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ECU/Alternator Problem

I have a 97' xj that has me baffled. I seem to be having intermittent computer problems that is primarily affecting my alternator. I first notice the problem when the alternator begins overcharging. The gauge will indicate that the alternator is charging at 19 volts which seems to indeed be the case judging by the way my headlights respond.

At the same time the ABS light will come on and the transmission will not shift out of a high gear. However, I can still manually shift the automatic transmission. These three things happen pretty much occur simultaneously every time the problem occurs. About the half the time the other gauges (speedo, temp, rpms, etc..) will swing across their full span wildly and repeatedly. Shutting down and restarting resets everything to normal until the next occurrence.

I initially hoped it might be a grounding issue as I had replaced the battery a few weeks ago, so I checked and cleaned the battery connections. That seemed to decrease the frequency to once/every other week for several weeks until the last few days, where it is now occurring nearly every trip.

I know the voltage regulator is in the computer, so I suppose I will need to replace it, but I wanted to make sure there was nothing else obvious I need to check? Could the alternator or maybe a bad ground freak out the computer like this or is the ECU puckered?

Reply to
ds1234567890
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Reply to
Scott in Baltimore

I use a multimeter and take is for a walk while looking for a bad connection.

With the engine on and the lights, heater fan on high, wipers, stereo, etc., all turned on I take a volt reading at the battery terminals. I then leave the positive on the terminal and walk the negative probe to the clamp, then the engine block, then the alternator case, then the vehicle body looking for a drop in voltage. If you see hundredths of a volt drop, no problem, if you see tenths, you have found a bad connection.

I also check the positive clamp and cable by leaving the negative probe on the post and walking the positive probe.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - G> I have a 97' xj that has me baffled. I seem to be having intermittent
Reply to
Mike Romain

Thank you for all the suggestions.

Still trying to figure this one out.

Last weekend, I probed the battery cables from the battery to the alternator, to ground, etc... checking for voltage drops. They appeared to check out ok, but I checked and cleaned ground connections all over the engine compartment anyway.

Next, I bought a used engine control unit at the junk yard for $25. After swapping them out, the problem still occurred.

My next thought was the alternator, so I swapped it out earlier this week. Here comes the head scratcher. The problem seemed to greatly decrease in frequency. However, instead of overcharging at 19 volts the needle will only breifly sweep up to 15 or 16 volts. Even more strange is that it only does it while accelerating through fairly sharp corners or turns. In the past few days, it has done this probably 3 or 4 times, but only swept high enough once to click on the ABS light and cause the shifting problems with the computer I described earlier.

I had replaced a dead battery a few weeks before this problem cropped up, so I thought perhaps the new battery had some intermittent shorting issue. Replaced it this morning with no apparent impact to the problem.

So to recap, I have ruled out the voltage regulator in the computer, the battery, and maybe the alternator (improved the problem, but didn't eliminate it).

Anyone have any advice or ideas?!

Reply to
ds1234567890

Ground strap across an engine mount to the frame bad or a bad body ground?

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06
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Reply to
Mike Romain

Found one body ground yesterday that I missed, so I cleaned it....have to wait and see if it works. Do you think the field driver that attaches to the alternator could be bad? What should the voltages be on the two connections?

Reply to
ds1234567890

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