I have a 94 ZJ with the inline 6. I've already replaced the alternator twice. I also replaced the battery and ECM and it's still not charging. It's put enough voltage out to run the truck, but as soon as I turn the radio, lights, or A/C on it just quits charging and the belt starts turning really slow. I'm really stumped. Can some please help?

 

check the connector on top of alternator. 2 skinny wire with plastic plug. One should be ground and another going to positive.

 

Check all your electrical connections. Make sure they are clean and tight. Get a volt meter and test the battery voltage. It should read around 12 volts depending on charge. Start the engine and test the battery voltage again. It should read higher then the first battery voltage test. Run the engine at a speed above idle and you shoud read 13-15 volts. This would indicate that the alternator is charging. If the reading is still battery voltage then you do have an alternator or regulator problem.

You can also test voltage directly at the alternator. Red meter lead to the large thick wire on the back of the alternator and black meter lead to a good ground. If the alternator is charging you should read 13-15 volts on the meter.