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Service 4wd-key Off Drain

philzinn

By philzinn
in Ask A GM Technician

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philzinn Enthusiast

philzinn

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I have a 2002 Sierra with AutoTrac 4WD. In June 2008, the battery started going dead if the truck was not started in 10 days. In December 2008, I replaced the 4 switch unit because the Service 4WD light was coming on when the buttons were pressed, $50 bucks because alot of people say those go. It did not fix the problem.

 

In March 2009 I noticed the transfer case started sticking in neutral occassionally when shifting from 4HI to 2HI. In May 2009, the battery would go dead in 3 days, so I tested the transfer case again, hoping they were related. The transfer case would always stuck in neutral when going from 4HI to 2HI, or out of AUTO to 2HI. I Finally diagnosed the key off drain to the ATC circuit using a light bulb between the battery and battery cable, took out the ATC fuse and let the truck sit for 2 weeks and it started right up. So I took the truck to a shop about the transfer case and they told me it was the transfer case encoder motor and wanted $1,014 to fix it. I said thanks, bought the part for $200 and changed it out by myself. So the transfer case shifting problem is fixed.

 

But I still have the key off drain. The Service 4WD light still comes on when any button 4WD is pushed, but goes out when the ignition is turned off, and the battery drains out in 3 days. An auto parts guy told me he heard there were problems with the front axle encoder motor connector, but I'm running out of ideas here. Any suggestions? :rolleyes:

philzinn Enthusiast

philzinn

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In looking at some other posts, i can add this.

 

I thought it might be the battery back in June 2008 since it was still the original, so I put in a new battery that had been sitting in my garage for a couple of months but not put in the truck. Still went dead, so in Jan 2009 I put in a Sears Die Hard Gold. I also checkd the diodes on the alternator to make sure I did not have a bleed.

 

That is why I finally did the light bulb between the battery and cable test. I don't have an ammeter, but I know the key off draw is over 500 milivolts because it blew out the fuse on my digital voltmeter.

  • 4 weeks later...
philzinn Enthusiast

philzinn

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Well I finally broke down and took the truck to the dealership because I replaced the transfer case encoder motor and only part of my problems were solved. For $218 I bought the encoder motor. The truck still drained the battery. Shifting to 4HI from 2HI would not work, AUTO 4WD was OK, and sometimes the lights on the swtich worked and sometimes not.

 

They say it is the Transfer Case Control Module or the Computer that has probably shorted out, hence the drain and that a re-program will not help it. Fixed out the door is $782. I don't know if I can buy one cheaper, but I am sure it would still have to be reprogrammed after I take an afternoon installing it. So I would have the charge from diagnosis and reprogram, plus the computer. Should I have them just do it?

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