My wife's a/c quit working and the vents would not stop blowing heat. Also we were finding oil on the floor in the garage which was coming out of the a/c compressor. we took the car into the local "Indy" for repairs.
He replaced compressor, replaced reciever drier, replaced heater control switch. Heat was still blowing out of vents. He tested Temp Controller motor & it was fine. He checked for fault codes again & a/c system would not talk to his code reader. Freon is new. I'm 1700.00 dollars into this & its still not working right. He currently has a/c working (with vents forced open I think) to give us a/c. He says we will not have heat if we need it. Anyone have any thoughts as to another cause. I took the car back after (4) days at the shop. Guy says needs a few more hours to trouble shoot I thought that was excessive considering the time already spent on it. He's also stumped. Any thoughts would be appreciated. Thanks.

Does it blow hot air all the time or only when it's sitting still? The reason why I ask is my wife's ML had the same problem but it occurred mostly when she was stopped or when she was in stop-and-go-traffic. The problem was the fuse to the auxiliary fan. It was blown. From my service manual, the original 30 amp fuse and wiring are not big enough to handle the fan load in all circumstances. Although our car's wiring had been replaced during a recall the fuse had not been upgraded to a 40 amp fuse. Once I upgraded/replaced the fuse in her car and the auxiliary fan started working again the A/C was cold all the time. Beyond that I had an SEL 500 that had a stiky heater valve that allowed hot water to flow through the heater core even with the a/c on... although I have no idea if our ML's have the same setup. Good luck