hey guys have a question, I have not seen here yet. My 99 ZR2 with Manuel controls, a/c is not switching positions. I understand the vacuum line checking but even with it in the off position it still will blow air cool or hot depending on where the temp control is set. Maybe in my head but it seems the air gets cooler coming out of the vent when I go up a long hill. The vacuum would be down then under a pull. Does this sound like a door not switching? I dropped the glove box door and there are two actuators there and they seem to work, at least the arms move. I can also see the fan turn and when I turn it off the door will close. If the door closes where is the air from the vent coming from when the truck is moving Are there more actuators under there?

 

There are 4 vacuum actuators in the heater case. Two of them have two vacuum lines, and two of them have one vacuum line each. The actuators operate doors in the heater case to control where the air flow is directed. The temperature control is done with a stepper motor, (electric). The stepper motor will adjust temperature even if the mode switch is in the OFF position. In the OFF position, air flow enters the cowl area and is directed through the dash registers. If the mode switch loses vacuum, the vacuum actuators will default to defrost.

 

Thanks Captain Hook. From what you're saying then my mode selector has vacuum because it comes out the vent, some at the the defrost and a little on the floor. Mainly the dash vents. I see the actuators behind the glove box and both of them seem to be doing their job. One closes the fan door and one with a white arm.Where are the other two and do all four of them get their vacuum from the same line or is there more than one line supplying them? Temp. control seems to work fine also because it gets hot inside and I can turn it to cool it down. Not really a big deal but I don't think I would have enough heat on the windshield in a heavy snow or ice to keep it clear and I would like for it to work the way it was intended to do so. Thanks again.

 

The other 2 actuators are on the driver side of the heater case, above and to the right of the gas pedal. They are "mostly visible" without any disassembly. The door under the glove box is the recirculation door. In the MAX AC position only, the actuator, (#13) has vacuum and the door closes off the outside air intake. The HVAC system then recirculates the cabin air. All 4 actuators get vacuum from the mode switch. In the OFF position, air flow should only be from the dash vents. If it's coming out the floor and or defrost, there is a problem with either the heat/defrost actuator, (#12) or the mode valve actuator, (#15) or the linkage to the door, or the seal on the door. This chart shows the vacuum lines, color of the lines, and the actuators. It also tells which actuator(s) should have vacuum in each mode switch position:



Unfortunately, if there's a problem with a door, or a door seal, and some other repairs, the entire dash needs to be removed before you can remove the heater case & split it apart to repair the problem... If you've not done that job before, plan on at least a day labor. If you end up removing the dash, do yourself a huge favor and replace the heater core while you're in there

 

Captain thanks for the charts, I will mark them for future reference. So with the mode selector in the off position there should be air coming through the dash vents, correct? Mine does that with a little coming out of the defrost and maybe the floor. However whan it is in the on position ,vent floor or defrost nothing changes but the air flow is greater because of the fan. Very little defrost or floor, mainly vent. I think the fan door also closes in the off position, actuator 13. Seems both of those behind the glove box work when putting the mode on vent because I see them move. So I must have vacuum under there so I can rule that out right? I will check the drivers side tomorrow and see if they are working. Can I tell by visibly watching them when I move the selector or mode? Once again, thanks.

 

The inlet door, (actuator#13) should drop down, (so you can't see the blower wheel) in all positions except MAX AC. In MAX AC it should lift up and expose the blower wheel.

Sounds like you have a problem with one of the actuators or doors on the left side, by the gas pedal. Using the chart: Check vacuum in all mode positions to each color vacuum line on both actuators. Make sure they have vacuum, and vent, when they're supposed to. Watch the actuators when you move the mode switch. Make sure they move when they're supposed to.

 

I am going to try to get to it this weekend. I will post what I find then. I have only had my ZR2 for a little over a month and have ran into various problems but none that I have not repaired up to this point. Sure have learned a lot though. Put it on the road last week and ran into a sticking rear caliper and replaced that and my abs light went out now I am getting a SES light I need to check out. This Blazer has set for awhile with a bad transmission I replaced so these problems will happen but it sure is running well now. I sure appreciate all your help.

 

Captain, did as you suggested, thanks for info, and it appears that the drivers side actuators are working but don' t seem to have a lot of travel. I am assuming that the door is probably not working. Not going to take it aprt at this time because it does have good heat and even though I am getting a little heat on the floor is there anything I can do to kinda of divide the heat evenly between the floor, defrost and the vent until I can fix it right? Right now the majority comes through the vents and some more on the floor would be nice when it's 15 degrees out like we have had here in Va. the last week or so. I can live with what I have because it will get hot inside. Once again thanks for your expertise.

 

The two actuators on the driver side have 2 vacuum lines each. When vacuum is applied to one side of the actuator, it only moves half of the full travel, (not very far). You can disconnect the linkage from the actuator with the yellow and brown vacuum lines, and try manually moving the door until the air flow comes out the floor. Do the same thing with the actuator that has the red and blue vacuum lines. If you can't get air flow to the floor by doing that, the case will need to come out, and split, to find the problem with the doors.

 

97' Blazer with 3 button 4x4 and manual HVAC control.
This thread is the best I can find in the forums that comes close to what I need.
I know it's old, but so am I.

My mode control is stuck on whatever default it goes to, which seems to be mostly defrost. I can see, but I can't feel my feet.
This model has black, blue, brown, grey and orange vacuum lines that go into the mode control.

The black line goes to the engine compartment, and it would not hold a vacuum. I took the mode control switch off so I could get directly to the manifold that all of the lines feed into plugged the black line under the hood, and blew smoke into the black line and it came out of the brown one. Using a vacuum pump on that connector, I get good vacuum on grey and orange, and on black and brown if I cover one port. Blue seems leaky, but I'm not 100% on the seal I'm getting on the test.
I've looked hard for a diagram, and the best one I found was here...but it's for a 99' and it's different.
I think I should be able to pull a vacuum on all the lines, including the blue one.
Can anyone tell me what that one serves?

Thanks!