Auxillary Battery Not Working ?

[trx680]

I have a 2001 GMC 2500hd. It has 2 batteries. I bought the truck a few months ago from a younger couple who did own the truck long themselves.

 

I was fooling around with my 2nd battery (on the passenger side) and checked it with my voltmeter and it only read 5.4 volts. The main battery read over 12V. Both batteries are the same and appear to be bought at the same time. I swapped them over to see if the truck would start, and it didn't.

I had to jumpstart the truck from the charged battery (which is now in the passenger side).

 

I would think that both of these batteries would be hooked up together so they both get charged and they both get used together....right??

 

So somethings wrong with the auxillary battery system.

 

I cleaned all four of the cables when I hooked the batteries back up, so I'll rule that out. Any idea what it might be? Is this a common problem?

 

THANKS !!

[Gregorio]

Start tracing the cables to see what device is isolating them. My guess is that it is not functioning anymore. It should keep them separated but charge them both from one alternator. What is connected to the spare? The trailer only?

[Z45]

The battery could be bad.

[trx680]

What is connected to the spare? The trailer only?

 

I have no idea whats connected to the auxillary battery. I thought everything was connected to both batteries. I figured they were hooked up in parallel to one another so that I still have 12V.

 

 

I'm charging the dead battery now. I'll find out if the battery is bad.

[pistonslap]

What is connected to the spare? The trailer only?

 

I have no idea whats connected to the auxillary battery. I thought everything was connected to both batteries. I figured they were hooked up in parallel to one another so that I still have 12V.

 

 

I'm charging the dead battery now. I'll find out if the battery is bad.

 

 

From what i read the aux battery is only for the trailer hook-up. It is not hooked into your starting circuit,but still gets charged.It is that way so if you run your trailer power too long with the truck shut off it will only kill the aux battery. I would like to have mine hooked in to the starting circuit with a switch you could enable if your pulling a trailer.

[Gregorio]

The battery could be bad.

 

 

Good point. Test the voltage at the aux battery with the engine running. If the voltage goes up to alternator output level, your charging circuit is fine but the battery is bad. If it stays low, both are bad.