I have a strange issue with my 1999XJ 4.0 2WD. I was driving down the road and at about 1/4 mile the battery light came on. I noticed that the headlights were a little dim. I pulled into the nearest gas station and checked the voltage at the battery while the vehicle was running. It read 12.11v. As I revved the engine the voltage dropped 12.10 then 12.09. I stopped revving the engine and the voltage went back to 12.11. I had just replaced the Alternator on Saturday. After replacing the alternator the volt meter read 14.20v. The vehicle worked fine all weekend and then yesterday I was headed down the road at about 1/4 miles and the battery light came back on. I repeated the previous voltage check and once again the voltage read 12.11v and had the same results as before. I was told by the Advance Auto Parts guy that the voltage regulator was in the PCM and this could be the issue so I just thought to wiggle the 3 cables that attached to the PCM. The voltage jumped back up to 13.80v and when I revved the engine it would go to 14.20v. This is a strange thing due to the fact that I had recently cleaned and reconnected the cables to the PCM along with putting dielectric connector grease to make a good connection. The battery light never came on again for the rest of the 25 mile trip to work. I need to know which cable has the wiring for the voltage regulator in it. It could have been a bad connection somewhere on that line or at the PCM. When I leave work today I will check the voltage again to make sure it is running right. All the connections are good at the alternator.

 

I would check to see that your dielectric grease isnt making connections between other contacts.
I would then follow the wire looms and look for any wear.

Clean grounds, retighten alternator connections.

The ECU rarely goes bad, So I wouldnt really be looking at that.

 

Well I went out to the Jeep when I got off and checked the voltage again. When I first turned over the vehicle it read 12.41v. I tried to wiggle the cables again which had no effect this time. I revved the engine and it slowly started to climb staying in the 12.5v range. After a few minutes the voltage started going up to 13.41v and climbed to 14.07. I drove the vehicle home and checked the connectors on the PCM, traced the wires and check for fraying or wear but did not find any. I pulled the plug off the alternator and put dielectric grease on the connector. I then did some voltage testing of the battery temp sensor. When I unplugged the battery temp sensor the voltage would drop from 13.89v to 13.71. I turned on the head lights and put them on high. The voltage was 13.59 at idle and when I unplugged the battery temp sensor it would drop to 13.41v. With it plugged in it would stay at 13.59v. Rev the engine it would go up to 13.80v depending on how much I revved the engine. I will have to wait till the morning to test the battery temp sensor when it is cold. Can anyone tell me if these are normal or not and why the voltage is so low when the car is cold?

 

This morning while the vehicle was cold the temp was 33F I checked the voltage after cranking the vehicle. It read 14.17v so I thought it was ok. I put the car in gear and drove 1/4 mile and the battery light came on again. I pulled into the same gas station and checked the voltage which read 11.89v with the lights and everything on then all of a sudden without doing anything I hear a sound like something kicked in and the voltage jumped up to 14.01v. This is getting to be very strange. When it snowed I could have gotten water in the PCM when I had the hood up slightly charging my battery. There is a little hole in the back of the PCM. I am going to have to take it off the mount and see if water comes out the hole.

 

Take your battery to your favorite parts house and have it tested.

 

I replaced the battery 5 or 6 months ago. The battery tests good and will take a full charge and still has 950 cold cranking amps. I had a starter issue which I thought was a battery issue. It turned out the starter seized. Could water in the PCM/ECU have any effect on the voltage issue? I would hate to have to spend another $190 on a rebuilt PCM/ECU.

 

A bad alternator can kill your battery.

Have it tested.

 

Well it looks like its the Voltage Regulator in the PCM/ECU. We did a lot of testing and it is coming back to the $187 PCM/ECU. Does anyone know of a cheaper priced on the web?

 

As the computer is pretty hardy, I would certainly consider getting a salvage yard one.

Probably in the $30 range. Check with your local salvage yard and even if they don't have one, they could probably have one for you in a day or two.

 

I would but the PCM/ECU has to match you vehicle exactly so each one is programed for each vehicle type, power requirements, and accessories.

 

A new PCM will need to be re-flashed by the dealer so that the VIN in the PCM matches the VIN in the gauge cluster.

Simply dropping a PCM in will cause the CEL to come on.

 

Out of curiousity...Have you checked the PCM Fuse in the Junction Panel?

 

I checked fuse 11 in the junction block inside under the left side of the dash and it checked good, there was no discoloring or cracks. It was making a solid connection.

 

I have found a used PCM/ECU for a 99 Cherokee 4.0 W/Automatic Transmission. It came off of one that was running fine until it was totaled. I haven't put it in yet but I will after work and see what else can go wrong.

 

the cel will come on, but it should run fine.
IIRC, you can take the jeep to a dealer and have the new ecu flashed with your vin to get the cel to turn off.