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2003 250 SD 4WD, manual A/C & heat controls.

A/C compressor works normally when A/C is selected.

Problem is, the truck thinks A/C is selected all the time if anything other than off is selected - it cycles on and off like the A/C is selected even when it is not.

Relay?

I'm no expert on a late model ford, but most compressors cycle on and off even if AC is not selected to reduce condensation or fogging (if you will) inside the cab. And they are also designed to periodically cycle even if the whole system is off, so that the seals and such get lubricated throughout the AC system.

Thanks, but I don't think that's it.

I can keep the cab cold inside on a hot day with the selector on a vent setting.

Then I think you need to check the vacuum lines and check valves.Or the control module on the dash is bad..