PSU88

I'm having an issue with my 1978 A/C blower motor when I engage the A/C from the HVAC contral panel and thought perhaps someone else may have run into this.

When I had the engine out, I replaced the A/C blower motor, resistor and relay. I don't think I noticed it until recently, but when I move the heater/AC control from "vent" to any A/C mode, the blower motor shuts off. I did install dual SPAL electric fans and have a relay that engages the fans when the A/C is powered. Just to make sure, I eliminated this relay to see if that was causing the issue...the blower motor still shut off as soon as the A/C was engaged.

Sitting in the garage with the car not running and key in the "on" position, I can hear the blower motor run on all speeds as it should when in the "vent" position. As soon as I move the control to A/C, I hear a "click" that engages A/C (of course, the engine isn't running), and at the same time, the A/C blower motor shuts off. When I move the control back to "vent", the A/C blower motor comes back on.

Before I start tracing wires all weekend, does anyone have any tips or ideas of what may be causing this?

Thanks as always!