Temperature Switch (Warning Indicator): Description and Operation

The coolant temperature warning lamp lights whenever engine coolant temperature is too high. Battery voltage is supplied to one side of the indicator bulb when the ignition switch is turned to the On position. The coolant temperature sending unit is connected to the other side of the bulb. The coolant temperature sending unit is a thermistor that changes its electrical resistance with changes in engine coolant temperature. The sending unit is installed in a threaded hole that penetrates a coolant passage of the engine. When the engine coolant temperature is too high, the sending unit resistance is low. This provides a ground path for the indicator bulb, which causes it to light.