Coolant temperature sensor function
Here you will find useful basic information and important tips relating to the coolant temperature sensor in vehicles.
Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for use by suitably qualified personnel only.
Coolant temperature sensors are used by the fuel management system to detect the engine's operating temperature. Depending on the sensor information, the control unit adapts the injection time and firing angle to the operating conditions. The sensor is a temperature sensor with a negative temperature coefficient. This means that the internal resistance reduces as the temperature increases.
The resistance of the temperature sensor changes depending on coolant temperature. As the temperature increases, the resistance is reduced, which reduces the voltage at the sensor. The control unit evaluates these voltage values, since they are directly related to the coolant temperature (low temperatures result in high voltage values at the sensor, and high temperatures result in low voltage values).
A faulty coolant temperature sensor can manifest itself in different ways through fault detection by the control unit and the resulting emergency program strategy.
Frequent fault symptoms are:
In addition, problems may also occur with the exhaust gas test cycle through increased CO values or intermission of the lambda control. The following entries can be stored in the fault memory of the control unit:
The last fault code can also occur if the coolant thermostat is faulty.
The internal resistance of the sensor is determined. The resistance is temperature-dependent. When the engine is cold it is high-impedance, and when the engine is hot it is low-impedance.
Depending on the manufacturer:
25°C 2.0 – 6 KOhm or 80°C approx. 300 Ohm
Please note the special reference value specifications.
Check the wiring to the control unit by checking continuity and short circuit to frame for every wire to the control unit plug.
Check the supply voltage at the removed sensor plug using the voltmeter. This is done with the control unit plugged in and the ignition turned on. Reference value: approx. 5 V.
If the voltage value is not reached, the voltage supply of the control unit and the ground supply must be checked according to the circuit diagram. If these are OK, a control unit fault comes into question.