i have a question on my 77 a/c heat. i belive i have a vacume leak, when on max a/c, air will come out of my vent but when u switch the control to normal a/c or vent, the air will slowly move to the lower heat vent. if u keep the r's up while driveing the air flow will keep coming out of the vent but if u switch it to heat ( it changes to the heat vent) and then when u cahnge it back to vent the air flow wont change back to the vent. it never goes to defrost. i think its a vacume leak. now here's the kicker. while the heat was on and the tempt. lever was on the hot side the car has warm air but all of sudden it changed to cool air. i ran the r's up some while driveing the air warmed back up but cooled back down when i slowed down. but most of the time it stay cool. the cable works, u can hear the slide move inside the box. so what can be the problem. my car tempt. guage is around 180 or so.

 

On the A/C selector there are small vac tubes that control the damper/blend doors and hot water flow through the heater core. The vac hose on the back of the intake is source of vac. The vac switch on the control is held together by a spring washer and the hoses are pressed on and secured with push on retainers on two post. You can use Vaseline to lube these areas for a tighter seal. Ck. Hoses for cracks. Sounds like loss of seal at control or hoses. Willcox has vac tube diagraham and I believe tech info on his website. Mds...

 

Switch should be first issue, see 2.
I wrote this up awhile back for this problem.

The problem is that heater is the default position (no vac. or vac. leak) due to being designed in the north. Vent should be default. Very safe bet the problem is 1 OR 2:
1) The vac. line from engine may be damaged from heat - a small plastic tube that goes in below the hood alarm switch into wiper valley. Replacing most of engine side w/ hose will fix. OR
2) The vac. switch itself is not switching due to vac. leak as the usually plastic rivet is not holding the 2 main switch parts together well. Besides replacing, it is also possible to tighten this &/or lube w/ a thin layer of white lithium grease to fix. Disassemble, clean & grease.
3) If you have a heater, the small vac. tube to heater hose valve underhood may be damaged from heat & need replacing. Block w/o heater.

PS The design problem comment is not intended to be offensive to any of our northern friends.



You may be able to hear a leaking switch. A test can be done by removing a console side on your year model & pressing the switch together (engine running).


Switch fix may solve Temp. change- check rad. level, vac. tube line to heater valve in heater hose. Further check that the temp. lever is moving the "door".

 

great info, i was going to try changeing the thermostat but since i have the consol out again i'll look at it the vacume lines there frist and then then the engine