4x4 GMC Sierra pickup truck 4wd problem


Old 09-12-11, 09:10 AM
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Join Date: Nov 2005
Location: Canada
Posts: 77
4x4 GMC Sierra pickup truck 4wd problem

I have a 1998 4x4 GMC Sierra Z71 off road SLE three doors pickup truck with about 268000 Km on it.
I have trouble getting into 4 WD.
When I press 4 Hi it does not engage the four wheels.
The last time that I tried it a few days ago, it would not work. But the four wheels engaged when I pressed 4 Lo.
When I disengaged ( I think ) the 4Lo stays on even though it seems to be driving only with the rear wheels. ( When it was driving in 4Lo, the 4Lo light did not stay on ).
What could be the most likely reason?
Thank you.
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Old 09-12-11, 09:22 AM
lawrosa's Avatar
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Join Date: Dec 2010
Location: USA
Posts: 17,607
I believe you need to start looking at vacuum leaks. Not sure if its like my S10 but I think they are all similar.

There are transfer case vacuum switches. The button type that you have incorporate a motor at the front differential that pulls a fork in to lock it into 4 wheel.

I think in part there is a diagnostic on how your lights are blinking or not blinking on the 4x4 buttons.

You have several devices in this link. You need to floow all vacuum lines if they go to any of them and repair. Then trouble shoot each device.

GMC Sierra Pickup Truck Transfer Case Motor Actuator Switch At Monster Auto Parts

Mike NJ

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