charging needle waiving back and forth

[customcruiser72]

My charging / volt meter needle has started waiving back and forth erratically. It goes from extreme Discharge to extreme charge and the needle moves so fast back and forth you can barely see it. The problem is intermittent and sometimes the needle is steady in the middle. It has been doing this for a few weeks and my battery has not been discharging and it is a daily driver. I thought it was the guage but recently I was driving in the dark and my headlights were alternating bright/not so bright. I have an external regulator which has been replaced a couple times over the last few years. I think I will either convert to internal regulated altenator or get a new external regulator but I was wondering if these symptoms are familiar to anyone.
I have a 71 GMC Longhorn C20 with a V8.

[68gmsee]

You should have an ammeter on your truck unless someone installed a voltmeter later. That's what the trucks came with. Voltmeters are wired in parallel and ammeters are wired in series.

First thing double check all connectors to make sure they are tight and clean. Make sure battery cables are new and posts clean. Battery cables can corrode internally if old and not visible from the outside. After this you should take both the alternator and regulator to a reputable alternator/starter shop for testing. Some local auto parts may have the equipment for testing the older alternators and regulators but not all of them.

(Important): Install several ground straps to insure you have a good loop to the battery. This is to eliminate any possibility of a grounding problem. Old bolts and rust don't conduct like they should. I installed 4 on mine:

-New one from rear of engine to cab
-under the rear of cab to frame
-rear of bed to frame
-radiator support to frame.

[customcruiser72]

Thank you for you input. I will check the grounds and connections tonight as a start. I like your idea of adding ground straps.

[68gmsee]

Your idea of going to an internal alternator is a good idea. It's fairly easy and lots of info here on this site and the internet.

I did that on my 69 chevy but I left my 68 GMC with the stock one. I did have to replace the external regulator on it twice (one was new defective) but haven't had any problem with it since.

[wesmigletz]

I recently had a new regulator fail. It may have been bad from the get go. I decided to just replace my alternator with an internally-regulated one from a mid-80s truck.

[customcruiser72]

Just to complete this thread, I have the problem fixed and it turned out to be a faulty battery. The battery in there was 1.5 years old so was replaced under warranty.

[customcruiser72]

Oops, spoke too soon. Replacing the battery did not fix the problem.

[wesmigletz]

Any luck with this yet? Have you checked the two short fuses in the wire harness? One is on the passenger side near the battery try, and the other is on the driver side not too far from the regulator. Maybe the fuse holder is corroded and has an intermittent connection.

[customcruiser72]

I ended up changing the altenator with a rebuilt one from a 1985 camaro with internal regulator and followed the posted instructions for converting. It initially seamed to have something wrong with overcharging but it settled down after a couple days, maybe the battery needed charging. I have put on 1000 miles over last 2 or 3 weeks and so far so good. The headlights are noticeably brighter. The charging guage on the dash shows very slightly off center toward the charge side.
J
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[wesmigletz]

Glad you got it fixed