auto temp control

I have 2 olds,a 95 with manual temp control and a 1998 regency with auto temp control.when I put the defrost on in the 95 the air conditioner compressor turns on and off when engaged,but when using the defrost on the regency the compressor doesn't, even when manualy engaging the AC button.The outside temperature is 34-36'F,any ideas? thanks.

The A/C compressor should not engage below 42 degress. This is done to avoid slugging it with liquid referigent