PolarGlade

Hey everybody,

Things are coming along really well with the '84/AT rescue. I have moved on to getting the HVAC functions working properly and need your help.

Here are the symptoms: (Hot and cold do work btw)

- Fan on high.
- Dash vents selected (white rectangle) blows great just from dash vents and doors.
- Down arrow selected, no change. Zero air from floor vents, only dash vents and doors. Can hear relays click when selected.
- Up and down arrow selected, no change only dash vents, no air from floor vents. Can hear relays click when selected.
- Up arrow selected, very little air to windshield vents mostly dash vents.
- Defrost selected, air goes hot like it's supposed to and stronger air flow to windshield but not great, still blowing strong from dash vents.

Where should I start?

The Forgotten On

Replacing the defrost and floor vent vacuum diaphragms would usually solve this.

But first, take a vacuum tester like a migh t vac and check the vacuum on all of the diaphragms first, as a failed one can affect the performance of the rest.

Mrmerlin

try reading this post,
it will give you all you need to fix your system,
and understand how it is supposed to operate.

https://www.google.com/url?sa=t&rct=...89947451,d.b2w

PolarGlade

Thanks for the quick answer on this. What's the location of these two diaphragms? Are they tough to get to?

PolarGlade

Great page this one! Reading now.

PolarGlade

Found this page too, in case anyone wanted more info. I will post my findings when I get this figured out.

http://dwaynesgarage.norcal928.org/1...0Procedure.htm

The Forgotten On

They can be a pain to get to if you replace them, you need to remove the center console to get to the floor vent valve, but you need to remove the dashboard to get to the defrost valve.

Testing them isn't too bad, you just need to get the bank of vacuum solenoids from under the HVAC box out ( 2 screws) and then plug the tester into each line and pull a vacuum on each valve.

If it holds a vacuum, the valve is good, if it doesn't it needs to be replaced.

PolarGlade

Well, I finally beat this thing!

Thanks for all the advice about the common vacuum issues. All the vacuum stuff was working perfectly. Had it not been from the Mityvac I would have given up.

Turns out there was no voltage getting to the floor vent vacuum control solenoid (the yellow one). I was able to test the voltage on the corresponding yellow wire and when the floor was selected, no voltage.

Once I had the HVAC head unit out and opened up on the bench, I found the little contact that drags across the traces on the board for the floor circuit wasn't even touching.

I bent it down after pulling the arm off, cleaned it all up and put it back together. Magic!

All I can say is if anyone reading this is having even the smallest issue with their HVAC flaps, stop fooling around and spend $35.00 and buy a Mityvac.

Thanks again everyone!.

Gman1003

I just realized I have the common HVAC diaphragm problem(s) too. I was getting the tapping/clicking noise from the center area of the dash. After reading what Mr. Merlin posted as well as the excellent guide from Dwayne's Garage, I'm pretty sure it's the diaphragm actuators for the comb vent and maybe others. I used the Mity-vac to test the vacuum and it is definitely not holding vacuum from the line leading from the brake booster to the dash. I plan to order the replacement items from one of the vendors on this site and then dig in to the dash. Without the help of all the posters on here, these repairs would take so much longer to diagnose and fix.