I'm having an electrical problem with my 89 12v. I have gone through 3 batteries, 3 alternators, and 2 external regulators. I have to jump the truck off to just even start it. After removing the jumper cables the voltage meter drops immediately then continues to drop until it drops below the 8 volts mark. Then it kills the truck( while it is running.) Driving either down the road or idling. This truck does not have a ECM like 91.5 -93. Please feel free to contact and share this thread ( if possible).

 

89, 90, 91 charging systems are very primitive but reliable, tell us what you have done to diagnose/repair it.

Check the fuse links on the drivers inner fender, also check the charge wire from the alternator to the battery, there are 2 bullet connectors on the wire that are prone to loosing connection, at the alternator about 1 foot down the wire, the other is at the battery terminal, think it is an 8.ga wire.

The only power a Cummins requires to run after it is started is the fuel shutoff solenoid "FSS" and can be powered virtually by flashlight batteries, you might also have something draining your battery additionally.

BTW 3 bad alternators from the auto parts unfortunately not uncommon.
Jim

 

Also make sure everything is grounded good. I don't even bother with parts stores alternators we have a local shop that rebuilds them.

 

I'll check my grounds again but I work at an auto part store and 2 of them were warranty. They would work good for about a couple days then it would start the same process of dying. Again thank you and I will check it out. I haven't done anything aftermarket on the truck it's all original. But I got ran off the road by a garbage truck and hit a huge ditch full of water suprisingly my truck stayed running and I checked the air filter and alternator to make sure it wasn't shorted out then a day or two later the charging problems happened and I didn't have that problem out of it before then

 

Maybe water got into a connecter?

 

That's what I was thinking but I just cleaned my connectors and I'm fixing to jump the truck off. And see what it does again and I went ahead and tightened everything down again so I will see what happens.

 

Best of luck to you.