Newsom09

I recently traded for a 96 Tahoe and when I first got the it, the battery would die completely overnight so I figured it was just a bad battery. So I put a good battery off of another of my vehicles on it and it had the same problem except it wouldn't be entirely dead overnight, it would still have enough power to play the radio of turn the engine over a time or two but that's all. I checked the alternator by unhooking the battery while it was running and it didn't affect my tahoe, so I was told to put a test light between the negative terminal and negative battery cable and see if it glows. I was told if it glows bright then I have a parasitic drain and to pull each fuse until it dimmed and that would tell me which circuit I had a short (drain) on, but if it just glows dimmly then it was fine, but my light glowed dimmly. I checked to make sure there was no lights that stayed on in it when the switch was off and made sure the radio went off and everything, so I can't figure out what's draining the battery like this. Any ideas??

Newsom09

Is there anyway the alternator could be going bad and not charging my battery good and causing it to die like this? like I said, I did unhook the battery while it was running and it kept running. I also hooked the battery back up and checked what kinda voltage the gauge was showing while it was running and it was showing 12 volts but when I turned the heater and headlights on, the voltage meter dropped quite a bit and stayed instead of returing to 12 volts. I also noticed a kinda whining sound coming from my alternator today?? Any advice would be greatly appreciated.

SabrToothSqrl

Advance Auto will check your charging system free for you, and give you a print out of what's not working... I'd start there... you can't beat FREE

Andrew Bewernick

Having a similar issue. About 2.5 A draw when all modules have gone to sleep. I pulled the fuse for the BCM, DDM (heard it clicking relays before), and "RDO" (I assume means audio system.) and a number of others. RDO fuse (42 I think) made about 1 A difference. Current did not return to 40 mA. One thing I found which is super strange is a kind of beeping coming from the engine compartment fuse block. I beeps faintly about 6 times, pauses then starts again. This went on for about an hour with a current of about 1 A. I tried pulling relays, feeling them, listening with a tube to try isolate the area. I think it's coming from the underside of the fuse block. I tried to recreate the beeping after it stopped but couldn't. Last I left the vehicle the current draw was back to 40 mA.
Does anyone know what could cause a beeping sound in the engine compartment? I know the horn works so I doubt it's the horn relay.

mountainmanjoe

Any aftermarket stuff? Alarm ....

Cusser

Maybe you got lucky, because that redneck "test" kills alternators. Gomer and Goober didn't know any better, and used the old hillbilly test for GENERATOR testing and ruined lots of alternators.
"Was we wrong??"
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One tests across the battery terminals with a DC voltmeter while the engine is running. It's a 1996: install a battery cut-off switch if you cannot find the parasitic drain. Many parasitic drains are caused by bad alternators; see beginning of my reply.

repairman54

Did you do a full code read of all 4 code families ? U, C and B codes could be present and are very helpful.
The radio wakes up the CD player in my '11 when I open the door, always has for example. Stuck CD's in player will cause power drains.
What year is this truck ? It makes a big difference.

Andrew Bewernick

Apologies for the long delay in replying. I don't check my gmail often enough. Anyway...
The Tahoe has tons of aftermarket stuff. It is used as an investigative vehicle for tracking radio signals as well as used as a mobile office. There's measurement gear, computers, radios, an inverter, VPN etc. I think I figured out what the issue is/was. The PC's soundcard output was plugged in to the Aux input of the vehicle's stereo. It created a ground loop through the battery isolator. As soon as I unplugged the Aux cable the BCM went to sleep and stereo stopped it's 1.9 A draw. So I think it's all good now. (Still need a new battery though. This one won't hold a charge any longer.)
Thank you for your interest. Happy new year.