A/C Climate Control on 1990 Mercedes-Benz 300SEL

I have a 1988 Mercedes Benz 300 SEL the A/C Blows Cold but you can not adjust the temperature in the car if you adjust the temperature the A/C blows hot air and you can not get cold air again until you turn the car off for awhile No one can figure out what the problem is!? The same thing happens if you turn the fan on low also

Asked by for the 1990 Mercedes-Benz 300SEL
AC control Module
how long will it take too put n a ac cotrol assembly n a 1988 300sel and about how much.