Problem:

When using the heat and having the air flow set to HEAT the air is still coming out of the defrost vent. This does not change regardless of having it set on "MIX" or "DEFROST" it still comes out of the Defrost vent. The only setting that works properly is the VENT setting. Any ideas??

88 3.0l

 

what year is your aerostar? sounds like your selector lever on the dash is not working.i think some are electric and cable wire(after 1991), and some are vacume and cable wire(1991 and older). mine is a 91, the switch had some hoses and cables and some electric connections i had a similar problem. it turned out to be a little vacume hose inside the dash board needed to be reattached to its fitting, it had come off. i had to trace it from the switch into the dash to a small silver canister it had been attached to. once i reconnected it, all was well.
first i would check the switch itself, see if you can remove it to check its connections, next check each wire if possible, if you have a newer aerostar and it is all electric you should be able to check the actual switch it self for faults. i just looked at all the wires and hoses and pushed everything back together. on my 91 the dash has a top panel that can be removed to gain access into it.i got lucky i think, found the hose sitting next to the thing it was supposed to connect to.
it could be the wirecable is off or something is in there jamming it. i'm always getting pens etc out of my vents(from the kids sticking things in there to see where they go.....hope that helps, i picked up a haynes manual at auto zone for reference. rick

 

Its an 1988. combo of vacume and electric. I had a feeling this was not going to be easy. I did look around to try and find a vacume leak. Guess I need to look harder. Thanks for the advise