Coolant temperature sensor - How It Works

General points

The coolant temperature sensor is used by the fuel induction system to record the engine operating temperature. The control unit adapts the injection time and the ignition angle to the operating conditions depending on the sensor information. The sensor is a temperature sensor with negative temperature coefficient: As temperature increases, internal resistance decreases.

Function 

The resistance of the temperature sensor changes depending on the coolant temperature. As the temperature increases the resistance decreases and with it the voltage at the sensor. The control unit evaluates these voltage values, since they are in direct relation to the coolant temperature (low temperatures result in high voltage values at the sensor and high temperatures in low voltage values).

Effects of failure

A faulty coolant temperature sensor can become noticeable in different ways through the fault recognition of the control unit and the resulting emergency running strategy. 

Frequent fault symptoms are:

■ Increased idling speed 

■ Increased fuel consumption 

■ Poor start-up behaviour 

In addition there could be problems with the vehicle emission test cycle due to increased CO values or the lambda regulation missing. The following faults can be stored in the control unit: 

■ Ground connection in the wiring or short-circuit in the sensor 

■ Plug connection or interrupted cables 

■ Implausible signal changes (signal leap) 

■ Engine does not achieve the minimum coolant temperature 

This last fault code can also occur with a faulty coolant thermostat

Troubleshooting

■ Read out the fault code 

■ Check electrical connections of the sensor cables, the connector and the sensor for correct connection, breaks and corrosion.

Testing takes place using the multimeter

1st test step 

The internal resistance of the sensor is determined. The resistance depends on temperature: when the engine is cold, resistance is high and when the engine is warm, resistance is low. 

Depending on the manufacturer:

25 °C 2.0 – 6 KOhm 

80 °C ca. 300 Ohm 

Note special reference value specifications.

2nd test step

Check the wiring to the control unit by checking every single wire to the control unit connector for transmission and connection to ground. 

1. Connect the ohmmeter between the temperature sensor connector and the removed control unit connector. Ref. value: approx. 0 ohm (circuit diagram necessary for pin allocation on the control unit). 

2. Use the ohmmeter to test the respective pin at the sensor connector and removed control unit connector to ground. Ref. value: >30 MOhm. 

3rd test step 

Use the voltmeter to test the supply voltage at the removed sensor connector. This takes place with the control unit inserted and the ignition switched on. Reference value approx. 5 V. If the voltage value is not reached, the supply voltage of the control unit including ground supply must be checked against the circuit diagram.