Whats a good voltmeter to get?

You can replace them all if you want but it’s not generally neccessary.
I’d recommened taking the batteries out and set them on a bench or a piece of plywood on the floor. Test each one individually with a load tester. They will tell you if it is bad or just needs charge. If bad replace if needs charge just charge it. May take overnight. The bad battery will draw down the good ones but once you get rid of the bad one the others generally are gonna be fine once charged.

 

I bought one from harbor freight and didn’t pay attention as to what groups it could test but it has a 1000 cca test capability on the meter so I have used it with good results so far. It looks like a knockoff of the Actron that Napa and others sell.

 

Some of them aren’t capable of putting enough load on a 31 battery. There’s a redneck way to do the same thing, but I don’t want someone blowing up a battery.

 

I would get it checked out

 

Alts not charging. Check output at back of alt and at batts. Should be very close. Alt should be putting out at least around 13.5 volts no more than 14. Batts should be gettin at least 13.5 to stay in good shape.

When you shut down engine and have key off does amp gauge go back to zero? It should if it is working. Volt gauge should show what batts are sitting at with engine off. Some trucks read hot with no key some need key to be on acc.
What does volts show engine off?

 

Also. With engine running. When you turn on 4 ways does the needle pulse negative and volts drop with the flasher?