HVAC Question about Vacuum Actuator for Vent(s)...

[Teach]

My HVAC system is blowing mainly to the defog vents at the windshield, regardless of where I tell the system to blow using the selector. I can reach up from underneath and manually move (hold closed) the arm on the vacuum actuator for the main vents and it will blow out of them. There is no vacuum line going to the nipple on the actuator. I thought it might have come unhooked, but can't find one anywhere. All of the underhood HVAC vacuum hoses/lines are intact.

Where does the line to the main vent actuator connect?

 

[tbirdman]

The default setting for the HVAC system is defrost through the upper dash vents if there is a loss of vacuum somewhere in the system. On my 85 I had a similar situation and discovered there was a break in the vacuum line going out to the AC reservoir in behind the passenger side fender apron. I cut a small section of rubber hose, cleaned up the vacuum line, joined the sections back together and that solved the problem.

What year Mustang do you have? Did the system ever work at any time before it started blowing air only through the defroster vents?

Vacuum leaks are generally going to be in the engine compartment somewhere, as these are 40 year old cars and the engine bay is exposed to heat/cold cycles and the elements. I know you said you checked the HVAC underhood lines/hoses. I would get an EVTM (Electrical, Vacuum and Troubleshooting Manual) for your year car off Ebay and see if you can track down all the vacuum lines with regards to the HVAC system in the engine compartment. The EVTM will give you information on what vacuum lines you should have coming from the HVAC control in the car out to the engine compartment.

 

[Teach]

Thanks, tbirdman. I should have replied to my own post that I got things figured out. Several of the vacuum hoses going to the control unit in the dash were disconnected. There were other similar issues. I suspect that maybe the heater core was replaced at one time and quite a few of the vacuum and electrical connections never made it back together. Getting those vacuum lines connected, replacing the resistor for the HVAC blower fan, and replacing (and adjusting the pressure for the converted 134a system) AC low pressure switch have things working great. Thanks again for taking the time to reply.

 

[tbirdman]

Glad to hear you got it worked out. I was kind of late replying to the original post anyhow. But at least this might give someone else with a similar issue an idea where to start.