There is something called the monovalve (a vertical can in front of the battery, you can trace a water hose to it, one end goes into the car), which opens and coolant water passes thru the heat exchanger inside the car. This brings heat thru your vents. The monovalve is opened by the pushbutton unit. If you measure the voltage across the terminals of the monovalve, you should get very low value (close to 0) when the heater is not needed. If you get close to 12V or -12V (depending on the polarity of your meter terminals), when you dont expect the heater to be running, then the pushbutton unit is sending the wrong signal. If you dont get +-12V and you still get hot air, then the valve is bad.

Another component that comes into play when heater is needed is the coolant pump. This is a pump that can again be traced thru the water hose connected to the monovalve. On my 87 260E it is located near the coolant tank. When heater is needed, this pump also comes on and pushes the coolant thru the monovalve into the heat exchanger. You can also measure the voltage across the terminals of this pump. If you get + or - 12V when heater is not needed, the pushbutton unit is sending the wrong signal.

Why the PB unit is sending a wrong signal ?

1. trivial - it is bad
2. it is receiving a wrong signal from the interior tempertaure sensor, so the PB unit thinks it is too cold and turns on the heater.

I am thinking you can test the interior temperature sensor using a hair dryer. The resistance across its terminal should change with increase in air temperature, most likely decrease with increase in temperature, I am not sure about this sensor, never touched it.

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Saumil S. Patel