I'm trying to help someone with their 2001 grand Cherokee. They said the problem is random and the volts are dropping and everything is dimming and the vehicle acts like it is dying. And it will come and go. I have checked both the alternator and the battery both on and off the vehicle both check out as a pass. I was wondering I have looked into the grounds some what and haven't noticed any issues. When they checked everything on the vehicle they said that the voltage regulator was failed. I just wanted to see if any one had any input into this issue. I did notice when I checked it out with a multimeter that it's resting volts were good in the 14 range and then after I started it it started just continually dropping until it hit 8 volts. I ran it up at that time to the local parts store to try to get it tested while the issue was there but their tester wasn't able to run the test due to low voltage. I want to say thanks in advance any help is greatly appreciated

 

Reads like the voltage regulator is dying. Some will go out completely without any indication and others will do just as you describe. Verify all grounds are present, clean, and secure. Best luck.

Charlie.

 

Actually, the voltage regulator is the PCM. It controls the output directly, BUT the diode bridge rectifier inside the alternator may be failing. Also, underneath the battery is a temperature sensor that tells the PCM to cut back the charge rate when the battery gets hot. That may be giving a false signal to the PCM resulting in it not charging.

 

I tried a second altanator and that didn't change any thing. Would the sensor under the battery throw a code? And I'm sure I could search all over for the answers but where are all the grounds that I need to check located at.

 

That sensor failed in my '99 and did not throw a code - it was causing weird charge profiles just like yours.

 

I was getting a serious corrosion problem around my battery. That white acid that eats through everything. I bought a "mat" that goes under the battery. It has the same chemical impregnated into it as the little red and green things that go on the battery terminals and eliminates this problem. I wasn't aware of the sensor that Dave spoke of. My battery ceased charging as the mat insulated the sensor from the heat on the battery. I didn't get any codes either. Might be worth looking into. it's pretty flimsy. Check to see it's there, in one piece, and connected. They are pretty cheap to replace, install and if you lift up the battery , easy to find.

 

So recent trip to the auto parts store they said that the voltage regulator failed on their test. I'm assuming it's the one in the pcm. Was looking at reman ones on line dealer wanted about 850 for the part and another 100 to flash it. Does anyone recommend one of the sites over another and also I was talking to them about the milage since the vehicle is still being driven does the milage need to match up directly or does it matter?

 

You took what to the auto parts store/ The PCM?
Some guy behind the counter told you the reg in PCM failed?
He knows because they tested it?

1. If Yes to all this.
2. I'd say, its highly unlikely a person working behind the counter at a auto parts store would know this without testing the PCM. And or douing diagnostics on the vehicle.
3. I am not aware that a auto parts store can test PCMs.

Lets start here.

 

Took the Jeep up to the auto parts store. While I was up there I had them run the battery and altanator test with both still in the vehicle. Just wanted to see maybe something was about to fail maybe I would get a different result. When they tested it it came up with voltage regulator failed.

 

Yeah, from their standpoint, and due the manner in which their tester is connected, that sensor can make for a result that looks like a failed regulator.

 

Just reading thru my FSM about the battery temp sensor shows up a couple of interesting things. Depending on it's output, certain OBD II diagnostic sensors are turned on or off, such as the canister purge and leak detection pump, and the 02 sensor heaters. These are turned off below 20*F. The voltage regulator circuit in the PCM is integrated into the circuitry and is not repairable. The PCM must be replaced if it malfunctions.

Every freaking thing on these jeeps is integrated into something else! It seems this sensor is more important than even I realized.

 

Sorry what do you mean by fsm and how do I find it?

 

I understand now. If its not the sensor and you need a new PCM. There are various after market places that sell them, including your local auto parts store. Iwent to the dealer to get mine. It was expensive, very expensive. Before you buy one from any place other than the dealer, check them out on YELP and often you'll find write ups from the local BBB. I didn't know about the ones sold through auto parts stores when I bought mine. The YELP and BBB reviews for so bad for all the others I went straight to the dealer for a MOPAR rebuild. Looking back, I'd give the auto parts store brand a shot in the future. I a can't remember who it is right now.

 

FSM=Factory Service Manual. I downloaded mine from pacificcoastmanuals.com for $7.95. It is year specific and is 93 MB in pdf format compressed files. It has circuit diagrams and diagnostic procedures about everything. Just the turn signal/hazard flasher section is 13 pages!

Thank you FredNOAZZ for suggesting this manual. It's the best I've ever seen!