what causes voltage regulator to keep shorting out. I changed the Alternator for the hell of it, but the old one was working fine. I've gone thru 2 voltage regulators in a week. I keep getting a constant drain on the battery. I'll trace it back to the voltage regulator. Once I put a new one on, no more drain and works great for a few days, and then wham the new one starts to drain the battery. Is it just cheap regulator made overseas?

 

I replaced mine with a AZ one long ago
and battery went dead. I just put old back in.
I need one but I'm afraid of the same thing again.

 

ok, replaced the shorted voltage regulator again. So far so good. question, is it supposed to get hot enough that you can't keep your hand on it? Voltage at battery with engine running 13.8 to 14v. I let run for about 30 min in driveway then shutoff. I disconnected the negative terminal and checked for battery drain. I had no battery drain this time. I'm just worried about the new regulator being too hot to keep your hand on. Is this normal or will I soon have another failure?

 

That's an interesting one. No, the regulator should not get so hot that you can't put your hand on it. Was this happening when you were running the previous alternator? If not, I would suspect that your new alternator has a shorted field terminal and is drawing too much current out of the field terminal of the regulator, eventually frying it. This is the only external thing I can think of that would cause the regulator to fail prematurely.

To check this, remove the connector from the regulator, and measure the resistance between the F (FLD) terminal of the wiring connector and GROUND; it should not be below 4 ohms. Report your results.

 

It ranged between 4.1 and 4.0 exactly. It was doing same thing with old alternator. I was hoping just maybe I had bad luck and got bad new parts on the VR Side.

 

Got it, back to the drawing board I guess. If the regulator's field output isn't shorted external to the regulator, then the regulator must be internally shorted somehow since it's getting so hot. It's not clear to me yet what outside of the regulator could make this happen more than once. Nonetheless, with the regulator disconnected, measure the resistance from each terminal of the regulator (I, A, S, F) to ground (regulator body) and report your results.

Does this truck have an ammeter gauge or an ALT light on the dash?

 

i had a similar issue when i put the alt from my 78 F250 on my 76 F150...both trucks have the same setup, but the 78 had a capacitor hooked to the yellow wire off the regulator but the 76 didnt so i left it off...after weeks of charging issues and burnt regulators i backtracked my work and noticed the capacitor, so i plugged it in and it still works fine to this day and that was over 8 years ago..before i swapped alternators both trucks charged fine the way they were....you might see if you have one or not...

here is a pic of mine

 

That capacitor has nothing to do with the charging system. It's an ignition noise filter to keep RPM-dependent noise out of the AM radio. You must have fixed some other intermittent issue by moving things around in that area, because it's electrically impossible for the absence of that component to cause the issues described here.

 

when removed my truck wont charge...freaky coincidence i guess...

 

mine does have the capacitor and it's hooked up. I will check the resistance on each terminal to ground and report my results. thanks for hanging in there with me.

 

Is the regulator supposed to ground to the chassis through its housing? Possibly just a bad ground.

 

ok, unplugged voltage regulator and checked each terminal on regulator to ground.

F=6.39 ohms
S=.99 ohms
V=.98 ohms
I=6.87 ohms (I is not utilized on my circuit)

 

Those resistance readings sound impossibly low - it sounds like your meter was auto-ranging, in which case those numbers would actually be in k-ohms, which would sound more inline with what you'd see with a modern electronic regulator, except the "V" reading (which is actually an A).

Can you confirm the resistance on the A terminal to ground (including the unit)? If you truly have less than 1 ohm to ground at the regulator A terminal, then this would explain why it's getting so hot (battery voltage is applied directly to the A terminal). However, you'd have a massive drain on the battery. With the regulator disconnected, the A terminal should be high-impedance (the meter will show nothing, or float around at a very high resistance reading). The A terminal isn't supposed to actually supply power to anything inside the regulator until voltage is applied at the S terminal.

czaroc also makes a good point, it would be a good idea to check how well the inner fender is grounded. The regulator relies on a groundstrap between the firewall and the engine block to ground, as do all the other loads at the front of the truck (like the headlights, etc). It's not clear to me how this would cause the issues you're seeing, but it's free to check.