As I've mentioned previously, my truck ('80 F-150 Custom, 300-6 engine) hasn't been charging correctly. After some limited troubleshooting, I decided to replace both the alternator and the voltage regulator - I was fairly certain the problem was with one of those two pieces, and it seemed like a good idea to just replace them both, given the truck's history (30 years old, the last 15 of which it was parked) and the fact that the parts are cheap ($72 for both at NAPA).

First thing I did today was swap out the regulator. Easy enough, although the electrical plug on the new regulator was narrower than the plug on the wiring harness. The electrical contacts themselves were identical, so clipping the side of the plug off on the regulator solved that problem. I screwed it back on to the fender, and started the truck.

At this point, the CHARGE idiot light comes on. This is progress! What annoyed me about the whole charging problem is that the light never came on - first I knew I had a problem was when the truck refused to start. I interpreted the light coming on to mean that functionality was coming back to the charging system, but I still had an alternator problem.

So I crawl under the truck to start changing the alternator, but I'm slowed down by the fact that the alternator is mostly hidden by the lower radiator hose and the fact that the bolts are rusted. By the time the penetrating oil sinks in it's getting dark, so I decide to wrap things up. While I'm down there, though, I notice that the alternator belts (why are there two?) are loose, so I swing the alternator out a bit to snug them up.

The idiot light stays on when I start the truck up again, but now when I give it a little gas it goes out, and stays out even when the truck returns to idle. It stays out while I'm test driving it, and then when I turn it off at the gas station it flickers on for a second as the engine spools down. As a bonus, after filling the tank I discover that the gas gauge is now working correctly, whereas before it had been reading erratically anywhere from a quarter to a half tank low.

When I come home, same routine...light comes on and stays on at start, goes out as soon as I give it gas, and comes on briefly when I'm shutting down in my garage.

Am I correct in interpreting all this to mean that the alternator is in fact partially operational, but perhaps at a low level? I still plan to replace it, but I'm thinking now I can wait until I do the water pump in the spring, when I'll have the radiator and hoses out and can access the alternator more easily. And what, EXACTLY, triggers the CHARGE light to come on? Is it a voltage threshold, current, or...?

 

You should try this 3-point voltmeter test that I have, It works every time for me.



I also have other troubleshooting help here;
Ford Truck Enthusiasts Forums - Grubbworm's Album: Troubleshooting help, for anyone that needs it.
Let me know if any of these help.

 

Yep, that was the guide I used for my initial troubleshooting. However, I ran into a problem when my battery was showing 13.5 volts or so when disconnected from the vehicle...so either I have some kind of amazing super-battery, or my ancient multimeter is inaccurate.

Now that the new regulator is in, I'll go through the test again and just assume that the meter is reading 10%-15% high and see what happens.

 

A fully charged battery should show 13.2V (2.2V per cell X 6 cells). Your ancient multimeter is likely better than 90% of the stuff that can be found today, with a "made in china" sticker.......

 

my truck had the same problem when i bought it thought the belts were tight but were hard and cracked so replaced them and tightened up and worked !

 

Mine are pretty hard and cracked, too. If the truck holds out until spring, I'm going to do the water pump, alternator, belts, and hoses all together. I'd do it now except it's just a little too cold in my garage, and it's not absolutely critical that the truck be fixed right away.