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Unread 01-05-2012, 05:14 PM   #1
PNWClimb
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Electric problems, shorting in the ECU?

Alright, here's the long saga leading up to where I am now: About a month ago as I was pulling into a parking lot I noticed the dash lights getting really dim and the engine was struggling to keep going without pumping the pedal, and it eventually died in the lot. It seemed like the alt had gone bad to me so I got a jump hoping to limp it home, but pulling out of the lot and giving it some gas it died again, so a friendly jeeper hooked me up to his rig and towed it home. I'm at school fulltime these days so I had the alternator replaced, and that seemed to have fixed but a few days after the fact it died again on accelerating. Then I figured it was the CPS, had it towed to the shop where they told me that someone had cludged the O2 sensors and that the driveshaft was rubbing on the wiring, causing a short killing the engine. So the O2 sensors were replaced and set into the correct positions, and it seemed to have fixed it again. Then a few days ago the voltage would be steadily at 11.5 volts after firing it up, then a few minutes into driving it would plummet to 9 volts, check engine and gauges light and would come on, but it kept running. I pulled off to the side of the road and turned it off to try and diagnose it, and when I fired it back up the shifter button was stuck and I couldn't get it out of park! Eventually I muscled it into drive and limped it back home again. Found the alt wasn't pushing voltage again, got a replacement alt under warranty and replaced it myself (tired of shops screwing it up) but whenever I tap on the ECU with a rubber mallet it looses voltage. Could it be a short? This has been such an expensive endeavor I really need the final fix. Thanks all, I know there's a ton of knowledge on this forum.

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Unread 01-06-2012, 12:58 AM   #2
optdoug
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When the wires on the O2 sensor short out, it is likely that the ECM maybe have been damaged as well. I had replaced something, I think it was the power steering pump. When I put it back together the O2 wires ended up being routed right next to the exhaust manifold and they melted and shorted. I had to replace the O2 sensor and the ECM to get it running again.
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Unread 01-06-2012, 01:08 AM   #3
firerescue98
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Depending in the year the charging system operates threw the ecu so to answer ur question and to help u out it most likely is the ecu
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Unread 01-06-2012, 01:09 AM   #4
firerescue98
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And to back up optdoug mine did the same as his same problem and i still havent got mine running cause i waiting to find a computer
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Unread 01-06-2012, 01:16 AM   #5
optdoug
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IMO, the junkyard is the best place to buy one.
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Unread 01-06-2012, 01:19 AM   #6
firerescue98
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Not around here i had to get one that was a hard find i had to get one from michigan i live in missouri
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Unread 01-06-2012, 02:24 AM   #7
PNWClimb
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Anyone know If I could crack the ECU and look around for something thats blown? The fact that it'll start, run and charge relatively well until it's vibrated gives me some hope. I'm pretty handy with a soldering iron and a multimeter. If not I'll start prowling the yards. Would I need a year specific ECU? Or just within 97-01? I'm guessing it needs have the same number of O2 sensors. Thanks.
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Unread 01-06-2012, 01:24 PM   #8
BlueRidgeMark
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Quote:
Originally Posted by PNWClimb View Post
Anyone know If I could crack the ECU and look around for something thats blown?

Fuggedaboudit.


You're dealing with solid state electronics. Most of the components can be blown without showing any external sign of damage. Even if you see an obviously fried component, you won't know if seventeen other components are damaged, too.

Without a schematic and the knowledge to go with it, you're shooting in the dark.

The most easily damaged are microprocessors, and you won't see any sign of it, and you can't find the problem with a multimeter even if you know what you're doing.
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Unread 01-06-2012, 11:21 PM   #9
Reptile
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The ECU does regulate the alternator output, and the shorting of the O2 sensors that outdouf and firerescue have experienced themselves would definitely explain it and it most likely sound like that's what's going on.

The other possibility is that the alternator issue could have finished up the battery... and a weak battery can cause the ECU to act up, which by the way 11.5V is not a sufficient voltage and isn't an indication of a healthy battery.
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Unread 01-07-2012, 05:06 AM   #10
tjwalker
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Quote:
Originally Posted by PNWClimb View Post
Found the alt wasn't pushing voltage again, got a replacement alt under warranty and replaced it myself (tired of shops screwing it up) but whenever I tap on the ECU with a rubber mallet it looses voltage.
This is by far your biggest clue. Gotta look at the "cause and effect" principle.

Get a salvage yard computer installed. It's the only reasonable economic solution as new ones are unbelievably expensive.....

Be sure it is compatible with your year, transmission, etc. The salvage yard should have the correct application data for you here.

Good luck and let us know if that resolves it. It most likely will.
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Unread 01-07-2012, 05:33 AM   #11
mj40
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looked up smc and cardone parts....smc no longer makes/rebuilds ecm for this..cardone list 79-1277 for a 97 4.0l AT and federal emissions...if you got cali emissions then its 79-1040...

http://www.cardone.com/find-parts/ve...okee&year=1997
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Unread 01-07-2012, 09:22 PM   #12
PNWClimb
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Drove down to Oly4x4 today and got lucky finding a pretty lil blue 97 and popped the computer out of it, and threw it in the heep, lo and behold it started up and was reading at steady 14 volts. Took her for a celebratory spin around the block It was idling at around 1k for a awhile but I'm guessing its just re-learning the maps. Thanks all!
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Unread 01-08-2012, 05:20 AM   #13
tjwalker
It's the crank sensor!
 
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Quote:
Originally Posted by PNWClimb View Post
Drove down to Oly4x4 today and got lucky finding a pretty lil blue 97 and popped the computer out of it, and threw it in the heep, lo and behold it started up and was reading at steady 14 volts. Took her for a celebratory spin around the block It was idling at around 1k for a awhile but I'm guessing its just re-learning the maps. Thanks all!
Glad to hear it is resolved and that we could help.

Just curious; what did they charge you for the used computer?
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Unread 01-08-2012, 06:14 AM   #14
PNWClimb
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$125 pulled myself, not too shabby considering the shop wanted $500 +labor to do it!
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Unread 01-09-2012, 09:04 AM   #15
firerescue98
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Im glad to here we could help and that u got it fixed thanks to chrysler and there fine enginering we half to replace computers because of small eletrical problems
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