The heater valve is pulsed when controlling hot water flow, except in the max position. In "Min" position the valve should be full closed. There are two wires to the valve. One should be battery voltage when the key is on and the other should be ground in the "Min" position. Verify that. If that is the case and you have heat then the valve is bad.

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