Oh, by the way, the default for the duovalve solenoid is 'open', so by disconnecting it electrically, you allow full flow through the heater core on both circuits. Normally, when the interior temperature is just being maintained, the solenoid valves will both be closed, and will briefly pulse open often enough to maintain the required heater core temperature.

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'96 C280 (gone)
'97 C36
'05 C230k