Alternator voltage swings

[Mark1968]

First post.

I have a '79 k20, the issue is the alt. seems to be hunting, driving at night with lights and blower on the voltage jumps from 10 to 17 randomly while above idle. I took it to the local autozone and they tested it both in the truck and on the bench, it bench tested fine but in the truck he saw the spike on his meter.

I am going to look at the grounds and the + battery cable then check voltage at the alt while under load. What else should I look for?

 

[BoondocK5]

The alternator my check ok for now, but the regulator part of it is going away, replace it.

 

[chulisohombre]

sounds like the regulator to me as well.it shouldnt jump like that.it will jump up to 14.5 or so after you first start it and then usually drop a lil after the battery is charged to normal again and stay betwen 13 and 14.5v.

 

[Mark1968]

Just swapped it for another I had laying on the shelf. Close to the same results. 15.7-8 running with lights and heater. Gonna go get a new one.

 

[diesel4me]

I sincerely doubt two altenators would show the same faults...I would suspect a loose connection in one of the two wires in the small plug on it,or a bad spot in those 2 wires,a short curcuit somewhere in the charging system wiring,perhaps a failing fusible link,---it could be the diode trio or regulator getting ready to fail,but I doubt two of them would do the exact same thing..a dead cell in a battery can cause weird charging symptons too..

 

[Mark1968]

Update, the issue persists. This has been put on hold due to a massive fuel leak overtop the drivers side tank. Also, battery is brand new and has been checked ok.

 

[Mark1968]

Well, my wiring is totally bumble fcked thanks to the many previous owners. No fusable links save the one at the firewall. I`m not seeing any voltage drops as i had expected, no missing or weak grounds that I can see. There is a mass of cut or bypassed wires under the dash, what looks like the kickdown for the th400 is unplugged cut and theres a wire spliced into it that runs to who knows where. Speedo cable unplugged, and many more wtf wires. What a mess.

I did get the radio to work though, plugged in the antenna.

 

[Mark1968]

Oh and I have a new fuel leak too, right overtop the drivers side tank. Not sure if it would be easier to drop the tank or pull the bed? It does look like I need at least one new strap as well.

 

[Mark1968]

Update

Issue seems to have gone away after I pulled the bed, cleaned the tail light ground and installed new bed bolts.