The monovalve is a good place to start. The valve gets a constant 12V and the control unit switches ground to close the valve. So if the valve does not have power and ground, it will always be open which allows hot water to flow to the heater core.

So the test is to remove the connection at the valve and remove the connector's plastic shell. With the engine running and the system operating, test the red/black wire for a constant 12V. If you have that, connect only the black/red wire and manually ground the other pin on the valve. You should hear a soft clunk. This means that the valve is operating, and the problem is with the wiring (unusual) or somewhere else.

Be very careful not to short the red/black wire to ground while you are testing. Also, you did check fuses 6 and 7?

The Somewhere Else could be the dash temp sensor or its foam tube, the temperature wheel, or the internals of the control unit.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe