Heater bogs down motor when turned on, it is a.

Sound like your a/c compressor is putting too much load on the motor. Not sure on your ranger, but when the defrost is turn on, it will kick the a/c on also. The sure way to tell is to see if the motor bogs down when the a/c is on and try to change the heater to the floor mode without the a/c on and see if there is any change on the rpm.

Tryed heat on floor and defrost does same thang.