Question about 1999 Jeep Grand Cherokee
The PCM has the voltage regulator inside it that controls the charge from the alternator to the battery, unlike older vehicles that have the voltage regulator directly on the alternator. Also there is a battery temperature sensor under the battery that the PCM uses to decide how much to send the battery. Possibilities; bad alternator, bad PCM, bad battery temp sensor, bad battery, and bad connections. Battery tests are easy check when fully charged should get around 12.5v then turn on headlights for 2 minutes and check again should still be around 12v. To test charging system start engine and test at bettery terminals should get 13-15v if not then something wrong with charging system take alternator to advance auto and they will test it for free if its good you are down to connections, PCM and battery temp sensor. test battery temp sensor at 75 to 80 degrees fahrenheit should get 9000-12000 ohms resistance (9k-12K ohms) if thats good check ground connections on vehicle body , connections on alternator, and plugs into pcm as well as the two above it (depending on model and year). If all that is good your PCM is bad get one on the internet preprogrammed for your vin number for $250 if you get one at the parts store it is $250 and then you have to get it programmed about another $80. You can also test the alternator your self by disconnecting the 3 tiny nuts and removing the connection from the field terminals (plastic piece with two terminals) and checking again at battery for 13-15v ( stay away from pulleys and belt,no loose clothing or long hair if you value your health). The PCM shorts the field connectors to regulate the voltage so removing that connector eliminates the PCM , battery temp sensor and the wire to and from them.
Posted on Mar 07, 2009
Finally found the problem! There is a wiring connector just above the computer that was corroded and loose with a bad connector. I took the connector apart and cleaned good and put back together using dielectric grease, then taped the connection so it would not come loose again. Problem solved.
Posted on Sep 15, 2008
Well first thing I would check is the alternator, Make sure the alternator has 12V on the big wire and 12v on at least one of the wires in the plug in connector with the key off and 12 on at least one wire in the plug in connector with the key ON There are fuses in the circuit Check them and replace any bad ones. Here is the wiring diagram for the circuit
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Posted on Aug 17, 2008
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