Alright for the last week or so of driving my 1979 F150 with a 400m in it, when i got on it to high rpms the headlights would dim. Well last night i finally noticed it got worse and checked out my aftermarket voltage gauge. which would randomly charge at just over 14 volts like normal, then the voltage would drop to like 10-12 volts and all the lights would get dimmer for a minute or a few, then it would go back up in voltage and stay there for a few to several minutes then it would occasionally drop again, it would especially do this at higher RPMS while also running poorly when the voltage drops.

is this my voltage regulator?

 

Check the alternator belt, make sure its tight and in good condition.

 

alternator belt is fine, it'll still drop in voltage just cruising along randomly. then go back up again and stay for a little while

 

Check the plug in at the regulator. It could be losing contact intermittantly.

 

Since the voltage is fluctuating, I would suspect the regulator. If it is external, it's an easy change. If it's internal, you may not be able to change it without dismantling the alternator and soldering the connections. If that's something you are comfortable doing, go for it.

 

I suspect the regulator as well, but you'll have to wait until it's stuck in the fault state consistently to troubleshoot. However, regulators are cheap enough that you're not out too much if it is in fact not the problem. It's your call, really.

 

So i put a new regulator on the truck for $40 and the voltage is still jumpy, especially when i give it a good rev it'll drop then go back up to 14 or so, and then at idle it will occasionally jump but rarely.. the belt is plenty tight before anyone else says to check it.

 

I had my battery tested and it tested at 13.1 not running. Charging system tested while running at 13.13 without a load, and 14.97 with a load, lights on heater blowing and revving it.

i tried the same thing while driving down the road, turned on my high beams and the blower motor on, and the radio shut off. and my voltage dropped to like 8-9 volts. as soon as i turned off the lights and heater, the radio came back on. and voltage went back to 12 or so volts. if the truck is idling or at low rpms it'll be at just over 14 volts. so why would it stay around 12 when im driving down the road at 30-50+mph? i don't get it.

 

It's still not regulating. It takes more than the regulator operating properly to regulate. Check the following:

1) Continuity between the wire corresponding to the 'F' input of the voltage regulator, and the wire boot corresponding to the FLD stud of the alternator. Regulator and FLD wire boot must be disconnected for this.

2) Continuity between the positive battery post and the wire corresponding to the 'A' input of the voltage reguatlor. Regulator must be disconnected for this.

If either resistance is no more than that of your meter leads shorted together, then the next time the voltage jumps, connect a jumper wire between either regulator mounting bolt and clean, unpainted metal on the engine (ground). Note any improvement in behavior.

The final thing to try is a full-field test. Try the above tests first. You need a multimeter.

 

I put a new alternator in and it fixed the problem, probably had a dead spot somewhere.

the regulator didn't fix it at first, but now I have a new Regulator and alternator. saved the old regulator just as a spare..