Introduction:
There are numerous temperature sensors in a vehicle:
- coolant temperature;
- oil temperature;
- interior / outside air and intake air temperature (possibly integrated into the mass air flow sensor);
- exhaust gas temperature;
- battery temperature in vehicles with hybrid or fully electric drive.
The aforementioned temperature sensors provide the control unit of the respective system with information. To give an example: the engine control unit uses the signal from the coolant temperature sensor to adjust, among other things, the injection, ignition, idle control, EGR operation (if applicable), and the cooling fan control based on the temperature. At a low temperature, enrichment occurs, and the EGR is activated to bring the engine to operating temperature faster. At a higher temperature, the control unit switches on the relay of the cooling fan. The most commonly used temperature sensors operate on the NTC principle.
Besides sensors that send information to the control unit, there are also safety sensors that function without additional electronics. In such a PTC sensor, the Ohmic resistance increases with rising temperature. An electric motor (such as the windshield wiper or window motor) and mirror glass are equipped with a PTC sensor. In some cases, a PTC sensor is used as a temperature sensor, but we usually encounter the NTC type.
Classic Coolant Temperature Gauge:
In older cars without control units and NTC temperature sensors, the coolant temperature sender works with a bimetal. The illustration shows the components of the bimetal gauge. A stabilized voltage source of around 10 volts is connected to the gauge. The bimetal in the gauge bends as soon as a (larger) current flows, causing the needle to move.a0
In the engine block, there is a temperature probe with a bimetal.
The temperature gauge comes into contact with the coolant in the engine.
The temperature at which the points open depends on the coolant temperature and the current strength. The average current strength is then dependent on the engine temperature. In some cases, the needle is in the maximum position when the ignition is off. The bimetal is then straight.
NTC Temperature Sensor:
The next image shows a simplified diagram of the ECU and the temperature sensor. The sensor (RNTC) has two wires. The positive wire is connected to the ECU and the negative wire to the ground. Inside the ECU, there is a bias resistor. The bias and NTC resistors are in series. The ECU supplies the series circuit with a voltage of 5 volts.
In a series circuit, the voltage is divided across the resistors. Part of the 5 volts is taken by the bias resistor. The other part is taken by the NTC sensor.
The bias resistor has a fixed resistance value; usually around 2500 ohms (2.5 kilo-ohms). The resistance of the NTC depends on the temperature. The voltage that the NTC resistor takes depends on the temperature.
The ECU measures the voltage drop across the bias resistor. With a temperature change, the voltage across RNTC changes, and hence the voltage across the bias resistor. In a series circuit, the voltage distributes over the resistors; if the RNTC takes up 0.3 volts more, the voltage across Rbias drops by 0.3 volts.
The voltage measured across the bias resistor is translated by the ECU into a temperature. Essentially, we now apply the NTC characteristic, with voltage on the X-axis instead of temperature.
At a high temperature, the least change in resistance occurs. The line in the characteristic decreases more sharply at a temperature from 0 to 20 degrees Celsius than from 40 to 60 degrees Celsius. For this reason, manufacturers often use a second bias resistor for the coolant temperature sensor. The bias resistors are placed in parallel and have different resistance values.a0
With increasing temperature, the ECU switches over to the other bias resistor, giving us a second NTC characteristic. This second characteristic will have a large change in resistance at a high temperature. This allows us to measure over a broader range and accurately determine the temperature during both the warm-up phase and at operating temperature.
The next image shows the actual circuit in the ECU, including the 5-volt voltage stabilizer (78L05), bias resistor (R), the analog-to-digital converter (A/D converter), and the microprocessor. More information about the analog signal transmission like that from the temperature sensor can be found on the page: sensor types and signals.
Diagnosing the Temperature Sensor:
In the event of coolant temperature sensor malfunctions, the following issues may arise:
- engine starts poorly due to additional injection for a cold engine, while it is actually already warm;
- overheating: due to a low value, the PWM-controlled cooling fan turns on too late or not at all;
- the engine idles roughly after a cold start;
- as the engine warms up, the idle speed increases;
- exhaust gas emissions are no longer satisfactory;
- black smoke due to a too-rich mixture;
- hesitation and stuttering with a cold engine;
- the air conditioning cannot be turned on.
The above complaints are often accompanied by an engine warning light, but this is not always the case. If a malfunction occurs where the signal from the coolant temperature sensor falls within tolerances, no fault code is generated.
In reality, the software in the engine ECU constantly checks whether the signal is plausible: with significant deviations relative to other temperature sensors, or a (too) strong rise or fall of the temperature, the signal is considered “not plausible.” This results in a fault code.
Using diagnostic equipment (a cheap OBD reader or an interface with software for the phone is often sufficient for this), the coolant temperature can be read out.
In the image, we see a temperature of -48a0°C.
The diagnostic program (in this case, the measuring value blocks in VCDS) often also provides a reference value that the temperature must meet. In the current operating conditions, the temperature should be between 80 and 115 degrees Celsius.
If it is suspected that a sensor value is incorrect, we can check the voltages with a multimeter. First, we measure the voltages over the sensor at three different temperatures. In the following three images, we see a readout computer that is connected via the DLC (Data Link Connector) via CAN-bus to the gateway. The gateway also communicates via CAN-bus with the engine ECU.
The paragraph “NTC Temperature Sensor” above describes that the temperature sensor is in series with a bias resistor in the ECU. The 5-volt supply is divided over the bias resistor and the NTC resistor in the sensor housing. When measuring a voltage of 2.3 volts over the sensor, the voltage over the bias resistor is 2.7 volts (2.3 + 2.7 = 5 volts). The voltage of 2.7 volts is converted in the A/D converter in the interface electronics of the ECU into a temperature. With a warm engine, the voltage over the bias resistor increases; this is seen in the last measurement. In that situation, this voltage amounts to 4.58 volts.
The images below show the live data and measurements with a broken ground wire between the sensor and the ECU. On the readout computer, a temperature of -48 degrees Celsius can be seen: the ECU measures a voltage of 5 volts over the bias resistor. The ECU generates one or more fault codes describing the sensor;
- signal implausible;
- signal below lower limit value;
- short circuit to positive.
Due to the interruption, no current flows anymore, and the NTC does not take up any voltage. The voltage difference between pin 1 of the sensor and pin 36 of the ECU is 5 volts: this is the supply voltage of the sensor. Through pin 35, 5 volts is supplied. Since the sensor does not take up any voltage, we measure a difference of 5 volts between pin 2 (ground connection) sensor and pin 36.
In the case where we measure a voltage of 5.0 volts over the temperature sensor (see the next image), we are thus measuring the total supplied voltage over the component. We are dealing with an interruption in the temperature sensor itself. The voltage loss over the positive and ground wires is 0 volts.
When we disconnect the temperature sensor connector and measure with the multimeter in the connector, the same value will appear on the multimeter screen.
With the result of this measurement, it is clear that the temperature sensor needs to be replaced.
