i have an 87' cherokee with a 4.0 4wd. my battery wont stay charged so i had it replaced (under warenty) and i bought a new alternator because the old one was bad, but the battery wont stay charged. any idea why?

 

Do you have battery voltage at the alternator stud with the key off? Do you have 12 volts or so at the yellow wire at the alternator plug with the key ON?

 

You either have an charging problem or a parasitic drain on the battery.

Do these tests for the alternator. Engine off, check battery voltage. Should be 12.4 or higher with a charged battery. Then check for voltage on the large terminal back of alternator. You should read the same battery voltage or close to it. This means there is continuity between the battery and the alternator.

Now start engine. Test battery voltage. Should be 13.5 to 15 volts. If you get this reading the battery is getting charged. In this case the problem may be a parasitic drain. If you still read the engine off battery reading the battery is not getting charged.

With engine running, with care, do the voltage test at the alternator. Again you should read 13,5 to 15 volts. If it reads O volts or low voltage the problem is in the alternator system.

If the alternator is charging but the battery is not getting charged, the problem is in the connnection between the battery and the alternator.