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Heating/vent Issue 2005 S-10 Blazer


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I was recently assigned a different work vehicle (2005 S-10 Blazer). It has about 148,500 miles on it and is in fairly decent shape. One very odd thing that's happening is the following:

 

When the heat-a/c selector is in the "heat" position (directing air on the floor at your feet) and the engine is placed under a load, the air will switch to blowing out the vents (in your face). The usual scenario is in the mountainous terrain where I live, as soon as I hit the gas more to ascend one of the ridges, it switches from blowing on my feet to blowing into my face. The second I top the ridge and let off the gas, you can obviously feel the air stop blowing in your face and revert back to your feet. This issue is annoying to say the least. I'm thinking it may be some sort of vacuum issue? Then again....who knows! Our one mechanic at work is a GM nut and would likely have the answer BUT he just took a job elsewhere. The other mechanic we have..............well.........no comment...

 

This is why I'm here asking the community!

 

Also, how do I get those stupid auto-headlights to NOT work????? :(

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  • 1 month later...

I had similar things happen in my '96 Sonoma 4X4 after my transfer case vacuum switch went bad and ATF got sucked into the vacuum lines. Eventually the vacuum reservoir filled with oil and I lost most of my "reserve" vacuum supply (a little dry rot in the vacuum lines didn't help, either). The way our vacuum system is designed, the engine provides the vacuum to control the HVAC system and 4x4 engagement. The vacuum is stored in a resevoir for when the engine is running at lower RPM's and isn't providing enough vacuum for all that is needed. In the later models, the resevoir is located around the front passenger side wheel well (older models had it attached to the hood). When the engine isn't running high enough to give the system all the vacuum it needs, the reserve supply of vacuum kicks in.

 

So it makes sense that the HVAC system does weird things when there's not enough vacuum. The vent positions in the cab are controlled by vacuum actuators that open and close various doors behind the dash. These doors direct the air where it's supposed to go. When you notice differences in where the air is coming out of the vents, this is because the vacuum actuators behind the dash aren't getting enough vacuum to stay in the right positions.

 

It's kinda odd to me that air would come out of the front vents when the engine is under load, when the control is set for floor heat. This is because to get air through the dash vents, 2 vacuum actuators have to be engaged: 1) the defroster vent door has to be pulled in, thereby blocking air to the defroster vents and letting it through the dash vents; and 2) the mode door near the accelerator pedal has to be pushed down to block air from coming out of the floor vents (this door can actually be pushed and pulled by the actuator). If you're losing vacuum, one (or both) of those actuators aren't going to get the vacuum they need, and will return to their "neutral" positions. When there is no vacuum applied to the defroster actuator, the spring-loaded defroster door will close and block air flow to the dash vents (comes out of the defroster instead). When no vacuum is applied to the mode door actuator, the door returns to a middle position where air can get to both the floor vents, and the defroster or dash vents.

 

It's confusing until you see how it all works behind the dash. Easiest way to tell if there's transmission fluid in the vacuum lines is to pull out the 4x4 vacuum actuator under the battery tray and see if it has any fluid in it (this is by far the simplest actuator to get your hands on). Or, pull out the vacuum reservoir and see if it's full of fluid. I did a little write-up on my website that has some pictures - check it out <here>.

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  • 1 year later...

On the drivers side near the fender, at the firewall, is a small check valve that has most likely failed, or one of the lines to it has cracked. Check that first, if it is a bad check valve should be an $8 part. It looks like a little black and white "T", and it will have three vacuum lines to it.

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  • 4 years later...

I have a 2000 Blazer and was having problems switching from heat to defrost also. My Blazer is 2WD and I believe the vacuum lines are slightly different but I solved my problem 100% by replacing a 2" long piece of vacuum line. The failed line was VERY hidden. To help you check yours I will explain the location. The ABS unit (Component with all the brake tubes coming out) is mounted on a bracket on the drivers side of the engine compartment. The bracket is screwed to the inner wheel well by 2 fasteners. Below the rear screw there is a nipple that comes through the sheet metal. That is where the hose was leaking. There is a bellows between the inner and outer wheel wells. That is what the nipple goes to. At least that is my belief . I have never actually seen it but that doesn't matter. We are not looking for how it works but rather how is breaks. Lol

 

 

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