While on a trip, I noticed the voltmeter in the dash drop to around 12 or so volts from the usual 14ish. A week later the battery was dead, it was 6 yrs. old so I bought a new one. Of course the next morning the truck wouldn't start, dead battery. I then charged the battery up over night. The next morning I started the truck, with the motor running, I have 12.8 volts across the battery posts. Measuring from the positive termanal to a ground on the motor or frame, I get the same voltage. With the motor off I have about 12.5 volts. I have 2 plugs on the alternator, I've removed them, blew them out with air and applied dia-electric paste. No change. It's only been less than 1 day since the over- night charge, but I'm sure the low voltage is a problem. Thanks in advance.

 

What year truck?

 

1989 f-150

 

Your alternator is not charging. You could try another one and see if that fixes it.

 

Agreed. An '89 would have an internal regulator, so there are less things to try.

 

Could be something as simple as dirty /corroded battery posts. Exactly where are you reading voltage from? Try reading from the positive connector, not the battery post. Just a little corrosion can act as insulation and prevent alternator voltage from getting to the battery post.
In fact, here's a post from yesterday on the very topic: https://www.ford-trucks.com/forums/8...ml#post7695050


Good luck,
Bunky Jones

 

I've cleaned everything really well, and put on the protective grease. I'd posted this question on another forum and someone mentioned to check to see if the plug on the alternator had shorted. I'd cleaned up the plugs but didn't really looked close enough or checked for continuity between contacts. I'll check this first. thanks

 

The large connections on the battery are good, since it will crank the truck.

 

"The large connections on the battery are good, since it will crank the truck."
I'm not saying the terminals is the OP's problem, but take my word for it, low voltage DC can do some pretty strange things.
However, since he said everything is clean, I'd suspect the alternator connector, internal diode pack, or the alternator itself.

Let us know what it turns out to be varunner696.

 

I'm good to go now. thanks https://www.ford-trucks.com/forums/8...ml#post7703773