A/C changes vent location on its own

Kevbz · Sep 7, 2019

I got my a/c going finally the other day. New compressor and condenser and blowing cold (previous owner said it just needed a charge my a@@)

Going down the road, windows up, cold air blowing I feel it blowing hard out of the dash vents, then all of a sudden a loss of volume, still cold, just not as hard. Could go 30 seconds or maybe 2 minutes like that, then all of a sudden it back blowing hard out of the vents. First I thought compressor kicking off maybe, but it was cold air, just not blowing. Thought, blower motor then? But it still had air coming out and if wasn't a off/on thing, but a gradual slowing down and coming back, almost like a variable speed blower.

Fast forward to going to dinner the other night, my wife is in it for the first time since I got the a/c going. Blowing cold and hard out of the dash vents, then the loss of volume. I start to explain to her that I have no clue whats causing it, and she tells me it blowing hard on her feet, she had on flip flops and could feel it. When the dash volume would go down, the floor volume would go up, back an forth slowly. I couldn't feel it because I wear boots all the time and my toes haven't seen the light of day in 10 years, lol

So long question short I guess...is this a vacuum issue, or mechanical issue with the hvac box?

75gmck25 · Sep 7, 2019

Its an issue with the vacuum motors that are connected to the system to switch between air positions. There are two more vacuum motors for the max A/C position, but these are not the culprit. IIRC, when there is no vacuum on any of the motors it will default to mostly heat and a little defrost. This design may be a failsafe so that it still works relatively well when it loses vacuum.

Do you still have the vacuum canister under the hood connected to the system? That canister retains enough vacuum so that the vacuum motors still work normally during short periods of low vacuum operation (full throttle, etc.). There should be a line from the canister running over to and through the firewall, and then connecting to the controls. The canister should have another line connected to manifold vacuum.

The repair manual for my '75 has an A/C operation diagram that shows which vacuum motor should have vacuum when the controls are in various positions. You should have a similar one for your year. It is initially somewhat difficult to understand, so don't feel bad if it does not make sense to you, but it will eventually help you in troubleshooting.

Take a look at the '75 diagram I posted below. The Plenum and Kick Pad motors are not the problem, since they are only used in max A/C mode to recirculate cabin air.

Bruce

RecklessWOT · Sep 7, 2019

75gmck25 said:
IIRC, when there is no vacuum on any of the motors it will default to mostly heat and a little defrost. This design may be a failsafe so that it still works relatively well when it loses vacuum.

Do you still have the vacuum canister under the hood connected to the system? That canister retains enough vacuum so that the vacuum motors still work normally during short periods of low vacuum operation (full throttle, etc.). There should be a line from the canister running over to and through the firewall, and then connecting to the controls. The canister should have another line connected to manifold vacuum.

Do you mean the big charcoal can up front? I took mine off years ago when I cut the EGR, I always assumed it was only emissions related. My blower works exactly how you said, mostly heater with a little defrost no matter which position the switch is in, only thing it does is turn the AC on/off depending on which position it's in. Now that you mention it, it's been acting like that since about the same time I unhooked the emissions stuff...

animal · Sep 7, 2019

Nope. Looking for the softball sized canister located just to the right and a little below the brake master cylinder on the firewall.

Craig 85 · Sep 7, 2019

It also can be the selector switch too. I just had to replace that after determining the vacuum pods were all good and no other leaks in the system.

This isn't the best picture, but these are the 2 pods you want to look at. The one boxed in red is between the firewall and A/C box. There are 2 nipples on it. It pushes the red door back and forth. The one below the yellow box diverts air to the floor.

If those hold vacuum, then its the valve on the bottom left in photo #2. Of course it's out of production and I could not find one. I had to replace the complete unit with a NOS one.

The valve number is GM 16037769. The complete control unit pictured is GM 16034591.

You must be registered for see images attach

Kevbz · Sep 7, 2019

75gmck25 said:
Its an issue with the vacuum motors that are connected to the system to switch between air positions. There are two more vacuum motors for the max A/C position, but these are not the culprit. IIRC, when there is no vacuum on any of the motors it will default to mostly heat and a little defrost. This design may be a failsafe so that it still works relatively well when it loses vacuum.

Do you still have the vacuum canister under the hood connected to the system? That canister retains enough vacuum so that the vacuum motors still work normally during short periods of low vacuum operation (full throttle, etc.). There should be a line from the canister running over to and through the firewall, and then connecting to the controls. The canister should have another line connected to manifold vacuum.

The repair manual for my '75 has an A/C operation diagram that shows which vacuum motor should have vacuum when the controls are in various positions. You should have a similar one for your year. It is initially somewhat difficult to understand, so don't feel bad if it does not make sense to you, but it will eventually help you in troubleshooting.

Take a look at the '75 diagram I posted below. The Plenum and Kick Pad motors are not the problem, since they are only used in max A/C mode to recirculate cabin air.

Bruce

I will check, I believe it is there, but I dont know the condition of it

Kevbz · Sep 7, 2019

Craig 85 said:
It also can be the selector switch too. I just had to replace that after determining the vacuum pods were all good and no other leaks in the system.

This isn't the best picture, but these are the 2 pods you want to look at. The one boxed in red is between the firewall and A/C box. There are 2 nipples on it. It pushes the red door back and forth. The one below the yellow box diverts air to the floor.

If those hold vacuum, then its the valve on the bottom left in photo #2. Of course it's out of production and I could not find one. I had to replace the complete unit with a NOS one.

The valve number is GM 16037769. The complete control unit pictured is GM 16034591.

You must be registered for see images attach

You must be registered for see images attach

OK, I think I may not have the pod in the red box hooked up. I say that because When I bought the truck in February I was doing some cleaning and I found a orange vacuum line behind the glove box. I know I have the pod under the yellow box hooked up because I can see it. I didn't know where the other line went. I actually thought it might of went through the firewall or the kick panel because I could see another pod there hooked up. Knowing there is one back there it now makes sense because the length is just right to get there. That I will go check out!

Craig 85 · Sep 7, 2019

The lines with the orange stripe go to the pod in the right kick panel and the pod in the cowl behind the hood (line goes through the firewall above glove box).

You must be registered for see images attach

Joshua Keith · Nov 23, 2019

Be sure there reservoir canister is connected correctly and that the appropriate check valves are in place.

A/C changes vent location on its own

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