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Old 08-22-2010, 06:30 PM   #1
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Little help with pcm

98 Wrangler, 4.0 manual

Think I have a PCM/ECM? problem, but want top make sure and am looking for a little help from some experts on here. Symptoms started about a month or so ago, fuel, voltage, speed, and RPM gauges dropped out a few times while driving, but only for a split second. Might have happened more, I guess it would be easy to miss. This week, it stalled while driving, and wouldn't start back up despite strong crank, no gauges or check engine light, but all other idiot lights and odometer working. First thought, fuel pump, but a little searching pointed to Crankshaft Sensor. Foolishly bought one and put it in, still nothing, so between tow and CKS, I'm already out some $$$. Saw a post somewhere that turn over but no start, with no gauge's or check engine light is almost surely the PCM. Also can't get a trouble code with the on/off ignition method. Other things I checked were fuses, relays, grounds, and unplugging harnesses to see if the gauges would come back.

I know the PCM isn't cheap, and I'd like to be sure before investing in one, and would like to avoid a shop which would involve another tow, and I live about 40 minutes from the nearest shop. Long story short, how can I be sure about this? What are the issues changing one out? I'm fairly good mechanically, brakes, radiators, starters, etc... but wouldn't mess with big jobs like engine or tranny work. And finally, where's the best place to get one?

Thanks in advance for your help, Rich

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Old 08-22-2010, 07:00 PM   #2
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You can't get the trouble codes with the key on-off-on method with the '98 or '99 TJ and possibly the '00 too. Only '97 and model years after '99 or '00 have that ability that was missing for a few years. You have to use an external code reader to extract any codes from those years. Changing the ECM is very easy, you only remove its cover and then its three big electrical connectors are easily removed to allow the new ECM to be installed. It can be swapped in 5 minutes from start to finish, nothing other than unplugging and reinstalling those three big connectors.

So none of the gauges even move when the ignition key is turned on? If that's the case, I would lean towards a bad ECM too. This assumes you have checked every single fuse behind the glove box and inside the PDC (power distribution center) on top of the passenger-side fenderwell.

Remanufactured ECMs can be found fairly inexpensively on eBay.

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Old 08-22-2010, 07:44 PM   #3
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Correct, the gauges don't even budge and I checked all the fuses in both locations, also swapped out the fuel pump relay and ASD relay with a known good one (the horn).

Good to know about the code, though it's kind of strange they would skip those years. I'll see if I can borrow a code reader from somewhere just to make sure, learned my lesson with the crank sensor.

Thanks for the input, I'll have to start searching for the part. Same yr., model, engine, tranny, etc... should work, correct?
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Old 08-22-2010, 07:45 PM   #4
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Quote:
Originally Posted by chowdnation View Post
Same yr., model, engine, tranny, etc... should work, correct?
Yes.
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Old 08-23-2010, 12:49 PM   #5
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OK, now I'm confused, where exactly is the ECU located on a 98 TJ??? I just looked quick on my way to work, and it isn't where I thought it was, driver side in back of the windshield washer tank. Then I saw this

WikiAnswers - Where is the ECM on a 1998 Jeep Wrangler Sport 4.0

and got really confused? Is it really under the carpet in front of the shifter? Is it easy to get at there? Are the powertrain unit and the engine unit the same thing? I need to get the numbers off the ECU to send to the part company, and want to make sure I'm on the same page as them. Will be pulling it as soon as I get home tonight, so any help ASAP would be appreciated.

Again, thanks in advance for any help
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Old 08-23-2010, 12:52 PM   #6
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The TJ's ECM is located on the passenger-side firewall, behind that protective aluminum cover. It has three large connectors connected to it. That object in front of the shifter, under the forward-most edge of the console, is the air bag sensor.

I went into Wiki and corrected that answer you found.
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Old 08-23-2010, 01:53 PM   #7
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You're the man!

Amazing how I couldn't find that answer, and when I did, it was the wrong one!

So it is what I suspected, I did find that in a TJ under hood diagram, but it was called the PCU, and I wasn't sure if they were one and the same.

Thanks again, hopefully I'm good to go on this now.

I'll have to dig this thread out if I'm not, amazing how fast these post's get buried 5-10 pages down!
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Old 08-23-2010, 01:57 PM   #8
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Another name for the ECM is the PCM (powertrain control module) but the PCM term is more commonly used with newer engines where the engine computer is more tightly integrated with an automatic transmission. For the 2002 and older that uses the "dumb" 32RH automatic transmission, ECM is the better term. For 2003 and newer that have the 42RLE automatic transmission which integrates its external computer (TCM... transmission control module) in tightly with the engine computer which is then more commonly called the PCM.
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Old 08-23-2010, 05:53 PM   #9
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Well I pulled the unit, and it took like 5 minutes, piece of cake. The unit has a sticker saying it was calibrated 3/2/09, which is literally weeks before I bought the Jeep.

I know that's no indication on whether it's bad or not, but the fact that it was serviced only like 18 months ago, does start making me doubt it is. I was thinking this part was sitting in there for the past 12 yrs. Are there local outfits that test these units? Dealers perhaps?
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Old 08-23-2010, 07:08 PM   #10
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You might want to try plugging those 3 connectors back in again, now that they are off.

Sometimes it's just a bad connection - especially since it was serviced lately. They aren't the best connectors, unplugging and plugging often get things working again. It's worth a try, and it's free.

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