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I have a 2004 gmc Sierra 4wd.. The selector switch lights up when I turn the key on and then go out. I also have no 4wd capabilities. I have replaced actuator and switch on dash.. What next?

Posted by Anonymous on

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6ya6ya
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Posted on Jan 02, 2017

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SOURCE: 4W will not turn on ford ranger 1999 4x4

that clicking sound you no longer hear is a relay switch behind dashboard ....aftermarket one's sold at all chain auto parts stores

Posted on Nov 25, 2008

  • 1 Answer

SOURCE: 2004 GMC Sierra 1500 4wd. Auto transfer case light issue

I have the same problem. It is intermittent and sometimes shows up with service transmission message. To get rid of it just put the truck in park, press the 2 high button and shift into drive while it is still flashing. I will worry about it more when it blows up. You can drive with the vehicle showing the solid neutral and it seems to make no difference. It has to be a wake up problem between the control modules. It only happens on start up.

Posted on Sep 06, 2009

Molson02536
  • 3854 Answers

SOURCE: Had 2001 GMC Sierra in 4WD Auto and went to put in

On the full size, the front actuator is electric, not vacuum.

When in 4wd mode, a switch on top of the transfer case conducts 12 volts out to the actuator. Wire #50 (brown) is the 12v feed, and it conducts to the light blue wire (#900) to the actuator.
When shifted out of 4wd, the switch opens up and no voltage goes to the actuator.

Check voltage at the connector for the front actuator (at the front differential - it looks like a large bullet that threads into the front diff.) If it has 12 volts, the actuator is the problem. If it does not have 12 volts, check the transfer case switch and the power feed to the transfer case.

I'll assume it is the actuator, if the plug to the actuator is getting 12 volts when in 4wd.
The way this actuator works, is it is a sealed chamber that gets heated by voltage. When it heats, the pin in the end swells and pushes a shift fork in the front differential. The shift fork engages a spline on the passenger side of the differential, and boom! you have 4WD.

GM offered an upgrade to this actuator that is a motor instead of a heated device. However, it requires a small harness addition, a spacer, and a new actuator. It is about a $150 to $200 option.

Good luck. The actuator is usually the problem 90% of time but recheck all of the wire harness just to maker sure there is no short before replacing any parts.

Posted on Jan 07, 2010

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you can hear the transfer case kickin in, but the front wont lock in. replace the actuator and ... service 4wd indicator came on after trouble trying to engage 4wd.

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You probably have a trouble code stored in the engine computer for a faulty transmission selector switch. The engine computer thinks the trans is in 3rd gear and not park. Or it really could be stuck in third gear.

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2005 gmc sierra 4wd warning light on and will not shift into 4wd


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2002 envoy code co550


you need to replace the tccm and get it programmed.


2-04-21-006E: Inoperative 4WD/AWD Lamps, Inoperative 4WD/AWD System, DTC C0550 Set No Communication with TCCM (Reprogram Transfer Case Control Module) - (Jul 20, 2006)
999999995.gif Subject: Inoperative 4WD/AWD Lamps, Inoperative 4WD/AWD System, DTC C0550 Set, No Communication with TCCM (Reprogram Transfer Case Control Module) Models: 2004 Buick Rainier 2002-2004 Chevrolet TrailBlazer, TrailBlazer EXT 2003-2004 Chevrolet Avalanche, Silverado, Suburban, Tahoe 2002-2004 GMC Envoy, Envoy XL 2003-2004 GMC Sierra, Yukon, Yukon XL 2004 GMC Envoy XUV 2002-2004 Oldsmobile Bravada with Electronic Shift or Active Transfer Case (RPOs NP1, NP4 or NP8)

This bulletin is being revised to add Subject information and change the labor operation. This bulletin cancels and supersedes Bulletin Number 03-04-21-006. Please discard Corporate Bulletin Numbers 02-04-21-006D and 03-04-21-006 (Section 04 -- Driveline/Axle).
ConditionSome customers may comment that the 4WD/AWD indicator lights on the selector switch do not work, or the 4WD/AWD system may be inoperative and/or the service 4WD light is illuminated. The condition is typically intermittent and always occurs at key-up. Upon investigation, the technician may find DTC C0550 set.
CauseThe Transfer Case Control Module (TCCM) has not received the wake-up call during key-up and has remained in the "sleep mode".
When the TCCM is in the "sleep mode", it will not communicate with the class II bus or the Tech 2®.
Correction
  1. Try to communicate with the TCCM. If no communication, go to step 2. If communication is possible, go to step 5.
  2. Remove the underhood (ATCM or TREC) TCCM fuse.
  3. Wait 30 seconds.
  4. Reinstall the fuse. The module should "wake up" and communicate with the Tech 2®.
  5. Check for codes in the TCCM. If a C0550 is found current or in history with or without other codes, replace the module and go to step 6.
  6. Reprogram the TCCM with the latest TIS software using normal SPS procedures.
  7. Verify operation and that no codes are present.
Important: For the 2002 model year, the TCCM must be replaced and reprogrammed.
Warranty Information

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