Alright, I'm not that good with electrical stuff but I've got to find the short in my truck's electrics.
1978 F350 former U-Haul box truck, 4 speed, 332ci
The alt light comes on, and will only go off at higher RPM. If I disconnect the negative terminal on the battery and put a voltmeter in there, it's getting voltage across it. The voltmeter does not show any current at the fuses.
What would be the best place to start?
I haven't had the truck long - the last owner said he replaced the alternator and it does look new.

thanks,
-Steve

 

The Alt light in my truck will come on randomly but my voltmeter reads good so I just ignore it to be honest. When you say you have no power at the fuses you mean the fuses on the firewall right? If you do I would just assume you have a broken wire going to them or a poor contact on them with your tester.

 

My alt light is always on.
My cheapie repair manual says I should disconnect the negative terminal to the battery and see if there is current between the cable and the battery when they are disconnected. There is, it shows 12v when I touch the (-) terminal and the disconnected cable.
It says to then remove each fuse and check the cable after each to isolate the circuit that is shorting. I figured I should just test it at the fuse box by touching the voltmeter to each end of the fuse connection. I did not find any current at the fuses.

 

So absolutely nothing in your truck which is connected to the the fuses works, correct? If so then I would think that the wire which goes to them is bad or cut somewhere. As for the alternator light always being on did you change the regualtor as well as the alternator?

 

The fuse pull test is used to identify a battery drain, but your problem seems to be that the alternator is not charging enough (not necessarily related). Does the issue with the ALT light coming on at low RPMs only happen when the engine is first being ran, or does it consistently happen the entire time the engine is running?

By the way, the fuse pull test can make it look like there is a battery drain if you have a door open, or something that draws constant power, like a clock.

 

Maybe I'm not being clear, I don't really understand electrical so well so maybe I'm confusing terms. Or maybe I have more than 1 problem. at once.
With the key out of the ignition, if I test the battery from the (+) terminal to the (-) terminal it shows about 12v. If I disconnect the (-) cable from the battery post and test between the disconnected (-) cable and the (-) battery post, it shows about 12v also.
This means there is a short somewhere, right?

Also, the ALT light is always on but will fade out if I rev up the engine.


This truck has been sitting for a long time and I'm just about done with the brakes. I've got to sort this electrical problem, then also the horn and turn signals so I can get it road ready.

 

If you are measuring voltage between the -BATT and Ground with the -BATT disconnected that would be normal because you have taken the current return path out of the circuit. Here's a few easy things to check.

1. Check the connections from the ALT to the +BATT side of the starter solinoid and check the fusable link. With the engine running you should have voltage there if you dissconect it from the +BATT. Be carefull not to short the alternator output to anything while its running. Remember electric current needs nice and clean connections so be sure everything is nice and pretty.

2. Replace your voltage regulator if it wasn't done already.

3. Check ALL grounds! -BATT to engine block, Block to chassis. I would even add a ground strap from -BATT to chassis. See photo.

4. Run a heavy load. Turn on headlights, Wipers, Radio. Does your engine drop a little RPM? Are your headlights bright or dim?

5. How's your battery?

 

When the trucks running get the volt meter and put on the positive and negative sides of the battery.

 

The truck had the ALT light on when I bought it. The previous owner told me he replaced the alternator and regulator but that it didn't fix it.

It only had one ground from the (-) terminal to the water pump so I cleaned it up, and I added another from the post to the body with a nice shiny connection. There's a small ground from the alt to the body but it was a very small wire, so I replaced it with a biggger new one.

The battery is good, but not as nice as the one in the photo. It's pretty new still and holds a strong charge.

The battery shows a little better than 12v if the truck is running or not.

I switched my voltmeter to ohms and it shows the connection from the solenoid to the alternator is good.



My Haynes book says that if a battery shows current from the (-) post to the disconnected (-) terminal that there is a short circuit somewhere. That's incorrect?

 

Yes, that's wrong. Just means the battery has full voltage. Sounds like your alternator isn't charging.

 

I'll take the alternator down to the parts store to have it tested and get a new regulator while I'm there.

 

Id suggest doing a 3g swap..

 

-The alternator tested OK. Reinstalled it.
-I've cleaned up the single ground it had to the water pump and added another from the (-) battery terminal to the body.
-I replaced the regulator. I sanded the contact area behind it and made sure it's installed snug up against the fender.

The ALT light is still on and I've still only got about 12.5v at the battery when it's running.


Wiring?

 

I dont know.. Most likely some wires broken or hooked up wrong.. I still suggest the 3g swap

 

disconnect the battery and check continuity from the B+ heavy wire on back of the alt to the starter sel. where the battery goes on. This checks the fusible link.