1994 Jeep Wrangler Alternator and Battery

URBAN750

MEMBER
1994 JEEP WRANGLER
4 CYL
4WD
AUTOMATIC
66,800 MILES

The car died while driving. First the lights on the dash went out and the tachometer and speedometer stopped responding, then it died.

A mechanic replaced the alternator and the battery and then the car started and we let it run for 20 minutes or so but the battery charge gauge never rose above 10v and was in the red. I was advised that the only explanation was the the gauge must be miscalibrated. I drove the car about 12 miles away and back and just as I was getting home the needle on the battery gauge/dial when to 0 and after I shut the car off it died. The diagnosis then was that it was a bad alternator and it was replaced but then the car still wouldn't hold a charge.

The mechanics of a Jeep wrangler are pretty simple and it's hard to imagine that anything could be draining the battery. The mechanic is trying to figure out what to do next. The wiring all looks good an intact.

What do you think is wrong? What is the next step?

2 Replies

CARADIODOC

MECHANIC
33,869 POSTS

Hi urban750. Welcome to the forum. If you have the little silver Nippendenso alternator, there are two small wires bolted to the back of it or in a plug. Measure the voltages on those two wires, but it has to be done while the engine is running. There will be 0 volts on both when the engine is not running.

One must have full battery voltage. The other one must have less than battery voltage but not 0 volts. The greater the difference in voltage, the bigger the electromagnet it is making. That's how it regulates its output. If those two voltages are correct, measure the voltage on the large output wire bolted to the back of the generator. If you find very high voltage, say 15 - 18 volts, you will also find it has 0 volts when the engine is not running. Look for a very large value fuse that's blown, possibly from the old alternator.

Caradiodoc