Hey guys, I have a situation where heater works on start up but if I put AC on then switch back to heat it stays cold. If I turn of engine and restart the heat works again. Any ideas ? Have a new climate control and supplemental water pump and have done a rebuild of the mono valve. I recently replaced the blower motor and black fan speed box behind the glove box. Any ideas of the next step?

MTC cartridge in the monovalve by any chance? They're known for doing exactly what you describe. The solution is usually an OEM cartridge which is a little bit more expensive, but seems to work properly.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Thanks Diseasel, I did a rebuild on the Mono valve . It looks good but maybe I need to replace the coil so I guess that means a new unit.

The coils rarely fail. Usually the part that gives problems is the cartridge (the internal part you replaced). Did you use an OEM Mercedes cartridge or did you use MTC? If the latter, buy an OEM one and be done with it!

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)