Electrical failure

[madmax]

Hi: I own a 1989 Comanche, 2.5 with 5 speed. This has been a very reliable, daily driver over the last year. I have even taken this truck on two 1000 mile trips. Anyway, tonight while driving home I experienced a complete shut down of all systems--one minute I was tooling along, lights on, radio blasting, and then suddenly everything shut off in an instant: motor, radio, lights. I coasted to a parking spot and then checked battery cables which seemed secure. There is absolutely no electrical power at all. Is this simply a battery problem? Could it be the ECU?? Please help!!!

[mvusse]

Headlights and radio do not go through the ECU. Also when the truck is running it should be able to keep running off the alternator no matter how bad the battery is. I imagine the problem is somewhere between the alternator and the fuse block.

 

Never really looked, but do our trucks have (a) fusible link(s)?

[freakjeep93]

ive never looked either but ive been told they do

[jimoshel]

I believe the '89 MJ has 3 fuseble links. Two green and one brown. Two between the relay and alternator, one each for field and armature, and one between the relay and main wiring harness.

[madmax]

Thanks for the input. I discovered that the positive cable to the battery had rubbed up against a bracket holding a relay--probably for the last 21 years! The insulation wore through and the cable shorted out the battery. It killed the battery, but everything else seemed fine after I replaced the battery. I used some electrical tape to repair the cable and now am back in business. It is a mystery to me why the motor and lights did not continue to operate off the alternator, but they died instantly along with the engine. In forty some odd years of driving I have never experienced such a failure.

[mvusse]

They didn't keep operating off the alternator because when the battery cable shorted to ground, it shorted the ENTIRE electrical system to ground, including the alternator.