Mechanics

DEAD BATTERIES

2002 GMC Sierra • V8 2WD Automatic • 76,000 miles

When the truck sits for 1 month without use, both batteries go dead. This is a Duramax. I do not have onstar. How do I trace down what is eating up the batteries power?
Avatar
Nmgmc
November 13, 2010.




There is a procedure for finding a battery draw like that.

You will need a digital ammeter and a jumper wire with clips on the ends to do this.
First rig any door switches so you can have a door open without triggering the

interior lights and unplug the hood light. Remove one battery cable and attach the

meter in series between the battery cable and battery post. Take the jumper wire

and also attach it the same way. Leave the jumper wire on for at least 10 minutes to

expire all the automatic timers. Now remove the jumper wire and read the meter.

Anything over 50ma is too much draw. The way you locate this is to start removing

fuses one at a time until the meter drops to normal level. This will be the circuit

with something staying on. Determine what components are part of that circuit and

check them individually until the problem is isolated.
AD

Wrenchtech
Nov 13, 2010.
After you shut it off disconnect the positive battery cable and put a test light on it. If it's on then there is a draw on the battery. Thingslike radios will draw like a couple of seconds. Then pull fuses one at a time until the light goes out. Try the ABS unit first. If you pull the fuses and the light is still on, disconnect the alternator as it may be drawing due to a bad diode.

Hmac300
Nov 13, 2010.
Hi guys. There's nothing wrong with this truck except for all the insane computers. The industry standard is it should start after sitting unused for three weeks. That's assuming up to a 35ma draw on the batteries for the memory circuits in the computers.

The test light procedure doesn't work anymore, again, because of these stupid computers. Some of them will draw up to three amps for up to 20 minutes until they go to "sleep" mode. Disconnecting the battery will cause them to wake up again and draw high current as soon as the circuit is reconnected with or without the test light.

The only way to test for excessive battery drain is to remove a battery cable, install a jumper wire first from that cable to its battery post, THEN install an ammeter switched to the 200 ma. Scale across that jumper wire. Wait at least a half hour to be sure the computers have powered down, then remove the jumper wire. That way the circuit will never be broken and the computers won't wake up. The jumper must be momentarily reconnected if you want to change the meter scale because turning the switch will break the circuit for an instant and the computers will wake up again.

The secret here is the truck sits for a month. While some cars and trucks will still start after sitting that long, no manufacturer guarantees it will.

Caradiodoc

Caradiodoc
Nov 14, 2010.

AD