Mono valve problem - no heat. '87 420SEL

[jriewer]

I have no heat because there is a constant current going to my mono valve keeping it closed. All the posting I’ve read said that as a fail safe, the CCU should fail in a mode that supplies heat if the defrost button is pushed before engine start up. This is not the case for me. No matter what buttons are pushed on the CCU, I continue to get a current to the mono valve keeping it closed. I have been driving around the last few weeks with the electrical connector disconnected on the mono valve so I can get heat. All the other functions of my CCU work. The temp dial operates to turn on the A/C as its supposed to. The blowers and vents are all working properly. I pulled the CCU and re-soldered the potentiometer connections, and a few other connections that looked suspect. There were no burn marks on the PCBs as other posts said was indicative of my aux water pump burning out. The aux water pump is working. Is there a temp sensor I should check that monitors the coolant temp and then tells the CCU that the coolant is hot enough and its time to open up the mono valve?