Vent??

1975vette · 08-09-04, 10:31 AM

hi when i turn my dial to vent, on my 75, it makes a vaccuum noise and doesnt work but ac works fine ....any suggestions. thanks

**Bob Chadwick** · 08-09-04, 10:38 AM

Are you saying it gets cold but blows out the bottom rather than the center of the console.

A common problem. The vacuum comes from the engine to a control on the bottom of the contols in the dash. Moving the control changes which one of areas the air blows out of. The actuator is on the driver side of the air stack behind the dash. You can see it with the dash in. Pull the vacuum line off the actuator and see if the vacuum changes when you play with the controls. It probably won't. You see a lot of posts about vacuum leaking from the area where all the lines come together and sealing it with vaseline.

1975vette · 08-09-04, 10:52 AM

good suggestion but cold air blows from the center and the bottom....when you turn the dial to vent...nothing

**Bob Chadwick** · 08-09-04, 11:15 AM

I have a diagram that VMRod put together showing the vacuum connections.

Can you reach up there and give it a helping hand? I've never been very clear on how a single vacuum line can control three positions. On/off, up/down, that makes sense. Up, down and middlle, now that has never made any sense to me.

**Bill75** · 08-09-04, 11:24 AM

Interesting question........ I recently heard the same thing on my 75. I was plagued by hot air in the cab, one day I shut the car off and I sat in the car for a short time thinking about where the air was coming from. Suddenly I heard vent door actuate. (engine off) I looked at my heater controls and everything was off. After looking things over and removing the passenger side kick panel to expose the vent door I found that with the vent control in the "off" position on the console, the door would open as soon as the engine was started allowing air to enter, if I positioned the control to the "vent" position, the door closed. I had this car all apart last winter and I suspected that I may have screwed something up in the console by putting the heater controls back together, but I can't figure out how I could have done it.

Anyway, 75vette, if I've understood your delema correctly, you might want to see if you're having the same experience as I am. For now I just positioned the vent control so the door is closed until I have the time to look into it further.

Bobchad, can that round connector with all the vacuum lines on the heater control be put on incorrectly so it's calling for the wrong thing to be energized?

Bill

Maybe someone else will respond also

**Bob Chadwick** · 08-09-04, 11:50 AM

Originally Posted by Bill75

Bobchad, can that round connector with all the vacuum lines on the heater control be put on incorrectly so it's calling for the wrong thing to be energized?

I don't know. To be honest it was such a PITA to take it all apart that I just spent the $100 for a new control unit. It fixed mine mostly. If the car sits for a while, it stays down low until I put it in the a/c position, then it comes to the vent and everything works fine.

**wishuwerehere82** · 08-09-04, 11:56 AM

Originally Posted by 1975vette

hi when i turn my dial to vent, on my 75, it makes a vaccuum noise and doesnt work but ac works fine ....any suggestions. thanks

You have a bad switch contact on your Vent position on the selector. Also a common problem. There are little springs that make contact between the wiper plate and the output contact points on the switch, and those springs get annealed after a few years with the power running through them. You could try pulling the module apart, and you will probably see some scorched areas on the P.C. board and probably a loose contact that is burned off and flopping around. A new switch module is a sure fix. Not the blower speed switch, but the A/C, Vent, Heater switch.

Pete.

**Bob Chadwick** · 08-09-04, 12:01 PM

So the vent/defrost/heater function is controlled by vacuum and and an electrical contact?

**wishuwerehere82** · 08-09-04, 12:07 PM

Yep, there's a switch module in that selector assembly that make contact with to the blower motor, as well as the vacuum valves that control the actuator on the heater box.
It turns the fan speed to High when you select MAX A/C and makes contact all the rest of the time when in normal modes.

Pete.

**wishuwerehere82** · 08-09-04, 12:22 PM

Here's some pictures of what they look like when they get crispy critter'd. The wiper arm fits into a slot on the selector arm and follows the vacuum valve as the selector is moved.
This one is off my '87 lesabre, but they use the same GM switch since the 70's.

**Bill75** · 08-09-04, 12:52 PM

Ahhhhhhhh!! Great infomation, I never knew there were contacts in that vacuum housing. I thought it was a junction point for the vacuum lines and was all mechanical. Thanks for the pictures.

And I agree Bob, what a PINA!!!!!!!!

**joe1975** · 08-09-04, 01:55 PM

1975 vette is that a stock color like maybe flame orange could you give me the code and maybe a pic of your car in process of paint right now looking for just the right Orange, thanks