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]

I’ve unplugged the Auxiliary Water Pump and the Monovalve and still get very little heat. I will be flushing the system first and pouring in new coolant (Zerex G-05). The car sat for quite some time so I may be fighting a partially plugged heater core. I’ve also chased down and resolved several electrical gremlins in the car in the two weeks that I’ve owned it. I will not be surprised at all to find that both the Auxiliary Water Pump and Monovalve are faulty.

Unplugging the Monovalve produces no change in the temperature of the air being discharged. Unplugging the Auxiliary Water Pump produces no change in the temperature of the air being discharged. Unplugging BOTH at the same time produces no change in the temperature of the air being discharged.