I have an 87 PU. The battery light is on and stays on. I have already replaced the alternator, but when I pull the positive cable from the battery the engine dies. I have not been able to figure out the problem. Anyone have any ideas?

 

Have you tested the alternators output with a volt meter? Test at battery terminals, should be around 13.5- 14.5 volts.

 

On this age vehicle it is possible the wire coming off the B terminal of the Alternator has just worn out and has become so brittle it will no longer conduct electricity .

This wire goes up to the power distribution block by the battery.

***Another thing if the Alternator is indeed good

is look at the ground cables these pretty much suck from new. Upgrading mine have solved quite a few charging and starting issues. On my Toyota fleet

 

Try checking your charging fuse and if its blown then replace it and everything should be back to normal.Mine had my battery light on even with a new alternator and battery.

 

I recently had the same issue with my 82 22R. Here's the sequence of what I did:

1) Original alternator - new regulator - NOT CHARGING
2) New alternator - original regulator - NOT CHARGING
3) New alternator - new regulator - NOT CHARGING
4) New alternator - new regulator; checked all circuits, plug connectors and wiring with VOM (good); checked all fuses (good); fuse links (good); tested battery (good); had both alternators tested (both good); unbolted, cleaned and re-attached ground lines and tested with VOM for continuity (good); detached, tested and re-attached wires to alternator, following FSM trouble shooting directions (good) - NOT CHARGING

After certifying that all wiring, fuses, and alternator(s) were working properly, I tried the system with a third regulator (easiest to change, and I had no other way to test the regulator), and identified that the original and first replacement regulators were not working! - NOW, THE SYSTEM CHARGING LIKE NEW!!!

 

I would check out all your ground wires. Ground from the battery to the motor mount, motor mount to the passenger side fender well. Also make sure the charge wire from the alternator is good and positive on the battery to the fuse box is good. I had a similar problem and it turned out to be the ground wires. I replaced all of them with 4 gauge from the welding shop. You local welding shop should be able to make all the wires you need for about 50 bucks. I went with 4 gauge cause I installed a 130amp alternator.

 

I figured it out. I pulled the alternator and the brushes are already worn out. There is a hesitation in the pulley about halfway through a revolution. Thank god I have a lifetime warranty on it.