Testing AC/heat vacuum system
Thread Starter
Drifting
Joined: Apr 2004
Posts: 1,422
Likes: 127
From: Mequon WI
Testing AC/heat vacuum system
I'm testing the heater AS vacuum lines for leaks. No engine yet so my only source of vacuum is a small hand held vacuum pump. When I supply vacuum to the black line outside the firewall I can build vacuum and hold with maybe a slow leak as long as the selector switch is set to off. As soon as I move the switch to AC, Vent, Heat it loses vacuum immediately. My little hand held pump can not build any vacuum while the selector is anywhere but off. Oh, this is a new selector switch. I also installed a new water switch as well.
I removed the plug behind the selector switch. When I apply vacuum directly to each line the corresponding damper opens. The vacuum will hold almost indefinitely.
Could my test method just be bad that the little hand held pump does not have enough volume to activate the dampers through the selector switch? Any other ides I might try?
Oh, this is a 78.
Thanks
I removed the plug behind the selector switch. When I apply vacuum directly to each line the corresponding damper opens. The vacuum will hold almost indefinitely.
Could my test method just be bad that the little hand held pump does not have enough volume to activate the dampers through the selector switch? Any other ides I might try?
Oh, this is a 78.
Thanks
Melting Slicks
Joined: May 2015
Posts: 2,365
Likes: 428
From: Cape Girardeau Missouri
A little hand vacuum pump will even operate the headlamp doors if you do enough pumping.
You obviously have a catastrophic leak somewhere in the HVAC system. At the "off" setting no vacuum is applied anywhere in the system* as the vacuum input in the main switch is blocked at "off". If otherwise working properly you should get air from the floor vents with no vacuum to the selector switch.
*a tee does give a separate supply of vacuum to the hot water shut-off switch. If you hold vacuum with the main selector switch "OFF" and the temperature all the way to "COLD" at least the basic supplies to the system are OK.
You obviously have a catastrophic leak somewhere in the HVAC system. At the "off" setting no vacuum is applied anywhere in the system* as the vacuum input in the main switch is blocked at "off". If otherwise working properly you should get air from the floor vents with no vacuum to the selector switch.
*a tee does give a separate supply of vacuum to the hot water shut-off switch. If you hold vacuum with the main selector switch "OFF" and the temperature all the way to "COLD" at least the basic supplies to the system are OK.
