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1979 Mark V ATC Question

First off, love your site! Saved so much time and money already and I've only had my car 2 months. One week after buying my Bill Blass Mark V someone hit the front end and Lincoln Land has been so helpful getting body parts. Though used, these parts look brand new!

Anyway, my problem is this: my heater, or ATC, only blows on defrost. The motor speed control works fine, and it heats up nicely, but I can't get it off the defrost setting, no matter where I set the control to. Where do I start looking for a situation like this?




Hi Brad -

We are pleased that you are enjoying the Blog and your quality parts from Lincoln Land. We certainly appreciate your continued business.

Your ATC issue may be easy to diagnose and repair thanks to your information regarding the front end accident. That ATC system defaults to DEFROST and FULL HEAT with the loss of vacuum to the system. The vacuum could easily have been disconnected somewhere under the hood during the front end repair. The vacuum reservoir is located under the hood with the vacuum check valve. You may want to speak to your body shop and they may be able to easily locate and repair the disconnected parts etc. for you. If nothing is found, the vacuum circuit will need to be traced from the engine vacuum supply on the intake manifold to the system controls as per the shop manual. Let us know what you find.