Thereby, it gives an indication of the temperature of the engine. The readout computer shows a temperature of -42 degrees Celsius: the ECU measures a voltage of 5 volts across the bias resistor.The Engine Temperature Sensor accurately measures the engine coolant temperature. The images below show the live data and measured values with an interrupted ground wire between the sensor and the ECU. In that situation, this voltage is 4,58 volts. When the motor is warm, the voltage across the bias resistor rises this can be seen in the last measurement. The voltage of 2,7 volts is shown in the A/D converter translated into a temperature in the interface electronics of the ECU. When we measure a voltage of 2,3 volts across the sensor, the voltage across the bias resistor is 2,7 volts (2,3 + 2,7 = 5 volts). The voltage of 5 volts is distributed between the bias resistor and the NTC resistor in the sensor housing. The section “NTC temperature sensor” above describes that the temperature sensor is in series with a bias resistor in the ECU. In reality, the software in the engine-ECU constantly checks whether the signal is plausible: in case of strong deviations from other temperature sensors, or a (too) strong increase or decrease in the temperature, the signal is considered "not plausible". Also, if a fault occurs in which the coolant temperature sensor signal is within tolerances, no error code will be generated. Often the above complaints are in combination with an engine malfunction light, but that is not always the case. the air conditioning cannot be turned on.hesitation and stuttering when the engine is cold.exhaust gas emissions are no longer in order.as the engine warms up, the idle speed increases.the engine does not idle properly after a cold start.overheating: due to a value that is too low, the PWM-controlled cooling fan starts too late or does not start.bad starting of the engine due to eg extra injection for a cold engine, when in reality it is already warm.More information about the analog signal transmission, such as from the temperature sensor, can be found on the page: sensor types and signals.įaults with the coolant temperature sensor can lead to the following complaints: The following figure shows the actual circuitry in the ECU containing the 5 volt voltage stabilizer (78L05), bias resistor (R), the analog-digital converter (A/D converter) and the microprocessor. This allows us to measure over a wider range and accurately determine the temperature during both the warm-up phase and at operating temperature. The second characteristic will have a large resistance change at a high temperature. This gives us a second NTC characteristic. With increasing temperature, the ECU switches to the other bias resistor. The bias resistors are connected in parallel and each has a different resistance value. For this reason, manufacturers often use a second bias resistor for the coolant temperature sensor. The line in the characteristic drops faster at a temperature of 0 to 20 degrees Celsius than from 40 to 60 degrees Celsius. In fact, we now apply the NTC characteristic, with the voltage on the X-axis instead of the temperature.Īt a high temperature, the least resistance change takes place. The voltage measured across the bias resistor is what the ECU translates into a temperature. After all, the voltage in a series circuit is distributed over the resistors if the RNTC absorbs 0,3 volts more, the voltage across Rbias drops 0,3 volts. With a temperature change, the voltage across RNTC changes and with it the voltage across the bias resistor. The ECU measures the voltage drop across the bias resistor. The voltage absorbed by the NTC resistor therefore depends on the temperature. The resistance of the NTC depends on the temperature. The bias resistor has a fixed resistance value usually around 2500 ohms (2,5 kilo-ohms). Part of the 5 volts is taken up by the bias resistor. In a series circuit, the voltage is distributed across the resistors. The ECU supplies the series circuit with a voltage of 5 volts. The bias and NTC resistors are connected in series. The plus wire is connected to the ECU and the negative wire to ground. The following image shows a simplified schematic of the ECU and temperature sensor.
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