HVAC vacuum pressure measurements
 

I have searched the HVAC troubleshooting guides and the service manual for this answer, but can't seem to find it.

Essentially I have an issue with my 1977 280Z mode door or my selector valve. I replaced all the vacuum lines about 6 months ago and the system has worked fine, now I have the dreaded cold air from the vents problem. As part of my troubleshooting to determine the likley cause, I want to determine the vaccuum throughout the system. This will not only tell me if there is a leak, but will also help me figure out where the leak is and if a component has failed.

My question is, what vacuum should be read throughout the system? The service manual says the acuators work at 8 in HG, so I know I need at least that amount. Should I expect the vacuum to be the same as the engine vacuum or is there a loss as you transition to all the tanks, hoses, and connectors?

The vacuum in the HVAC system will probably never get to the same level as the engine vacuum. However, the engine vacuum will go well below -8mmHG. It just won't necessarily stay there long. Between the bottle and HVAC panel, there shouldn't be any appreciable loss.

What is the condition of the vacuum bottle? I believe there should be a check valve, too. Could any of the replaced lines have been nicked? Are you missing any firewall grommets? Did any of the vacuum lines get damaged by being near a hot part?

One way you could test components is to use a vacuum pump like a Mity Vac. See how well the components hold vacuum, moving toward the intake manifold. When you see the vacuum decaying rapidly, you found the problem.

Well...its fixed. 4 hrs of frustrating under dash work. I started by reading the vacuum in the engine compartment. It was ~12 In HG all around. Then moved into the cabin. Pulled the mode door actuator which wasnt working right (no vacuum). Check and no leaks between the selector vavle and the acuator. I then pulled the HVAC selector valve, cleaned and re-lubed it, fiddled with it for 2 hrs, but coudnt get it working right. D1 was always low. It didn't occur to me that D1 has a side tap for the actuator that controls the hot water to the heater core. Getting to that acutator is a major pain, but after tearing half the car apart, I found a cracked vacuum hose near that actuator.

Needless to say, now it works...kind of. I think I put the selector valve in a tooth or two off. Defrost works correctly, recirculate works 1/2 inch right of the detent, heat just blows cold air from the vents, and A/C works correctly.