Constant hot air

[ol'blu72]

I have a problem with my 72 blazer. I get constant hot air out of the vents. I have tried different settings and nothing stops it. BTW its an AC truck.

[70k52wd]

Not real sure, but it sounds like a vacuum operated door is not shutting all the way allowing heat to enter vents. Could be debris, weak vacuum(cracked,broken hose) or a seal. Good luck.

[ol'blu72]

I did find a vac tube just hanging under the dash but I have no clue where it is supposed to go. I pluged it thinking I was loosing vac from it but it didnt help. I know notning about the heat system on these. Does anyone have a diagram of how it is supposed to work?

[70k52wd]

I don't have a diagram, but if you search for heater box repair you may find a diagram. I think LMC trucks may have the repair parts and a diagram that might get you started. Here is a link to LMC heater parts.

http://www.lmctruck.com/icatalog/cb/full.aspx?Page=129

[71swb4x4]

I am pleasantly surprised; I thought you were starting a thread about my wife.

[mikajo39]

Isn't there a hot water valve in the engine compartment thats controlled by vacuum to shut off the hot water to the heatercore when the A/C is on? Mine was stuck open and leaking so I just put an On/off valve in where the vacuum valve used to be. Turn it on during the winter turn it off during the summer, problem solved!

[tim21391]

i put in a cheap aftermarket shut off in from autozone.. al lyou do is cut your heater house and its just a straight threw with a shut off valve on top u just tighten to shut it off

[ol'blu72]

The shut off valve sounds like a good fix. I do want to find out where that vaccum hose in the dash goes. I think if i hook it back up it may shutoff the valve. Is there a source for the valve if it is stuck and needs to be replaced?

[prostreetC-10]

Oh......when I saw the title of this thread I thought it was about in-laws!

[easyrider]

It does sound like your vacuum canisters aren't closing the heat valves. On my 67 with the all season heat/air the vacuum lines go from round vacuum canister beside your under hood a/c unit to the vacuum switch on the back of your a/c ,heater control (in cab), then there are two lines coming out of that switch one goes to the vacuum door behind your glove box and the other goes back out under the hood to the vacuum door on the top outside of your a/c unit. I'm no expert,so I hope I helped a little and didn't confuse you more. Good Luck!

[haysonj]

Originally Posted by ol'blu72

The shut off valve sounds like a good fix. I do want to find out where that vaccum hose in the dash goes. I think if i hook it back up it may shutoff the valve. Is there a source for the valve if it is stuck and needs to be replaced?

On the passenger side vent there is a is a vac pot that brings in air when the AC is on and that is probably where that hose goes or it goes to one of the ports on the distribution valve on the controls.

[ol'blu72]

That is exactly what I was looking for thank you. Now where is the "control head vacuum switch" is that on the backside of the heater controls im assuming.

[haysonj]

Originally Posted by ol'blu72

That is exactly what I was looking for thank you. Now where is the "control head vacuum switch" is that on the backside of the heater controls im assuming.

Yes it is in the controls

[moon]

I think your hot water valve is stuck open.

When working correctly that valve closes when the vacuum is cut off and then the hot water supply to the heater core shuts down. If I’m correct, then the unplugged vacuum line would not be the reason for the heat.

With vacuum: The hot water valve opens, the outside air vent closes and the inside re-circulated air duct opens.

Without vacuum: The hot water valve closes, the outside air vent opens and the inside recalculated air duct closes.

Check the water heater hose coming out of the valve …. is it hot when the engine is running?

My truck had the opposite problem …. Previous owner broke off the top control level in the control head (top lever works the control head vacuum switch) with the “Heat” in the ON position (no vacuum) …. and his fix was to replaced the water valve with a simple in line hand turned valve.


Disclaimer - I just figured the above out, on;y by reading this forum ... I am a complete student here and as such am prone to being wrong.