Refer to Electronic Engine Controls in Section 4 for information on servicing the Engine Coolant Temperature (ECT) sensor.
The engine coolant temperature gauge uses a heat sensitive sending unit to transmit an electrical signal to the gauge. The sending unit is a heat sensitive variable resistor that is located on or near to the cylinder head and threads into an engine coolant passage. The sensors are a Negative Temperature Coefficient (NTC) type. As the temperature increases, the electrical resistance of the sensor decreases. As the coolant temperature changes, so does the resistance of the sensor. The gauge is calibrated within the operating range of the sensor and interprets the resistance value to display the coolant temperature.
On E36 models, the coolant temperature sensor electrical connector consists of two wires. One wire is a ground from the instrument cluster, typically a brown wire with a yellow tracer, that supplies a ground signal to the sensor. The sensor, depending on its temperature, changes the resistance and supplies a signal to the gauge via a signal wire, which is typically a brown wire with a violet tracer.
On E36 models prior to 1996, the fuel injection temperature sensor and coolant gauge temperature sensor were separate units. The coolant gauge sensor housing was typically black, where the fuel injection temperature sensor housing was blue. Beginning with the 1996 models, the two sensors were combined into one unit. The sensor instead of being a two-wire version, was now comprised of four wires. The operation remained the same, however the value of the two sensors are different from one another and they should not be substituted for one another. Furthermore, the cylinder head castings on the 1996 and later E36 models have but one threaded hole for a sensor, unlike the previous models which had two. If installing an engine from a 1996 or later E36 into an earlier E36 model, the cylinder head does have an acceptable area that can be drilled and tapped for an additional sensor, however this procedure should be done by a qualified machinist to ensure a proper fit and avoid the chance of coolant leakage.
The engine coolant temperature gauge sending unit locations are as follows:
S14 engine: The coolant pipe above the exhaust side of the engine, just forward of the DME sensor