Transfer Case Switch Observations

Around the same time I was testing my knowledge of the infamous transfer case switch on S-series vehicles, using my 1996 Sonoma as a pawn, I knew something was up with my 2004 Blazer's HVAC system. Throughout the winter of 2010-11, I began noticing that air flowed out the defroster vent when the controls were set to floor vents. I was still getting heat to my feet, but also warm, dry air into my eyeballs. I had a pretty good idea what the problem was.

I began tearing into the vacuum lines under the hood. First thing I checked was the front hub locker vacuum actuator. Like the Sonoma, it was located under the battery tray. The actuator tested fine for vacuum and had no oil inside. Next, I checked the vacuum supply cannister to see if any transmission fluid had made its way there. Unlike the Sonoma, with its round ball hanging from the hood, the Blazer's vacuum cannister was almost entirely hidden between the inner and outer body walls near the anti-lock brake booster/motor. The only evidence of its existence was a vacuum hose connecting to what appeared to be a piece of black plastic stuck to the inner wall. It's not easy to remove. The metal brackets securing the anti-lock brake apparatus had to be removed, the fuse box had to be temporarily set aside, and the cannister itself then had to be pulled out of its mount holes. The plastic rivets GM used to secure the cannister were apparently designed to be "one and done." Once they go in,the rivets are nearly impossible to remove without breaking them off (which I did). That's ok though...a little duct tape did the trick when it was time to reinstall. Turns out the cannister had no fluid inside.

Apparently I caught the Blazer's transfer case vacuum switch failure much sooner than the Sonoma. While there was evidence of fluid in the vacuum lines, there was much, much less of it.

Above: Accessing the recirculation actuator and the heater slave valve actuator is as simple as removing the glove box. The connector block for the colored vacuum hoses is also back here. Both of these actuators tested fine for vacuum. When I blew out the lines with compressed air, only a small amount of fluid came out. The heater slave valve actuator is an extra 4th actuator that isn't present in my Sonoma. This was added sometime in the late 1990s.

Helpful hint: You do not have to remove the glove box to access this area. See that hook in the upper left? Just lift up on it and the glove box door will swing all the way down and out of the way.

Above:

Here is the mode door actuator and the defroster vent actuator. These are accessible from under the dash hear the accelerator pedal. Unlike the Sonoma, it's difficult to see the action of the mode door itself. But not having to drop the dash was one of the better design changes on these later-2nd generation S-series vehicles. The defroster vent actuator has red and blue vacuum lines attached. The mode door actuator has a yellow and brown lines attached.

These actuators are not the easiest to remove, because of the plastic clips that hold them in place. They were clearly designed for ease of installation, not removal.

The vacuum hose connection behind the AC/heat controller was not making a good connection, resulting in a hiss that wouldn't go away. I replaced the controller with a new one and I regained control of the vents. With the transfer case vacuum switch replaced, I averted some potentially more significant problems down the road.