03 Duramax batteries discharge

My 2003 Chevy 2500 duramax has a battery draw somewhere that I cannot figure. I can use it for a few days in a row, then it sits overnight and batteries are dead. Have replaced them 4 times in 18 mos. Chev dealer thought it was a relay(?) that activated the glow plugs-replaced it-Nope. I replaced the original alternator-Nope. Local mechanic replaced a relay that controlled exterior lights, unhooked brake controller-no luck. I don't think this is just a small current draw, it seems to discharge pretty fast when it happens. It's useless to take on a trip with this problem. Any suggestions, previous experience like this?

I've used a $10 battery disconnect to temporarily stop a battery from discharging until I could find and fix the real problem.

Try removing the fuse for the radio for few days and see if that helps, Excessive parasitic draw is a common problem with GM cars and trucks of that era. You can Google and read for days on various forums.

It's not easy to troubleshoot since radios and other power equipment are controlled by BCM, ECM, etc, and not a simple on/off switch. It is normal for them to continue to draw power for some time after shutting down but then they are supposed to go to "sleep".

If pulling radio or OnStar fuse fixes problem you will have to decide what to do, probably trip to dealer to see about TSBs,reflashing, etc. It can be a (expensive) pain or else live with no radio which might also disable warning chimes, turn signal clicker sound, clock, etc.

Start with radio and OnStar fuses, one at a time. Good luck.

Just use a disconnect or take cable off see if battery holds then.

Pull cable off one battery, then put milliamp meter on the other, pull fuses until current drops off on remaining battery.

You can post back the results.

Just be careful not to burn out or blow fuse in ammeter. Watch out with lights coming on or things that happen when you open the door. Start a multimeter on high range and work down, keep the key out, etc.

Good point, might want to put a test light in series.

Thanks for the replies. I will start with the onstar and radio ideas first. Have tried the milliamp checks, but it seems pointless unless you can catch it at the right time, as it seems to be an intermittent problem. Also, as I said, it draws down pretty fast when it happens. How would you do the battery disconnect idea with 2 batteries? Thanks

Not only do you have to catch it at the right time but you have to wait for some of the circuits to go to sleep before the current drops real low. A technician will sometimes have a closed switch in the circuit parallel with the ammeter, then let it sit for a few hours or overnight; then you open the switch and measure the current without everything starting up again from scratch.

Do you have, or did you have, the OnStar subscription active? I have heard of batteries running down when the OnStar subscription expires, especially if you had the phone service through them. I believe there are dealer bulletins about reflashing certain modules to fix this.

On the gas trucks sometimes the fuel pump keeps running in the tank (stuck relay). If the diesel has a lift pump might want to check that, pull fuse overnight or replace relay. Didn't some GM diesels also have a fuel heater to prevent gelling in cold weather, could be a relay sticking on that.

But start with radio and OnStar because at least you can still drive truck with that stuff turned off and it doesn't cost you any $$ to check out. That fuses may be labelled DIC on some years. Good luck.

I would not fart around with a disconnect, find the issue and fix it.

Do you have the trailer plug on down below the bumper like everyone else? Just replace the wiring to the trailer plug on my dodge as it was shot (gravel roads) and my intermittent dead battery issue has so far disappeared...

Disconnect your batteries and hook up your battery charger as a power source and look to see how many amps it draws. This will give you a good idea what's draining your batteries. It's worked for me more than once. Good Luck.

You could try a compass. Just walk around it and the compass will point towards it, if it's a short. They also have these clamp on amp meters,but were still pricey last time I looked.

Have several Chevy trucks and this problem occurs often. In the 2 trucks that discharge the problem is in the steering column. The shifter handle doesn't quite go into park all the way and doesn't allow the key to shut completely off leaving some circuits energized causing the batteries to drain. The key appears to be off but isn't.

Pulled the fuse on the OnStar and have had no problems for 2 weeks. This fuse only supplies onstar and the rear entertainment console (a factory radio tuner with headphone jacks, so no big deal.) Am keeping my fingers crossed. 4020, if the shifter is out of whack, can you still remove the key, yet still have "key on" power? Or would being able to remove the key mean power is shut off, as it should be?

That's what we have been doing but employee's tend to loose things😳! Glad you found it.

You can buy a device to plug in that will fool the vehicle when the battery is disconnected. I would suggest you go on youtube and check motor age training as well as Ericthecarguy and others on how to test for a parasitic drain. Hope you find it.

Ok I think I found it. 4020 put me on to it. I found that sometimes that the ignition switch would not be completely off even when the key is removed. I don't think the shift selector was the problem, just the key switch. I noticed the little red battery icon would stay on after removing the key (sometimes) and this was when the batteries drew down if left overnight. Just putting the key back in and removing again would shut the icon off, so I think this was the problem and why it was so intermittent. My ultimate solution--I traded it in on a 2014 GMC duramax. I wanted a Ford but the wife won out... It's a nice truck, and I still have my 2000 7.3 powerstroke....