85 Heater issues

[71Chevy4-speed]

Hey does anyone have an exploded view of the HVAC system and/or a vacumm diagram of it? Truck won't select from vents, to floor, or defrost, took the glove box out and found two linkages, one is to hot/cold and the other just seems to select vents or floor, but doesn't seem to change to defrost either way I push it, took the dash apart and see that the select switch has a bunch of vacumm lines running to it, one line runs down to a vacumm servo it looks like and the others run behind the heater box, haven't taken the heater box out yet, little to much white stuff on the ground and no shop sucks. Thanks

[MylilBowTie]

I don't think there is a need to remove the heater box. Look under the hood around were the heater core is. There should be a plastic vacuum line that control the door flaps in the heater box. It probably got brittle and broke or came off its vacuum source.

My 86 doesn't have A/C so it all mechanical.

[87 STEPPER]

Check that switch with all the vac lines on it. they have a small plastic snapping lock button in the center that holds the 2 halves togeather and when it breaks the 2 halves will not seal and cause that problem it happened to me on my S10.

Some times if i moved the sw lever back n forth it would suck it back togeather and make a good enough seal and work.

Also you may be able to hear it leaking or sucking.

[grinch4om]

odd, default mode(no vacuum) usually goes to defrost . find the vacuum pod that controls it, unplug the line. if vacuum is present regardless of switch position, suspect the switch. or you could manually hold the door to the defrost. also remember the door can break...tis plastic

[1BadC10]

I recently had this issue in my '86. Vacuum source can be taken from the intake (for my newly "swapped" powertrain). Do as previously posted. There's a vacuum line that passes through the firewall via grommet near the heater core. If the line (very small) is old/brittle, be careful. There may no be enough of a seal joints where the line attaches to the rubber sleeve.

I had nothing but lower vent blowing heat at my feet. Fixed that connection, and I have full vent functionality. Now... I just have to fix my A/C.

[71Chevy4-speed]

Thanks guys, got it fixed, wish I would of asked here first haha, oh well under the dash got the dust cleaned off it, somewhere along the lines I apparently unhooked the vacuum line through the firewall must of been when I built my throttle bracket and disconnected all the A/C stuff, A/C is leavin when I get serious about the motor in it. Thanks Cole.

[worthirt_99]

I'm having the same problem, but don't have the part to connect it to my intake at the moment. Question is: Can I hook this vacuum line into the back of the carb?

[tylerg85]

Been having this exact same problem. Guess I know where to start looking after work