Alternator charging 20 volts

[fordpickupman]

I recently picked up a fairly clean 2001 5.0 XLT for cheap due to it having an electrical problem.....
I had to put a different battery in it because it had smoked the other one.
I started trying to chase the problem down and determined that the voltage regulator must be fried as it was charging over 20 volts on a good battery. So I replaced the regulator, but no change. I had an alternator shop test the alternator and they said it was fried and not rebuildable. So I ordered a new one, they tested it and it was good. I put it on, no change.
It is getting over 19 volts at the alternator on all 3 leads while it is running. I have searched and searched on here and other forums trying to figure this out.
I also have a ‘97 5.0 XLT that has a “custom” wire ran from the Yellow/white wire of the alternator to the fuse box. I’m wondering if it had the same problem and this wire bypassed something under the dash that was causing grief.
Im at a loss at this point.

 

[1998Exp]

Does anyone know if the 2001 model have a PCM-controlled alternator?

 

[RickOTR]

Sometime a bad Negative cable can cause a voltage drop on the negative side leading to an overcharge of the alternator.

 

[RickOTR]

Does anyone know if the 2001 model have a PCM-controlled alternator?

No, the PCM does not control the alternator

 

[410Fortune]

I do not believe the 5.0 trucks ever had the smart charging system that came in the later fords

ground cable suspect as our research shows and as @RickOTR so nicely pointed out

I work on many 5.0 trucks, I have not run into this one before so I would not say it is common, more likely just age
Check the ground cables, or heck bypass them
One solid ground to the engine, one to the rad support and one to the frame, you can ground the alternator body for peace of mind also since the alt ground must come through the drivetrain (no ground wire)

the 2001 5.0 stock ground goes here

timing cover stud, hard to see in the pic but it is behind the bracket that holds it in place

 

[fordpickupman]

I found it!
The wiring for the 2way radio added by the previous owner must be shorted somewhere.
I pulled that inline fuse and all was well in charging system land. Phew!
I hate being in electrical diagnosis limbo

 

[CDW6212R]

Well done, I hadn't heard of that issue before either. I would have said find all non stock wiring and check those, plus the main power wires.

BTW, you have a 4G alternator, being the 5.0 V8. The 3G only came in the OHV 4.0's, same used on the 1st gen's. The circuits of those differ, and the mounting holes don't match to be able swap them.