Center dash vents blow both Heat and A/C

Mark Pifer

Jun 25, 2018
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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!