Having problems with your 2001 GMC Sierra Crew Cab ?

Had 2001 GMC Sierra in 4WD Auto and went to put in 4 hi. then Service 4WD light came on and selector wouldn't let me select anything and the truck stayed in 2 hi.

Posted / edited by AnonymousUser on : 16-04-2018

Answers :

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\015\012volts out to the actuator. Wire #50 (brown) is the 12v feed, and it\015\012conducts 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\015\012front differential - it looks like a large bullet that threads into the\015\012front diff.) If it has 12 volts, the actuator is the problem. If it\015\012does not have 12 volts, check the transfer case switch and the power\015\012feed 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\015\012by voltage. When it heats, the pin in the end swells and pushes a shift\015\012fork in the front differential. The shift fork engages a spline on the\015\012passenger side of the differential, and boom! you have 4WD.

GM offered an upgrade to this actuator that is a motor instead of a\015\012heated device. However, it requires a small harness addition, a spacer,\015\012and a new actuator. It is about a $150 to $200 option.

Good\015\012luck. The actuator is usually the problem 90% of time but recheck all\015\012of the wire harness just to maker sure there is no short before\015\012replacing any parts.
Repair Help & Product Troubleshooting for 2001 GMC Sierra Crew Cab

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