If you see the compressor clutch cycling on and off rapidly, the system is low on refrigerant. That is real common on the Ford products that use quick-connect fittings on the hoses. They leak, and replacements will leak too.
The common complaint of switching to defrost has to do with acceleration more than engine speed. Under load, intake manifold vacuum goes down. Most heater systems are spring-loaded to default to defrost position if there is a problem with that vacuum. Your feet might freeze, but your windshield will be clear. Look under the hood for a small hard plastic vacuum line running to the firewall. There will be a check valve in that line. I am more familiar with those on Chrysler products. The valve is the size of a stack of three nickels. The dealer has a replacement that is about two inches long and 1 1/4" in diameter. It has a storage canister built in to prevent switching to defrost during those brief hard accelerations. The Ford dealer's parts department may have a similar solution.
Friday, June 23rd, 2017 AT 6:33 PM