Heater vaccum line schematic?

[Datsunboy]

Hey guys, my heater constantly blows out the defrost. I read in the archives that the default preset is defrost for the system when no vac. is present.

I applied vac. with my mity vac to the purple line (main vac line) and it holds pressure fine on defrost setting, but the other seem to bleed down. I think somethings wierd here. Can anyone tell me what the different colored tubes go to, and how to vac test everything to find my culprit?

Thanks - Evan

 

[Turbo Brian]

This is from a 1986 Cutlass GM shop manual. The G body cars should be the same:

The line from the vacuum tank sends vacuum to the purple line connecting to the Heater/A/C control assembly.

The dark blue line supplies the defrost vacuum motor. The Defrost vacuum motor gets vacuum when MAX, Normal, Bilevel, or Vent is selected.

The red line goes to the heater door. It gets vacuum when the switch is Off or in Heater.

The yellow line goes to the back side of the heater door. It gets vacuum in MAX, Normal, Vent, or Defrost.

The orange line goes to the outside air vacuum motor. It gets vacuum when Max is selected to bring in outside fresh air.

The gray line goes to the water valve (engine compartment). This valve has vacuum when Off or Max are selected. This closes the heater valve and blocks flow into the heater core.

Make sure all lines are connected from the vacuum tank into the purple line behing the glove box.

Another potential vacuum leak could be in a vacuum actuator. Remove each input line and apply vacuum to each actuator to see if it holds pressure.

If all actuators and lines are good, the problem is probably with a check valve in the vacuum tank or the vacuum control on the back of the heater/A/C switch assembly.