No problem. I apologize that I'm being a little slow. The truck isn't my daily driver and I have a few other vehicles with issues so I've been forced to prioritize. With that said I did do a little troubleshooting. I'll try to answer your points with the testing I have done.

I checked the DC output of the alternator with a meter and then I did put the oscope on it also. However, I honestly wasn't paying very close attention to the actual charging voltage. I can confirm that there is no excessive ripple on the output -- as long as the ground is properly connected (of course). Meter reads 0 AC and the scope shows the same. Of course if the scope probe is floating then I see the rise and fall of the generator, something like 12 Volt swing (this is peak to peak). I took a picture but I don't have it with me so the voltage I report isn't very accurate. Now with all that said I didn't pay any attention to what the actual DC voltage was, so I will have to go back and check that sorry. The battery is old. I have never replaced it, so it is certainly suspect.

As I said before I didn't see a lot of ripple, but I would still like to check (with a scope) at both the input and the output of the regulator, but I haven't ripped apart the dash yet so that may have to wait until this upcoming weekend. This could certainly still be a bad regulator.

I could absolutely have a bad oil pressure gauge or sensor, etc. That's also a possibility.

All these problems ...

OK just a few more comments.

After I checked the battery output I did my "turn on all the electrical stuff" test and the gauges went up super high. The truck doesn't start running rough while the electronics are on. Strangely enough it only appears to be running rough when I shut things off. As the needles fall, the thing starts running rough and then putters out and dies. It's really weird.

Based on your battery suggestion though I can very easily run the truck, turn everything on and measure that voltage and see if I have just been timing my "turn everything off" properly so the battery is dying just when I go to turn everything off.

I will try to do that tonight so I can report back in a timely manner. Still I probably won't be able to take the dash apart this week.

ok So I will: Run the truck and check the DC charging voltage. Turn all the crap on and see how much that charging voltage drops. If it doesn't drop I can try changing different heater settings / high beams / whatever to see if there is some place that causes a charging problem. If it still doesn't die / run rough then I can try shutting everything electrical off. So far this has been what kills it. I can then check and see what happens there.

 

Ok here are the results.

With the truck running at idle the charging voltage was 15.5 Volts (all measurements at the battery).
Turning on the lights increased the voltage to 16.5 V.
Turning on the heat (full blast) further increased to 17.5 V.

I left it running like this for a while and it didn't die. Shutting everything off dropped the voltage back to 15.5 and then the truck died. This time I was only able to get the truck to die this way once. All other attempts the truck stayed running without issue and the voltage simply dropped back down to 15.5 V.

Why is the voltage increasing as I increase electrical load? This seems backwards to me. Is the alternator regulator performing more poorly as current increases ... somehow?

Either way I'm pretty sure 17.5 is too high. Alternator regulator at fault here?

 

Wow!

Yeah way too high.
It should be at most 14.65v

Your regulator is at fault for, or not sensing load properly.

 

Yeah, voltage regulator - look at the wiring to it, too...

I'm used to ~14.2V at idle.

 

Agreed! Sounds like you are on track to fix it now. In addition to the bad voltage regulator, your battery may be toast if charged that high for any length of time. If not a sealed battery, at minimum check the electrolyte level.

You mentioned the battery was old, so I have no problem spending your money to suggest simply replacing it as a precaution. I've seen many apparent "charging" problems exacerbated by a weak battery.

Chances are your gauges will now behave themselves, too. Keep us posted.

 

ok thank you gentlemen. I'll change the alternator and let you know.

 

Alternator? Does it have a built-in voltage regulator? If not, it's not the alternator that's the problem....

 

All of the alternators that I've seen always have a little rectifier/regulator module integrated into the back of it, so I was thinking it was a module in the alternator... I guess I have to remember this is a little older than most vehicles I'm familiar with. Now I"m looking at pictures of the alternator I think it might be somewhere else. Let me check further.

 

If the alternator has separate wires connected to it look for some of them to connect to an external regulator (tin box)

If the alternator has two plugs it is a second generation alternator with internal regulator.
In this case you should definitely change the rectangular plug with the orange/black wires.
There is a Ford TSB on that.

 

The high voltage could also be caused by a sensing issue. All components may be fine, but due to a bad connection, the regulator thinks voltage is lower than it really is, and mistakenly adjusts the alternator output to compensate.

One common source of such an error is a bad ground at the alternator case. A quick and dirty check for this is to run a test jumper between the alternator case and the battery's negative terminal. If the readings now come down where they belong, fit a permanent jumper to ensure a good ground at the alternator case.

 

Just to keep you guys informed...

I actually have a couple of voltage regulators.
I have a F250 from the same year (85) and I found another regulator in my "stock".

trying both regulators produces absolutely no difference in charging voltage behavior (still wicked high).

I'm going to try kr98664's grounding idea.

After that I guess I'm chasing wires...

 

For some reason your regulator isn't sensing state of charge or system voltage.
The alternator is putting out all it can at whatever rpm's it's spinning.
This is going to boil your battery and warp the plates.
Check that the regulator itself is grounded (scrape some paint or rust, whatever) though I expect if you've swapped it it should be okay there.
Do you have an ammeter or idiot lights on your dash?

 

The yellow/white wire must 'tell' the regulator what the system voltage/state of charge is.
It would usually connect to the hot stud of the starter solenoid.

Dave Franklin offers a good explanation and diagrams here:
https://www.ford-trucks.com/forums/449569-1g-alternator-and-regulator-wiring.html

 

When I ground the body of the regulator the charging voltage drops to 14.5. Impedance measurement between battery negative and that fender (bolts that hold the regulator) is high. Alternator impedance to ground is 0 so that looks ok. So it looks like I got it. I have a bad ground to the passenger side fender.

It's dark now so I'll either find the ground cable and reconnect it or make one tomorrow.

Hopefully that fixes all my problems at once.

 

That's really crazy.
The starter solenoid grounds through its mounting ears.
You shouldn't be able to get the truck to turn over if the fender has no ground.