I picked up an 82 Euro 500 SEL last year and have it road worthy, but am stumped with a climate control issue..

The AC works fine, but the heat will only work for about 5 min, then blows supply air temp. At first I thought that the mono valves (this one has two) were cutting out, but all it takes is for me to cycle the ignition and I have heat again for 5 min. I have done this while driving... turn the key off and back on and hte heat instantly comes out again for 5 min +-..

The climate control interface has three wheels, one temp wheel for each side and an A/C wheel in the center..

Any thoughts??