me correctly the "ECM" stands for "Emission Control Module" NOT "Engine Control Module"; at least according to the CHILTON manual.

On my '89, the ECM is a huge POS!!

The engine on an '89 has an Ignition Module and an ECM. The "Nutter Bypass" is to eliminate the ECM from the ignition system totally (which I did) and I am VERY HAPPY with the way the engine runs now!!!

If your '91 is normally asperated (carb not injected) don't be too anxious to put the ECM back on!!

Good luck;
Kahuner
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Old 11-07-2011, 09:00 AM   #16
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Originally Posted by Kahuner View Post
If my memory serves me correctly the "ECM" stands for "Emission Control Module" NOT "Engine Control Module"; at least according to the CHILTON manual.

On my '89, the ECM is a huge POS!!

The engine on an '89 has an Ignition Module and an ECM. The "Nutter Bypass" is to eliminate the ECM from the ignition system totally (which I did) and I am VERY HAPPY with the way the engine runs now!!!

If your '91 is normally asperated (carb not injected) don't be too anxious to put the ECM back on!!

Good luck;
Kahuner
It's Engine Control Module (referring to the computer that runs the engine).

And the ICM (Ignition Control Module) sits near or under the coil pack (s) normally.
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Old 03-06-2013, 02:16 PM   #17
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does anyone know where it is on a 90?
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Old 03-06-2013, 03:24 PM   #18
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Originally Posted by Kahuner View Post
If my memory serves me correctly the "ECM" stands for "Emission Control Module" NOT "Engine Control Module"; at least according to the CHILTON manual.

On my '89, the ECM is a huge POS!!

The engine on an '89 has an Ignition Module and an ECM. The "Nutter Bypass" is to eliminate the ECM from the ignition system totally (which I did) and I am VERY HAPPY with the way the engine runs now!!!

If your '91 is normally asperated (carb not injected) don't be too anxious to put the ECM back on!!

Good luck;
Kahuner
Close, but for an 89 the ECM is (Electronic Control Module). The Ignition Control Module controls "spark". The nutter bypass eliminates the ECM and ICM.
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Old 03-06-2013, 03:31 PM   #19
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does anyone know where it is on a 90?
If you have a carb'd 4.2 the Electronic Control Unit or Electronic Control Module is behind the dash, approx. center left. The Ignition Control Module is under the hood below the coolant expansion tank.
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Old 03-06-2013, 03:41 PM   #20
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soo if i buy a ecm from jc whitney, that fits for a 91 wrangler, theres a possibilty it could fix my cold start problem?
Probably not. That's most likely the last thing that would initially fix a cold start problem. There are a number of other components that have to be first eliminated as the cause. There have been so many running changes to the electronic management systems for YJ's that the only true way to know the exact replacement for any particular year and combination is to remove the Unit/Module that was installed at the factory and match (cross reference) the replacement based upon the unit/serial number from the factory unit. For example, in 1989 alone at least two different Electronic Control Units were utilized based upon VIN #'s during production. Over the next few years there were ongoing production changes due to the ECU/ECM's becoming more sophisticated.

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