Where to start, A/C controls

[solidaxel]

I am having trouble with the A/C vents not going from bi-leval to vent only. The max normal vent and heater def work fine, but it will not shift from bi to max vent!
Is the vaccum motor not working or is the slide switch not working !!
Help need more AIR.......

[solidaxel]

I guess I should have mentioned this is on my 78 Silverado that I can not get the vents to work on Max air

[78SilveradoSWB]

With the engine off and the power on, I would think you should hear the vacumn actuator open and close. The cable might be kinked or the vent door mechanism stuck?

[apstguy]

The vaccumm motor might be bad. Do you mean no air comes out at MAX air or just the door will not open? My door makes strange noises when it opens because of the rusty spring in it. BOING!

[solidaxel]

The Max air works as I can here the change in volumn of air, but there is about 40% of the air going to the floor vent and not the dash vent, which used to have 100% of the air in Max setting. Heater is the same way so it must be a door hung up or there is not enough vaccum to open-close the door, I think !!!

[apstguy]

Oh ok. That means either dash vent/floor vent mix door mount is broken like mine was. It is like the only thing that is plastic in that system! Go figure! All you have to do is remove the section on the a/c box in the cab where it sticks out to go to the upper vents. It only has three screws. Once you open that up, you will immedeatly see your problem. It sounds funny as I typed it so if you need help PM me and I'll walk you through.

[solidaxel]

O K, so when I find this plastic piece it is it a delear item or do I have to manufacture something to work!!
Thanks, I see a Sat. job coming up!!

[87 STEPPER]

Ok guy's first thing there's no vaccum motor in the A/C system the vaccum come's from the engine manifold and as long as the engine isn't blown up then the vaccum sorce is not the problem.

And it wont be a cable problem as there is no cable in the vaccum part of this system.

The cable operate's the mixer valve and is attached to the slide lever for when you want to go from cold to warm to hot air (ie mix valve). You should be able to hear this door operate just by moving the slide and you should be able to feel a slight resistance.

Now you stated the problem exist when you select form bi-level to vent this is the mode selector and it is a vaccum rotory switch.

This could be the problem i had one go bad a long time ago and it would work fine in one mode and not in another.

It is a two piece sw. and it's held togeather by a small rivet type snap fastner and that rivet will break and allow the rotor to be cocked at an angel and the vaccum port's will not be sealed thus not allowing that mode circuit to be completed.

Remove the A/C system mode selection controler from the dash and look at where all the differant colored little rubber vaccum hose's attach and see if that rotory switch look's like it's coming apart move the selector thru the full selection range while looking at the rotory sw. to see if it seperates in any of the positions if it does then replace it.

If it looks good then one of the vaccum modulator's that operate the diverter valve's have gone bad.

Or one of the rubber vaccum hoses may have come off some where or it's been cut or damaged in some way.

[jimfulco]

I had a similar problem on a 77 Nova. The A/C compressor ate itself right after it started so I didn't care. Soon, the power brakes quit working except after a long stretch of non-use, and then they only worked once. Had 19" vacuum (6-cyl. engine) so I replaced the booster but no help. Finally figured out there was 19" of vacuum but only flowing about one cubic inch per hour. The vacuum passage in the intake "manifold" was coked up almost solid. I wallered it out with a screwdriver & a drill bit, and not only did the brakes work, but the A/C controls went back to normal.

[apstguy]

Thank you Bruce! You made it much simpler than what I said. When I said vaccum motor I meant the diaphram thingymambober that opens/closes the vent/floor mix door. I just can't remember the proper name for the thing. long week... Mine snapped at the tab where it was screwed into the door making air flow to the floor at all times and never shutting the dash vents off completely either. That rivit "barb" like thing was very hard to find for my dealer as they only sold them to dealerships in 100 count bags. (My dealer didn't want 99 extras just because of one customer which I can understand). I had to get the barb thing because I cut it off with a dremel as mine didn't want to come out easily no mater how much I banged on it or cussed at it.