2008 GMC Sierra 2500 HD 4wd

[ajot4u]

Hi. I love this truck once it starts. Several mechanics have checked it out. New battery, starter and alternator fine. After 3-4 days of non-use, battery is drained, no lights or anything, totally dead. Nothing coming up on the service shop codes. Any other ideas? Thanks. 

[Jsdirt]

Your guess is as good as mine. There's at least 15 modules in there, and any one of them could be drawing. Alternator could've failed causing a draw. Could be a short in the battery cable, or wiring to the fuse box under the hood or in the cab. Could be a light that's staying on when it shouldn't. Could be the instrument cluster, or radio. Literally hundreds of places it could be.

 

You'll need to pinpoint which circuit has the draw. If you happen to work somewhere that has access to a thermal imaging camera, you can shoot the box with it, and the fuse or relay with the draw will stand out like a sore thumb. That's the easiest way possible.

 

There's also a millivolt test across the fuses that'll pinpoint the draw, but you'll need a chart to figure it all out - can find all the info on that test here: https://us.autologic.com/news/testing-parasitic-draw-via-fuse-voltage-drop

 

You can do it with a multimeter, but it's a pain in the ass. An amp clamp is 200x easier. Sometimes disconnecting the battery can either fix the problem unexpectedly making troubleshooting impossible, or will cause other problems like bricked modules - best to leave the battery connected and use an amp clamp these days. Once you find the problem circuit, you can troubleshoot from there.

 

You'll more than likely need a wiring diagram unless it's something obvious on that circuit. Do visual inspections first. If you find yourself getting nowhere, then buy a wiring diagram online. I've had good luck with AllData DIY in the past. 1 vehicle for 1 year is about $15- $18 last I checked - could be more with the mega-inflation lately.