Four wheel drive is not engaging

Tiny
JESSICA KELMAN
  • MEMBER
  • 1998 GMC JIMMY
  • 4.3L
  • V6
  • 4WD
  • AUTOMATIC
  • 173,324 MILES
Four wheel not engaging when trying to switch into 4lo or 4hi. 2hi stays lit and if I press 4lo or 4hi it flashes twice and remains unlit and will not engage. Where should I start to find the problem?
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Sunday, April 30th, 2017 AT 9:17 PM

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Tiny
STEVE W.
  • EXPERT
These are known for having a few issues.
Bad selector switches, failed transfer case control module, failed transfer case encoder/actuator motor being the common ones that cause the light to flash.

First thing. Do you hear the transfer case try to shift or make and noise when you push the button? If not check fuse ATC 3 (20 amp) in the under hood fuse box.
Next you can try to re-seat the connectors and look for corroded/broken wire.
The switch is easy to access and if you are careful can be removed without removing the dash trim, it has plastic fingers that hold it in. Use a thin tool to reach between the panel and switch to depress the fingers enough that the switch comes free.
The TCCM is located behind the passenger side kick panel. The encoder motor is on the transfer case. Check the harness from the encoder into the body, they get rubbed between the case and the pinch weld area above the transmission and can cause problems.

If things look okay but none of that helped you have a couple options. Swap the switch, actuator or the TCCM out one at a time until you find the bad part, or take it to a shop that can scan the transfer case modules and actuate them and test which part is bad. This is especially true for the transfer case control module. They typically fail with an internal fault or failure to communicate with a scan tool, but you need a scan tool to find that out.
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Monday, May 1st, 2017 AT 2:11 AM

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