Unplug the monovalve and go for a drive. If you have full heat all the time, it's a control issue. If you have intermittent or random heat, the monovalve cartridge is bad. The SDL will heat fine with the Aux pump dead, so a failed one will not cause a lack of heat.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

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

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]