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GMC Forum: 88 gmc sierra 4 wheel drive problems?
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  #1  
Old 01-05-2010, 09:11 AM
Joseph Joseph is offline
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Default 88 gmc sierra 4 wheel drive problems?

I have a 1988 GMC Sierra 4x4 with a 350. The motor was just replaced about 12 thousand miles ago. i also just replaced the transmission about a month ago. It recently snowed where I live so I had to drive in 4 high. I was driving down the rode and all of a sudden my heat turned off and then the light went off on the 4 wheel drive, so I stopped on a side road and put it back into 2 wheel drive and then back into 4 wheel. Now the 4 wheel drive wont engage. I replaced the fuse for my heater and now it works but every time try to put it in 4 wheel drive the fuse pops. The 4 wheel drive is manual not by a button.
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Old 01-05-2010, 04:36 PM
Molson02536 Molson02536 is offline
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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.

If I were you, I would buy a new heated actuator. They are available at most auto parts stores, and are about $90. They are also on eBay for about $55 plus shipping. These are very easy to change - just thread out the old one, and thread in a new one.

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.
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Old 01-05-2010, 07:15 PM
Big_Ed Big_Ed is offline
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Say, by the way, do you know what the part number is for the upgraded actuator? If mine goes out (hopefully it won't though), I might as well upgrade to the more reliable one. Thanks.
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Old 01-05-2010, 08:46 PM
Molson02536 Molson02536 is offline
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88-UP C/K ACTUATOR UPGRADE

PURPOSE: IMPROVE AXLE CONNECT ENGAGEMENT TIME AND
COMPONENT RELIABILITY.

SYMPTOMS: NO FRONT DRIVE AXLE ENGAGEMENT WHEN SHIFTED
INTO 4X4.

Obtain the following GM part numbers:
(1) 26060073 Actuator
(1) 88959465 Harness/Spacer Kit
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Old 01-05-2010, 08:55 PM
Big_Ed Big_Ed is offline
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Thanks! I hope I won't need to get it, but if my actuator fails, I'll get the upgrade.
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