Dual battery question
 

My 8.1l has two batteries. The truck is dead after sitting for a day. I`m not sure how the batteries are wired.. Do they both start the truck in series?
Guess I could get them both load tested and figure out if I need to replace just one or both..

They are wired parallel. The cable from the passenger side battery comes over to the main battery on the driver side. Check the ground on the driver side battery to the block. I've seen 2 rot off.

If one is weak it will take the other one down over time. I would have both batteries tested (you will have to unhook for accurate reading) and if they are ok you have an open draw somewhere.....which is always fun to track down.

Weak batteries? Time for a new duramax!! Haha good luck Dan. How old are the batteries?

I had a bad blower motor resistor that was draining my dual batteries on my old 2001 8.1 2500hd.

How the heck did you/someone find that? Or is it common?

Batteries, alternators, starters...
Sometimes the symptoms can point to one of those 3, but actually be another of the three. One of those bad can also kill the other two.

Don't have Advance Auto do the load test in the parking lot. It's really not a good test of anything.
Pull them out, and take them to Sears. They have a diagnostic station they call the "microwave" as that's what it looks like. The batteries go in there for 10 minutes for a full test (and hyper-charge if good). They do charge for this, but it's worthwhile IMHO.

FWIW, Sears Die-Hard Gold in everything I own with tires.

It's common, there not actually a resistor. They are a pulse module. It pulses the ground to control speed, and gets stuck on. Blower just keeps going even with the key off.

Last time I jumped it, the HVAC blew full on heat on the drivers side and full ac on the passenger side.lol Good thing it fixed itself once I shut the truck off.

The batteries are 9 years old and I think this winter finished them off as the truck sits all winter.

thanx for the advice .. I`ll check that ground

9 yrs OLD! lucky You I would say you got your moneys worth. My 01 8.1 needed them about every 5 yrs. Didn't use much so probably didn't help Batt life.

Blower motor would not hardly turn off or change fan speeds. When I could get it to shut off, sometimes the blower just come on during the night and drain the battery.
For my current 2007 Classic 2500HD, they sent me out a bulletin that some also have issues and they will replace it up to 150k miles.

Did you check for a draw with the test lamp like I showed you?

I think its time for some new batteries Stanley.

Disconnect the negative cable. Clamp the test lead to the negative post on the battery, then probe to the negative cable, which will be disconnected. If the test light illuminates, you have a draw. Then start pulling fuses until the test lamp goes off. That will help narrow it down.

If no illumination, no draw. or at least not a big enough draw to kill the battery like it is. Either way, the batteries were stamped Jan/09. 5 1/2 years old, they are prob done anyhow. :bong:

in most case's if factory installed dual batterys one battery goes to the starter and the other one goes to a solenoid to keep the lights from dimming..this would be called the plow package..if they are tied together like a diesel then it was most likely done after market like i did to mine..most likely a bad battery taking out the good one..

Got 2 new Interstates, lets see what happens..

thank you for the replies.

the test light worked ok on older cars,newer cars and truck could show a draw for a while until all the modules go to sleep,
the best way to check a draw is with a DVOM set on amps,it might read high at the beginning but after a while (up to 20-45 minutes)it will drop down,rule of thumb is anything under 20 milliamps is ok, be careful not to turn on any accessory's you might blow the fuse in your meter

I recently traced down a parasitic drain on a 2005 Ford using the test light method. It had an aftermarket radio, which must have been shorted internally. Unplugged the stereo harness and test lamp went off and no more dead battery after sitting for 2 days. Your way is better though....Most of the stuff I have is still from the cassette player days !