Originally Posted by MB-Owner-in-ind

It does need a thermostat if this is the typical reading after 20 min of running in non-freezing weather. I assume you have checked the power at the aux. pump plug and found none after it warmed up. The system does have a shut off relay that won't allow the heating system to work until the engine is 125 degrees. I am not sure where it is located on the newer cars but it might be under the dash on the passenger side. That was where I found it in a 1977 280se. I would replace the thermostat, flush the radiator and check for power at the aux pump.