ZJ voltage regulator mod: charging system problems
I bought a 93 jeep grand cherokee laredo from my roomie. she just put new tires on it and I bought it for $600 bucks. I changed the oil and spark plugs. She said it had some electrical problems. I cranked it up and it worked just fine. I drove it for two blocks then when I pulled into the driveway the meter on the dash went to 9volts. I have learned from the forum that the computer controls this. I took the battery and alternator into Orielys and had them test them. The battery was bad. I replaced it. I cleaned up the engine compartment in that area, it was really cruddy. The alternator tested good. I put everything back together and the voltage meter on the dash still read low and then it kicked out a code 41. I revved the engine and the volts kicked into 14 on the dash and I read 14.5 at the battery with my voltage meter. Then the meter would coast down to 9v and then back up to 14.
Now the meter is locked into 9v on the dash and the check engine light is lit. I checked the fuses in the PDC.
I checked the voltage on the back of the alternator when the engine was running and the asd relay was 11.5v and the other post was zero volts.
I checked coninuity from pin 20 to the field ckt post on the back of the alt and it was making a solid connection, so the wire is good. There was some corrosion on the pins in the pcm connector and i cleaned them up. I found a testing flow chart that basically says the pcm needs to be replaced now. Which makes sense because it controls the voltage to the field of the alternator and there is zero volts going to it.
I went and picked up an external voltage regulator from Oreilys and I ordered the pigtail. That will be in tommorow.
I was wondering if anyone else has done this mod?
The things I am concerned with/wondering: what is the deal with the asd voltage going to one of the two pegs on the back of the alternator? if that voltage wasn't there, does that somehow disable the alternator, just wondering. i was planning on removing on one of the field pegs and putting one of the leads of the new external vr on there- then taking the other peg and wiring it to the other post of the v.r. just like in the above link. i know the asd system is important- so will that system still work if the lead is disconnected from the alternator?
the other thing is: i know that the check engine light will be permanently locked in if i go this route- i just want the jeep operational until i can get the $200 bucks for a new pcm.
Looking at the back of the alternator, there are two posts. Going clockwise the first post should have keyed 12v. The second post is a variable ground to control alternator output.
Before you do any modifications ground the 2nd post and start the truck. If the alternator goes to full charge then you do have a bad regulator. If the alternator doesn't go to full charge then you've got an alternator problem or bad charging wire/fuse. Parts store testing is not 100% accurate and the act of removing an alternator and bumping it around can make it start working long enough to test good. Bad wires will also start working when they're bumped around. You need to ground that post before going further.
Thanks foundrydude. I haven't done that test yet. I am busy next two days so can't work on it. So, I need to ground the wire that comes from the PCM to the alt and check voltage. If its high, then alternator is good. If not then the alternator is bad. Also if its high, then the pcm is bad. Got it. Thanks for the info. I've been working on this problem for over ten hours, with lots of research on the net.
also, i have a digital voltmeter and checked continuity of all of the wiring on the back of the alternator. and continuity for ground cables. All checked sat. Right now the check engine light is locked in with 9v, last time it was code 41, so i assume its the same. I will ground the post out and check voltage when i get home today. thanks for the help.
I just installed a new alternator after trying to track down a low-charging problem. This made it worse, now when I start up the engine idle is very low and when I snap the throttle it revs up and drops down to about 300 RPM. I've read that the regulator being faulty is either a good or bad situation; no halfway stuff. Is this true? When I go to do the regulator/alternator test (where you ground the second post on the back of the alternator) it blows the engine controller fuse (20A) in the PDS (this would be a giant flag if I had the right wiring diagram) . The alternator tested good off the shelf and the problem seems to be more prominent in the heat. Something resistance related? My manuals don't have good charging system schematics so it's hard to tell from bits and pieces that have been posted what actually goes where.
The most charge I've seen since the new alternator was installed is 13.9v; within specs. However, the night of the installation it seemed to just be running on battery voltage. I do get some light flicker at night when I rev it up, but no dead battery or stalling. Is there any harm in trying the voltage regulator mod?
I've gone through 2 93 ZJs with the exact same issue. I put a multimeter right to the battery on both and several different spots. Read low on the gauge but fine on the meter every time. I just ignore my volt meter now. Even with new battery, alternator, comp, all charging wires replaced with 4ga, extra grounds.... Gauge still reads low. Mines been like that for the past 10 years! I gave up on it long ago.