Battery guage going crazy!!

[1968_K20_4x4]

Hello everyone.

So I recently rewired my 1968 K20 and I wired up the alternator exactly like the diagram attatched. Except I think I used 10ga wire from the alternator to the main junction because thats what I had. I never had a working battery guage before, but now with the truck running the guage moves extremely quickly back and forth between the half way mark and the 3/4 mark. With the truck off the guage sits still at slightly above the half way mark. Pull the light switch on ( truck still off) and the guage moves just slightly to the left. Turn the headlights off again and it moves slightly back to the right.
The P.O. had completely bypassed the voltage regulator when I got the truck, so I'm assuming my alternator has an internal voltage regulator but I'm not totally sure and don't know how to test it. I do have a volt meter.
Looking for input on what is going on here and what to check next, and I know you guys will have the answers I need.

Thanks in advance!

[kwmech]

Did you check both of the fuses? And how did the PO wire in the alt and what were done with the blue and white wires

[dmjlambert]

It sounds like your gauge is working as expected. All the battery gauge does is show you the voltage difference between the screw junction at the battery and the soldered junction near the alternator. This shows you direction of travel of the current on the 12 gauge wire that goes across the top of the radiator support between those 2 junctions. It will tell you if current is flowing towards the battery because the battery is being charged, or if current is flowing out of the battery to power something. If the meter reads on the D (discharge) side, that means the battery is being discharged. If the meter reads on the C (charging) side, that means the battery is being charged. Right after starting the engine, it will read charging because the battery has just been partially drained by the starter. The meter rarely reads on the D side while the engine is running, because the battery isn't really used for anything significant while the engine is running, the alternator provides power and the battery just sits there or gets charged to recover from the startup.

Now the needle bouncing probably means the alternator is turning on and off. That may happen at low idle speeds, depending on what type of alternator you have. If the alternator turns on and off when driving, then it could be you have an alternator that is going bad and you may need to have it tested. I have heard one wire alternators need higher than idle speeds to start supplying power. If the alternator is a regular internally regulated 3 wire charging setup, the brown wire going to terminal 1 from the cab provides a signal to tell the alternator to start working. The wire going from terminal 2 to the junction near the alternator measures the voltage on the wiring harness to give feedback to the alternator about how much power it should send.

The 10 gauge wire between the alternator and the soldered junction near the alternator should be fine and I don't believe it will affect the battery gauge. I don't know for sure, but I don't think it would cause a problem related to the alternator turning on and off.

With the battery disconnected and truck off, does the meter still read slightly above the 1/2 mark? Mine does. I think the needle is just bent off center. When the meter has no power at all, it should read dead center. If yours doesn't, I think it just is a little off but doesn't severely affect the usefulness.

[67 Burb]

Originally Posted by 1968_K20_4x4

Hello everyone.

I'm assuming my alternator has an internal voltage regulator but I'm not totally sure

Here's the quick way to tell. Check where the connector plugs in.