When I park my 1999 Suburban on my 15 degree inclined driveway (nose up), the battery dies overnight. When I park on the flat street, or nose down on the driveway, it can sit a week or more with no trouble. I assume something is shifting towards the rear and draining the battery, but I’m at a total loss as to what that could be. Battery is an absorbed glass matt (AGM) so no battery acid shifting issues possible. Any ideas?
Try unplugging the underhood light. if the underhood light switch is out of adjustment/faulty the light might be coming on when pointing the front end up hill.
Tester
Is there an anti-theft device installed (after market)? I’ve heard that if a car is raised up in the front, anti-theft devices are supposed to disable. They think the car is being towed. Yours might be shorting out. Just a shot in the dark.
To track it down, I would park the car that way, then pull fuses to see which one sparked. If one of them did, I’d leave that out overnight to see if the battery still drained. If not, whatever’s on that circuit is the culprit.
Hmm. Good ideas. No anti-theft device that I know of. What would I look for?
I tried pulling each fuse and even in the dark, did not see any sparks indicating load. It’d take a month, but I suppose I could pull one each night. It literally kills the battery overnight, versus no problems for weeks parked nose down.
Open the hood and the underhood light automatically comes on. There’s a switch that controls at what angle that light comes on. Unplug the electrical connector from the light. If the problem goes away, the hood switch is the problem.
Tester
yeah. Did that. We’ll see tomorrow.
I was wondering if the truck could have an anti-theft device that I’ve never noticed or didn’t know about.
Hopefully Tester’s good suggestion will solve this trouble but if it doesn’t, here is a good link for advice on how to track the trouble down.
OK, well, unplugging the hood light MAY have done the trick. But, I also undid the courtesy light fuse because I’d had trouble with the door switch sticking in the past. I’ll put that fuse back and watch it for a week.
I had to wait a while and be sure it truly fixed the problem, and indeed it did. I knew about the under hood battery light, presumably on a mercury switch, but I would have thought it would be noticeable if the light was on when the car was parked on the incline in the dark. Apparently not. For a month I’ve had no problems. I realize the full diagnosis would require plugging the hood light back in and confirming that it kills the battery, but it’s winter in Colorado and that’d be a foolish thing to do 
Thanks to all!
Thanks for letting us know how it turned out. Glad it worked for you.
The angle that light hangs at can easily be adjusted, in fact it was common to bend the bracket when you have the hood up while the car was in the shop so the light stays off. Think of the angle the hood is at when it is fully open and the car is parked on level ground, now think of how far off the bracket would have to be bent to create the same angle only by altering the angle of the ground the truck is parked on. If the bracket is not bent the hill angle would have to approach 60 degrees before the light would come on. It certainly could be tested and the lighted mirror on the passenger side visor is very prone to staying on also.