How the BMW Thermostat Works

Posted 3/9/2010
By Robert Jacobs

Thermostats have been around a long time. They've always served to properly hold engine operating temperatures within a defined range, thereby helping to maintain proper temperatures for best combustion and emissions. So, you open the hood on a BMW and find the thermostat. It appears to be just like most other thermostat housings. It has a nice roundish-shaped body and a couple of big coolant hoses going to it. It also has a "temperature sensor" on it. That's where some confusion might come into play. The temperature sensor is not really a temperature sensor. It is a heating element.

The heating element can be thought of just like an oxygen sensor heating element. An oxygen sensor receives battery voltage on one wire of the heating element while the other wire receives a computer-controlled ground to maintain the oxygen sensor temperature for best operation. Why does a thermostat need a heating element?

The engine produces plenty of heat and a lot of that heat goes into the cooling system. Just like a normal thermostat, coolant temperature will cause the thermostat to open and regulate engine temperature. The added feature of the heating element on the thermostat will allow the engine computer to control the opening of the thermostat during set conditions. Notice that I said, "control the opening." The computer cannot control the closing of the thermostat. The coolant temperature will need to cool to a level that allows the thermostat to close, just like a conventional thermostat. When working normally, the computer can activate the heating element in the thermostat. When activated by the engine control module, the thermostat heats up from the heating element, not engine coolant, and will open.

The computer will activate the heating element if:

• Engine temperature is greater than 235 F.

• Radiator outlet temperature becomes too high (Note: An exact temperature is not defined by BMW).

• High engine load of injector pulse width greater than 5.8 ms.

• Intake air temperature is greater than 125 F.

• Vehicle speed is greater than 110 mph.

If one or more of these conditions are met, the engine control module will switch the ground side of the heating element so that current will flow through the element, causing the thermostat to get hot and open, allowing coolant flow.

For testing purposes, the thermostat can be checked for flow just like a conventional thermostat. Activation of the heating element by manually grounding the control wire for the heating element does not guarantee that the thermostat is opening. It can stick closed or open just like any other thermostat. Electrical testing involves disconnecting the connector at the heating element and verifying that one wire has battery voltage with the key "on." The other wire needs to have continuity back to the proper pin at the engine computer. The heating element can be checked for resistance, typically in the 10 to 20 ohm range.

Be careful not to rely too much on a resistance reading of the heating element, although an open or shorted element is an obvious failure of the element. The heating element is not replaceable as a separate component. A new thermostat comes as a complete unit of the housing thermostat and heating element.

Robert Jacobs is the Identifix European team leader. He is a certified Infiniti master and ASE master with L1 certification.

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