|Type:||NTC temperature sensor|
|Signal type:||Amplitude varying|
|Signal level:||0.2 V hot to 4.0 V cold|
The coolant temperature sensor has a built-in NTC resistor. The resistance of a Negative Temperature Coefficient (NTC) resistor decreases when the temperature increases. The resistor in the sensor together with a resistor in the ECU forms a voltage divider which is fed by a 5 V power supply. If the temperature changes, the resistance and the signal voltage at the dividers output changes. The ECU is able to determine the engine temperature from the sensor signal voltage and adjusts the injector opening time. In this measurement example the signal voltage of the coolant temperature sensor is measured.
Correct functioning of the coolant temperature sensor can be checked by measuring the following signal voltages, see figure 1:
|1||Sensor output signal||4 V|
|Ground at battery|
Figure 1: Measuring diagram
The lab scope is connected to the coolant temperature sensor via a Measure lead TP-C1812B and Back Probe TP-BP85 and set to recorder mode. In recorder mode a streaming measurement is performed, continuously displaying the signals live on screen. Because the measured signals vary slowly, the Automotive scope ATS5004D is set to a slow measuring speed.
Figure 3 shows a waveform of a coolant temperature sensor during the engine warming up. This signal can be downloaded and used to correctly set up the lab scope or as reference signal.
Figure 3: Lab scope measurement of coolant temperature sensor
Channel 1 (red) shows the signal of the coolant temperature sensor during a warming up of the engine. At the beginning of the measurement the key is switched on and shortly after the engine is started which can be seen from the spike in the signal voltage. After the engine has fired up, the voltage starts to decrease due to the warming of the engine. This measurement is made with an outside temperature of 17°C which corresponds with a signal voltage of 2.5 V. The measurement is taken until the engine has reached a temperature of 82.5°C. The sensor signal has been converted to the actual engine temperature with a Gain/Offset I/O. The purple line shows resulting engine temperature. The settings of the Gain/Offset I/O in this measurement example are specific for the type of temperature sensor used in the particular car that is measured.
Signal values may differ on different types of engine control units and coolant temperature sensors.
The following signal deviations can indicate a problem:
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The information in this application note is carefully checked and is considered to be reliable, however TiePie engineering assumes no responsibility for any inaccuracies.