I'll go for the monovalve. Check the control! One of the two wires should be battery voltage and the other should be ground when in the "min" position. If you are missing either you will get full hot. If you have both and still have heat, the valve is defective. Usually an insert will fix it. The insert is easy to replace, the valve not so easy.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician