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Unread 04-11-2012, 10:12 PM   #31
elCapitan28
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2000 XJ Cherokee 
 
Join Date: Apr 2012
Location: Provo, Utah
Posts: 32
Yep, the new computer fixed it. No problems. ...except for the normal gouging you get for having to go to a dealer, but after this much trouble, it's nice to just have it fixed. FYI, through a corporate accout, it was $485 plus about 1.2 hrs of labor (that includes tax), so total for me, $623

Still not completely sure if the problem was the welding, or bad O2 sensor wires, which someone said they had heard caused fried ECUs, but the check engine light that indicated an O2 problem is no longer showing, so... whatever. It's fixed.

I don't have a pic of the fusible link, but it's the one that goes from the fuse box (over the passenger side fender compartment, under the hood) down to the alternator. If you have shielding on it, and peel it away, you'd likely think it looks like any other 10 or 12 gague (I'm guessing) wire, but mine had a 2" black portion that looked sort of heat-shrinked over it, near the fuse box.

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Unread 04-11-2012, 11:21 PM   #32
Reptile
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1991 XJ Cherokee 
 
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Location: Bakersfield, CA
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Good to hear!
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Unread 04-12-2012, 01:23 AM   #33
5-90
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Join Date: Sep 2006
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Posts: 5,344
Gah.

1) "Brand new" is not always "known good" - especially with the chain stores. Noviry v proviry - "We trust but we also verify." If the unit hasn't been wrung out by you or someone you know and trust, it's not "known good."

2) The chain store benches don't test worth a damn, I don't consider the results there to be reliable. They'll test output voltage, but not the current - and the current is what's critical (those benches test for output voltage at 1.0-1.5ADC - and that won't even run the flashlight on my belt...)

3) There's also the possibility that you've got a shonky connection somewhere - again, "brand new" from a parts house isn't always "known good," since they don't QC aggressively. At best, it's "Statistical Process Control" - meaning they test a sample to figure a failure rate, adjust process to get the potential failure rate down to an acceptable level, then just test samples after that. I deal with a shop that tests everything that goes out, and I'll do a T&I on everything I've got before it goes out as well.

If you want a good qualitative test process, search up my process as posted in whatever the OEM Tech board here is (keywords are "battery", "alternator", "starter", "test".) That will allow you to wring out your system and see what's going on, you'll be done in between 10 and 30 minutes, and you need nothing more advanced than a voltmeter (length of time depends on how well you want to wring your system out.)

If you want a quantitative test, find a local shop that does naught by starter and alternator (or DC motor) rewinds, and have them go through it all for you.

(For instance, the shop I use has a test bench that looks like something out of a Frankenstein movie. But, it will give accurate test results to the hundredth of a volt and in 10mA increments - and tell you what the full output of the thing is - actual, not rated.)
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Unread 04-12-2012, 07:00 AM   #34
elCapitan28
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2000 XJ Cherokee 
 
Join Date: Apr 2012
Location: Provo, Utah
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@5-90, not sure what you're trying to get at. Sounds like you're saying I should have done more thorough testing on the alternator. But the same problem was occurring whether I had the old or the new alternator installed. That's when I had them both tested at AZ, and they both came out fine.

So I actually returned the new alternator and replaced the old one before troubleshooting other things. It was only when I replaced the computer that the issue resolved itself. The AZ test told me enough to know the alternator wasn't the problem at all, so I don't think a more rigorous test would have helped at all.

Unless you're saying that a more thorough test could have told me from the beginning that it was the computer, but that seems pretty impossible if you're testing an alternator disconnected from the rest of the system. You'd need a computer diagnostic tool for that.
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Unread 04-12-2012, 09:10 PM   #35
Reptile
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Quote:
Originally Posted by elCapitan28 View Post
@5-90, not sure what you're trying to get at. Sounds like you're saying I should have done more thorough testing on the alternator. But the same problem was occurring whether I had the old or the new alternator installed. That's when I had them both tested at AZ, and they both came out fine.

So I actually returned the new alternator and replaced the old one before troubleshooting other things. It was only when I replaced the computer that the issue resolved itself. The AZ test told me enough to know the alternator wasn't the problem at all, so I don't think a more rigorous test would have helped at all.

Unless you're saying that a more thorough test could have told me from the beginning that it was the computer, but that seems pretty impossible if you're testing an alternator disconnected from the rest of the system. You'd need a computer diagnostic tool for that.
Don't mind my friend 5-90 here. He's a smart guy and all but he always sounds like a robot. Your fix is fine.
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Unread 04-13-2012, 09:52 AM   #36
15tcherokee
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Location: Woodford County, Kentucky
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I don't have a chip key, so my question is what year ECM will work in my 2000 xj
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Unread 04-14-2012, 10:34 AM   #37
elCapitan28
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Join Date: Apr 2012
Location: Provo, Utah
Posts: 32
I'm no expert on this, but from what I've read/been told, ECMs can have a number of pieces of vehicle-specific information. Obviously, most of the data will be similar as long as you get a computer from the same year/make/model, but even then, the VIN, mileage, and SKIM key info (if it's been programmed that way) are all vehicle-specific...

So, best bet for 0 problems is to buy new, like I ended up having to. But if you really want to try a used computer, I'd go for one from a vehicle as similar to yours as possible. 2000 xj, CA or federal emissions (whichever yours is), no SKIM programmed, etc. Good luck finding that! Even then, you may run into problems if other modules have info that doesn't match the ECM. No guarantees.

Someone with more expertise, feel free to chime in
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Unread 09-10-2012, 07:53 PM   #38
94 greenmachine
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94 xj, had the same problem, pcm fixed it. Been dealing with one or the other alt fuses blowing randomly for two years. Put a voltmeter to the pos terminal on the alternastor when youre having issues. If its more the 14v, or 24v spikes like mine, its either a bad field control wire or the computer. Simple as that. The computer is the voltage regulator as opposed to damn near every other vehicle out there so you know if the output is fluctuating or not correct that the alternator may not be the issue.
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alternator , battery , volt meter , voltage drop

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