Engine Coolant Temperature Sensor Inspection

The engine coolant temperature sensor provides information to the electronic engine control module for controlling the ignition and fuel injection systems.

It is located on the left side of the cylinder head, in the thermostat housing (4A-FE engine) or in the coolant outlet pipe (3S-FE engine). The sensor contains a thermistor, an electronic element whose electrical resistance decreases in proportion to an increase in coolant temperature.

The engine ECU supplies a reference voltage to the sensor and then determines the engine temperature by measuring the current flowing in the sensor circuit. This information, combined with other input data, allows the system to control fuel injection.

If the sensor is faulty and does not output a signal appropriate to the temperature, the ECU backup circuit bypasses the sensor signal. In this case, the ECU uses a predetermined value, which allows the fuel injection control system to continue operating, although with reduced efficiency. When this occurs, the warning light illuminates on the instrument panel. For troubleshooting, refer to a service station equipped with Toyota diagnostic equipment.

The sensor itself can be checked by removing it from the engine and heating it in a container of water. Measure the sensor resistance at several temperature values. Resistance values for the 4A-FE engine at temperatures: 20 °C - 2...3 kΩ, 40 °C - 0.9...1.3 kΩ, 80 °C - 0.2...0.4 kΩ. For the 4A-FE engine at temperatures: 20 °C - 2...7 kΩ, 40 °C - 0.9...1.3 kΩ, 80 °C - 0.2...0.4 kΩ.

Do not attempt to check the sensor circuit directly on the engine without disconnecting it from the control system, as you risk damaging the engine ECU! Removal · To improve access, remove the air cleaner housing.

· Drain the coolant.

· Disconnect the sensor electrical connector.

· Unscrew the sensor from the housing.