Cooling temperature sensor test -symptoms, failure and failure exclusion

Here, you will find useful basic information and important tips related to the cooling sensor of the vehicle coolant.
Cooling temperature sensor notify the engine temperature notification of the engine to the engine control unit so that the fuel amount and ignition point can be adjusted accordingly. On this page, we will explain to you the functional principle of the cooling fluid temperature sensor and show you how to correctly locate the cause of the fault in the process of failure.

1. Cooling temperature sensor function: function principle
The fuel management system uses the coolant temperature sensor to detect the operating temperature of the engine. According to the sensor information, the control unit will adjust the spray time and ignition angle according to the operating conditions. The sensor is the temperature sensor with negative temperature coefficient. This means that the internal resistance decreases as the temperature rises.

The resistance of the temperature sensor changes with the temperature of the coolant. As the temperature rises, the resistance decreases, which reduces the voltage on the sensor. The control unit evaluates these voltage values because they are directly related to the temperature of the coolant (low temperature causes high sensor voltage value, and high temperature causes low voltage value).

2. Cooling temperature sensor failure: symptoms

The temperature sensor with faults can be expressed in different ways by controlling the failure detection of the unit and the emergency program strategy generated by it.
Common fault phenomena are:
● higher idle speed
● Increased fuel consumption
● Poor startup behavior

In addition, due to the increase of the CO value or the interruption of the control, the exhaust gas test cycle may also occur. The following items can be stored in the fault memory of the control unit:
● Wiring to short -circuit or sensor short circuit
● Positive malfunction or wiring disconnection
● Unbelievable signal changes (signal jump)
● The engine does not reach the minimum coolant temperature
If the coolant constant temperature device fails, the last fault code will also appear.

3. Check the cooling fluid temperature sensor: failure exclusion

Troubleshooting:
● Read the fault memory
● Check whether the electrical connection of the sensor wiring, plug and sensor is correctly connected, disconnected and corroded.

Use a multimeter for inspection
Test step 1
Determine the internal resistance of the sensor. Resistance is related to temperature. When the engine is cold, it is high impedance and low impedance when the heat is hot.
Depending on the manufacturer:
25 ° C 2.0 - 6 kohm or 80 ° C. 300 ohm
Please pay attention to the special reference value specifications.

Test step 2
By checking the diversion and short circuit of each wire connected to the control unit plug, the wiring of the control unit is checked.
1. Connect Ohm table between the temperature sensor plug and the control unit plug. Reference value: about 0 ohms (the circuit diagram of the pin allocated on the control unit).
2. Use Ohm table to check whether the corresponding pins on the sensor plug are grounded and remove the control unit plug. Reference value:> 30 mohm.

Test step 3
Use the voltage gauge to check the power supply voltage at the dismantled sensor plug. This is completed when inserting the control unit and turning on the ignition switch. Reference value: about 5 volts.
If the voltage value is not reached, check the voltage supply and ground power supply of the control unit according to the circuit diagram. If these are normal, there will be a control unit failure.