Warn’s Fix for the Infamous “Thermal Actuator”
on General Motors’ Full-Size 4X4s

My friend, Ken Niemann had a nice Chevy 4x4 that he used every day. During the snow this winter, so he needed 4-wheel drive to get into his uphill driveway. After shifting the transfer case into 4-wheel drive, he had to wait for several minutes for the front axle to engage so he could finish his commute home.

In the morning, after he got back out on the dry pavement, he had to wait several minutes more for the unit to disengage after shifting the transfer case to 2 wheel drive again. I had similar problems with my ‘89 GMC 4x4.

We are not alone with this problem.

Why? Because General Motors uses a thermal actuator to engage the center-disconnect front axles of most of their 4x4s. (Keep reading for a list) It uses heat to turn a liquid to a gas. This expands and pushes a piston against an internal fork that engages the right axle with the center differential. Until it gets warm enough, you don’t have power to both front wheels. Then they stay engaged until it cools down and the gas become a liquid again.

The time delay can be rather inconvenient if you are towing your Jeep and have to pull up a loose driveway. If you need 4-wheel drive, you might be faced with your Jeep out in the street while the thermal device gets hot enough. The time gets longer, depending on how cold it is outside.

If this device needs replacement, figure on just under $100 for the part, then about an hour of shop time with diagnosis and labor to replace it. Based upon an earlier article in Four Wheeler, I understand over 190,000 of the thermal actuators were replaced in 1994-1996.

Enter Warn Industries of Oregon. They now make a vacuum activated replacement for the thermal activator. Ask for part #34760. We installed it on my ‘89 GMC 4x4 and the results are definitely worth the effort. It’s dependable, easy to install, and looks like a factory installation!

No additional cables or switches are necessary. However four-wheel drive now engages within about 2 seconds and disengages as soon as you shift it back into two-wheel drive and let off the throttle.