Here's the gist:

1978 F100 short bed with 302, auto trans, RWD.

A while ago, out of the clear blue sky, my alternator light came on. Check alternator wires and connections, all good, battery showing ~13v off and ~14 at idle. Then the automatic choke stops working when the alternator light is on. Over a few days, the light goes from coming on and staying on, or simply not coming on at all (at random, whenever I turn off/restart the truck) to doing a ****ing disco show. Replaced voltage reg with ZERO change in behavior.

Fast forward to now: Disco show:


NOTE: That video was shot while driving down the road, NOT at idle. The behavior is incredibly inconsistent usually, and may or may not show up at all on any given drive, though it's almost a guarantee that it will at least flicker a few times.

Now the headlights seem VERY dim, to the point that I'm not comfortable relying on them. Tail lights are dim as well.

The interior dome light pulses in the same rhythm as the alternator light, even if the alt light is not on.

The idle audibly, though slightly slows down when the headlights are turned on. This makes a notable difference when the truck is cold. It idles fine with the lights off, but starts to sputter if the lights are turned on.

Turn signal blinks slower at idle than while moving

WHAT HAS ALREADY BEEN DONE:
- Replaced voltage reg
- check alternator/battery, all is charging as it should
- check battery-block ground
- check manifold-firewall ground (Shielding on the wire is effectively gone, dry rotted away. Wire is rusty. Contacts cleaned, no change in behavior)
- checked wires running through the firewall best I could. None appear damaged, all bundles are run through rubber grommets or are otherwise enclosed.
NOTE: there appears to be mouse damage here and there, which is why my right turn signal is not working at the moment. No obviously chewed-through wires otherwise, though.

WHAT I NEED TO KNOW:
- I have been unable to find the bed-frame and body-frame grounds. Info on where those are located would VERY MUCH help me.
- Potential causes, advice on troubleshooting.

Any help at this point would be great. I'd fill it with gas and burn it if I didn't ABSOLUTELY have to have inclement weather transport.

 

bumpbump..

 

Rectifier is bad. One or more of the diodes are bad causing the light to cone on. Get it tested and see if that's it.

 

How do you get a rectifier tested? As I said, this one is brand new, replaced when this issue started, and it had no effect on the problem.

 

bumpbumpbump

 

Here are a couple good troubleshooting pages:

http://www.aa1car.com/library/charging_checks.htm

http://www.aa1car.com/library/2002/cm10220.htm

As you know, have to use a process of elimination to determine the source of the problem. Start from scratch.

Check to see that the battery itself is not defective, and is fully charged. It may have a bad cell? Just pointing out everything needs checked. Not saying that's the problem.

Ground straps don't really need insulation, but it's an indication they are old and have seen better days. It's important that cables are replaced before they get to that stage. Any resistance in the battery and ground circuit, barely measurable, will cause a noticeable slowing of the starter and cripple the alternator. May not show up at idle but it will choke under load.

Check charging voltage at idle, then turn on all the lights and accessories and heater blower on HI, and run the RPM up to say 2000 or so and measure voltage at the battery again. It will sag a little but should maintain 13.8 to 14.3 under load.

Look at voltage drop testing, very useful, outlined here:

http://www.aa1car.com/library/voltage_drop_testing.htm

 

The diodes are part of the internals of the alt.
different from the voltage regulator.
I'm not sure if the box stores can test directly for that or not, but it might fail under load there.

 

Diodes only conduct one way. Should be an easy test with a DVM. Could be wonky slip ring or stator, etc? Sounds like alternator is junk.

Pretty much any test they do, can be done on the truck, and it doesn't need to be removed. Just use your noggin.

 

Yay Tedster! I'm able to see your posts!

David