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1970 GTX, A/C issue on center vents

Mark Pifer

FBBO Gold Member
FBBO Gold Member
Local time
4:54 PM
Dec 6, 2021
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Venice, Florida
Hi, I have a 1970 GTX with air conditioning.
I have an unusual problem.
Currently when I run the regular a/c, not the max a/c, the center dash vent blows cold air on the left hand side vent and hot air on the right side vent. This is unique as both vents are tied into one duct which feeds them both coming off the air handler box.
Now a little background.
We replaced the original dual heater core with a new Old Air Products dual core with valve. We did this because on the original core somebody had the valve shunted out and it could not be made to operate again. So we replace the old brass core with a new aluminum core and new valve.
When we select the Max A/C, the two center vents along with the two side vents blowing cold air.
When selecting regular A/C the left side center vent blows cold air and the right side blows hot air. The issue is both left and right vent are supplied by the same center duct.
Everything does works using the heater or defroster.
Has anyone run into this issue? What is hooked up wrong? What did I miss?
Thank you in advance for you help!
The only normal operational change between MAX AC and AC is the fresh air door position. In MAX the fresh air door is closed, only allowing cabin air in, in AC the fresh air door is opened to allow outside air into the evaporator. The position of the bypass valve or water valve does not change. Do you have the bypass valve correctly attached to the air conditioning door (large door that feeds the AC ducts). Not sure how the aftermarket cores are built, but the original two stage had an outer path and an inner path, the bypass valve closed off the inner path when the AC door was open. I wonder if the aftermarkets are plumbed right and left, which may partially explain the issue.
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