Whats a good voltmeter to get?

Oh, if the truck is running and it's in the negative, I'm leaning towards alternator again.

But like the guys have stated above, it's cheap and easy to pull your battery cables and check each battery before you throw money at an alternator.

 

Might be a bad gauge just lying to you.
I don’t know how they do the new ones but on the old Petes and most of the vintage trucks I’ve worked on all the dash electrics and ignition was run off of the back of the amp gauge. A 6ga wire came in off of alt to the gauge than a 6ga wire from gauge to circuit breaker box. If the gauge got stressed it would just die and read randomly frozen but still let full power go thru it. Everything would work as normal but the gauge.

 

No. Each battery needs to be tested separate with a load tester. You can get a good idea which one may be bad by using a voltmeter, but they need to be left to set individually,preferably overnight.

 

Yes but disconnect all the batts from each other by taking off all the cables and test individually.

 

Lol! @062 and I said the same thing about testing them just used different words to make it sound like we disagreed.

 

Don't take my word as the Gospel, but here's what I would do -

Pull all the cables. Make sure you know what you're doing, because once you start "welding", you're probably going to blow up a battery or at least melt some stuff. If you're unfamiliar with how to put all the cables back on, then take a picture first. Some might tell you to wear safety glasses.

Buy a cheap load tester. Looks like an analog gauge with a space heater built into it. Very rugged looking. Hook it up to each battery individually, and hold the toggle switch. The needle will tell you if it's bad or not.

 

You’ll need a load tester that is capable of checking group 31 batteries. Truck pro or Napa will be your best bet,I don’t think harbor freight has one big enough.
You just follow the directions for what ever model you get. Most have a button or switch and you take a reading over a few seconds time while watching the meter.