1968 LeMans A/C and Heater Control (Vac Canister?)

C

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The vacuum-operated heat and A/C control in my ‘68 doesn’t work. The vacuum source is being taken from the base of the carb and feeds directly to the control without benefit of a vacuum canister. This doesn’t seem right. I’ve looked high and low in the service manual but haven’t been able to find reference to verify the correct vacuum line source, or confirm there should be a canister. I’m sure the source should be ok, but what about a canister? I checked for vacuum leaks downstream of the carb with a gauge and the system seems to be pretty solid (very small drop, a minute or so to lose all vacuum). So my follow-up question is this: at idle should the control work without a canister, or do I have a deeper problem? There are no vacuum sounds when the control is moved between settings. Thanks!
 
Did you check for vacuum at the hose connected to the carb at the control head?
Did you look for a canister on the drivers side inner wheel well near the firewall partially covered by the fender?
 
No to the 1st question. I “assumed” there is vacuum at the control since the expected back check is inline before the firewall grommet and its the only vac line penetrating the firewall. I will confirm tomorrow. Yes to the second question. I did a forum search before posting my questions and saw a previous post indicating it might be there but it isn’t.
 
The oldest factory manual I have is the 70. In the AC section they show the vacuum line on the engine side of the firewall going to the left of the brake master, when sitting in the seat, and down. Did you look at the diagrams in the 68 factory manual AC section?
www.thepartsplaceinc.com

The Parts Place

1968 Pontiac GTO/LeMans/Tempest PLASTIC VACUUM CANISTER - MOUNTS ON THE FIREWALL OF A/C CARS AND STORES VACUUM TO OPERATE AIR CONDITIONING COMPONENTS. THIS CANISTER HAS 2 VENTS
www.thepartsplaceinc.com www.thepartsplaceinc.com
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I’ve gone over the AC section thoroughly several times. The only vac line diagram is under the dash from the inside firewall, to the control, then out to the diaphragms. Strange. I looked at the same plastic can as you during my research. Should have time tomorrow to dig further.
 
Got it working as designed. It seems the back check was stuck in the closed position. I blew/sucked the inlet and outlet, reinstalled it and the control started operating. Thanks (again) for the advice and help! New horns from Ames are scheduled to be delivered tomorrow and that SHOULD take care of the last of my current issues... for now.
 
BTW, the heat/AC system will operate on engine idle vacuum. I ordered a canister for over the road operation.
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Glad to hear you found the cause.
 
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