As you'll notice,one wire from the Mono-valve goes to the CCU
and the other goes to the Auxiliary Water Pump.
(AND the Aux Pump feed back into the CCU.)
Aux Pump may be BAD
OR [And, I hope not]
The CCU is NOT grounding the Aux.Pump/Mono-Valve Circuit.
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD1/Program/ETM/ETM.pdf