Electrical problem

[zac]

1979 K20 Camper Special. Had lots of PO wiring issues, think I have corrected them. Has new battery cables, new alternator harness, inspected headlight harness and see no issues. Tried 3 different alternators with same result. Driving with headlights on, brightness is very erratic, can help things by goosing the accelerator. But issue doesn't seem to be RPM related. Sitting at idle it's fine many times, except when it's not. Driving at 65-70 you notice lights start to dim, so you play with the pedal and you get a temporary fix. When lights are dim, voltmeter shows low, just above red arc. When they're bright, it shows 12-14. Other accessories make voltmeter dip as well, but seems more normal. The headlight issue is clearly abnormal. Running out of ideas. Could this just be the light switch?

[79 silverado]

My questions are the following: is there any aftermarket accessories that draw electric current to where maybe your alternator isnt high enough amp for what is being drawn at certain times, such as when the rpm the engine turns drops? (Stop lights or at idle on highway would be times it may drop rpm and not spinthe alt as quick...)

Also, are you using the stock pulley on the alternator, and when you swap those three alternators are you swapping the same pulley between them? It may be an undersized pulley which means the brushes inside do not spin as fast at a given idle as they would with a larger pulley, especially possible if the PO swapped cheapo chrome pulleys on the engine...

[zac]

Anything PO's could have done wrong, they did. The only thing this truck had going for it was a good body with a near perfect bed, a th400 and a full floater. It had a cd player with some of those ridiculous speakers--I took all that out. Nothing else out of the ordinary. The alternators I'm swapping in are from mid 70's Caddys so they should have plenty of capacity, although I have a very low opinion of rebuilt stuff in general. I had not even thought of engine pulley size, I will check that out. The alternator pulleys seem to be relatively similar.

[zac]

Bing. Water pump pulley seems small, 6" or so. One on my Burb is 7". Crank pulley looks right. I will find a bigger one in the "pile" and swap it on tomorrow. Thanks.

[79 silverado]

You're welcome... its always the last thing you'd think of... one of the reasons I overpaid for my 76 when I bought it was because the same guy had it for 36 years and hadn't mickey moused stuff. Hopefully this will be the last of your issues from here on out that they messed up on you.

[76Charlie]

Maybe a bad ground somewhere, and when you fluff the throttle you cause the motor to twist and it moves the wires slightly you know? I had a bad alternator ground before, not pretty when it sparks haha

[zac]

OK, I'm back. Replaced dimmer switch, headlight switch, alternator #4, changed pulley. Same deal. Another twist I forgot to mention before, when you pop the light switch on oil pressure gauge drops from normal to almost zero, temp gauge goes from normal to max hot. At highway speeds volts at 10-11, coasting into my driveway it popped up to 13-14. Same thing coasting into a sharp right turn, then dropped and lights dim as I accelerated after the turn. Maybe short in headlights somewhere? I thing my grounds are OK but I will re-do everything. I noticed some headlight harness grounds I will re-do also. After that I'm out of ideas.

[bilfman]

Check head light switch as well.

[KQQL IT]

Sounds like a bad ground
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[motornut]

go look here,bout a foot from the cluster plug
they all go to one,pink is cluster power