Truck Repair: Battery problem, prefferably, battaries

Question
I have a 2000 Chevy Turbo diesel.  Has two battaries.  Recently went dead, needed a charge.  Thought I had left something on.  Now one battery is completly dead, will not take charge and the other takes one.  I put another battery in (temporalily) in it's place but it doesn't show charge on the instrument panel, stays at a constant 7 on the discharge side. However, it does start up and run with no problems other then the rpm gauge doesn' work?  What could the problem be?  How do I check the altenator?  I have both batteries under warranty but I'm on vacation and the dealership is 400 miles away with no areas here, They are DayStart batteries.  Please help.   Dan

Answer
Hi Dan,
You need to check them with a volt meter, or better yet, a battery tester.
They should each show 12 volts or more each, when the cable between them is disconnected.
Then, after connecting that cable, and starting the truck, the voltage should go higher...prefferably to about 14 to 14.5 volts.
If it goes up, the alternator is probably ok.
If it stays the same, either there is a loose connection, or the alternator isn't charging, which would allow the truck to run on just battery power for a while, till it is pulled down.
Van