Im stumped! HVAC switch not changing location of air output

[Chevy3+3]

This is on an 88 1 ton crew. I have all my fan speeds working correctly but my air is coming out at the floor. I had tried ever position on the switch and nothing happens. I have power to the switch through the brown/white wire. The green wire coming from the switch does become energized when the switch is in Max AC, Normal A/C, Bi-level, and Defrost.

I dont know what to look for next. This is my first problem like this with the square bodies. I guess i have been lucky.

Any thoughts? Thanks

[4x4k30]

I would check under the hood first to see if you have vacum going to the heater box there should be a vacum canister bolted to the fire wall that has engine vacum going to it rite above the fan motor then the vacum goes thru a small vacum hose into, the heater box under the dash

[Chevy3+3]

okay I will check that in a few minutes but have a question. If it is vacuum controlled then why is there an electrical switch involved? There is only 4 wires out of the switch (one being power to the switch) so how can 2-3 wires change 5-6 output settings?

[kalbert]

I agree, check to see that there is vacuum available to it. There should be a reservoir under the hood near the master cylinder, and a check valve. If any of it is leaking or plugged the blend doors won't move.

The electric switch controls the fan speed, and when the blend lever is moved to the AC positions it turns on the A/C clutch. The hot/cold is a cable driven door, the mixing between defog, vent, floor, etc is done with engine vacuum.

[Chevy3+3]

Okay thanks to both of you. I went and looked at the vacuum lines again and everything appeared to be correct until I noticed the vacuum line going to the intake manifold had fallen off the nipple. I connected it back and WALLA! Everything works now.

Thanks a bunch. I know it was an easy fix but its funny how I overlooked something so simple.

[kalbert]

Always good when it's a free repair!

[Chevy3+3]

Originally Posted by kalbert

Always good when it's a free repair!

You betcha!

Do you feel like helping out on another problem. I found another vaccum line unhooked. There is 2 premolded vacuum line out the front of the TBI unit. The tube on the left side of throttle body is not hooked to anything and appears to be broke in half but the problem is I cant find the other half, lol.

Its been a few years but I think that tube may connect to the MAP sensor? Does that sound correct?

[kalbert]

Generally the vacuum to the MAP sensor is a short line that comes from the rear port on the TBI, and without it the engine won't run or won't run well/long enough to drive. I believe the one you're seeing on the front would connect to the vent canister mounted on the drivers side core support? I think this is the correct diagram for your system, if you don't have an air pump just ignore that portion of it.

http://repairguide.autozone.com/znet...52800a800a.gif