Instrument panels contain a number of indicating devices (gauges and warning lights). These devices are composed of two separate components. One is the sending unit, mounted on the engine or other remote part of the vehicle, and the other is the actual gauge or light in the instrument panel.
Several types of sending units exist, however most can be characterized as being either a pressure sensitive or a heat sensitive sensor. Sensors used for warning lights, such as a low oil pressure warning light are essentially on/off sensors. These sensors function similar to an electrical switch. The switch is operated by pressure and is either on (closed) or off (open). In the case of an oil pressure switch, it is designed to warn of low oil pressure. If the engine's oil pressure is within the manufacturer's recommendations, a properly working oil pressure switch will be in the open (off) position. If the oil pressure drops below a safe operating pressure, the oil pressure switch is closed (on) and completes the electrical circuit for the low oil pressure warning light, and the warning light should be turned on in the instrument cluster.
The other type of sensor used to monitor engine operating parameters is the variable resistance sensor. This sensor can be either pressure or temperature sensitive. For these sensors, as the pressure or temperature changes, the resistance in the sensor changes. This type of sensor is ideal for gauges and the Engine Control Module (ECM) because it can be used to monitor changes in pressure as in an oil pressure gauge, or temperatures, as in an engine temperature gauge. The sensors used by BMW to monitor changes in temperature are the Negative Temperature Coefficient (NTC) type.
A temperature sensor changes its resistance based on the temperature of the sensor's probe. As the temperature of the probe increases, the resistance of the sensor decreases. If a sensor's probe is installed in an engine's coolant passage, it can be used to by the coolant temperature gauge to transmit the engine's coolant temperature.
Pressure type sending units convert liquid pressure into an electrical signal that is sent to the gauge or warning light. Resistance type sending units are most often used to measure temperature and use variable resistance to control the current flow back to the indicating device. Both types of sending units are connected in series to the gauge or warning light by a wire. When the ignition is turned ON , current flows from the battery to the gauge or warning light and on to the sending unit.
The basic operation of a sending unit can be summarized as follows:
The basic wiring circuit for a gauge or warning light consists of:
Sometimes a variable and an on/off sensor are combined, such as the case with an oil pressure gauge with a warning light. The variable resistor portion of the sensor operates the gauge, the on/off portion of the sensor is used to operate the warning light, should the oil pressure fall below a safe level.