So I swapped my auto out for a T-5 in September, and ever since the swap, my heat has been stuck on defrost. The car is originally an AC car (AC compressor has been removed but controls are original) and I read on here my issue is that I lost the vacuum source to the HVAC controls, which is very possible I may have damaged a vacuum line during the swap, as I was laying on top of the motor trying to reach the upper bellhousing bolt.

Where does the HVAC vacuum supply go thru the firewall?

My only other thought was that some hardware may have gotten dropped into the defrost vent and it may have become lodged in the selector door. Is this even possible? Could a bolt that fell in the defrost vent lodge itself in the door, I'm not sure what kind of path the air takes in the dash.

Any input would be appreciated. I want all my vents to work again

 

on my 91 z28 it comes in with the wire cluster on the right hand side of the ac and heater fan in the engine bay, almost right at the top

 

Yep, the vacuum line was disconnected, all fixed now. I feel so silly.

 

I have the same problem but I can't figure out what happened. I had the old a/c heater box and everything worked fine. I put in an a/c delete box and now all I get is defrost. Where does the vacuum line tie into the heater box?

 

I must have bumped that hose on my '88 Formula, I am having the same problem, (airflow defaulted to defrost only) I can't find the line though. any chance of posting a pic?

 

2 hard lines come out of harness by firewall and heater motor. One goes to the Heater control valve, the other goes to vacuum source.

 

Hard line at firewall to Check Valve and Plenum Vac Source

Heater control Valve, 2nd Hard Line

Another View of Check Valve


Hope this helps


Bob

 

I dont remember having the vacuum line with the check valve. Mine was a carb 305 originally…could that be a difference? I literally hooked up the ac delete box the same exact way as the original ac box was. This is really driving me nuts!

 

The pre '90 ac delete boxes are not vac controlled, all done with cables. Did you replace the whole heater core/selector box or just the blower on the firewall?

 

I bought the ac delete box from Hawks. It had a blower motor in it, all I did was put my resistor in it and went.

 

You only changed the blower housing. You still have the vacuum controlled vent /heater core box under the dash. You have a vacuum line coming through the firewall that feeds vacuum to the control box under the dash. Maybe the PO cut it off.

 

Mine originally didn't have vacuum lines run to the ac heater box. I can change the air from hot to cold but when I try to change where I want the air it does nothing. I checked the cables under the dash and everything seems to be hooked up right. I'm running out of patience with this damn car!