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How to check the sensor with the SENT protocol?

11.07.2023

Although it has been several years since the new standard of sensors appeared on popular models, their existence has not broken through to wider awareness, and the typical mechanic knows almost nothing about them. In the following material we will show how the SENT standard works and how to deal with the new sensors.

Small but significant

Sensors are an important part of engines, providing the controller with a variety of information about the operating status of the drive unit. In order for the ECU engine controller to receive the information, it must be encoded in a way that it understands, either in analog or digital form.

Many sensors such as pressure sensors (e.g. MAP, DPF) or position sensors (e.g. throttle, EGR or variable geometry) use an analog signal in the form of a voltage in the 0-5V range to transmit data. To use such a signal in a controller, it must first be processed by an analog-to-digital converter.

Signals in digital form, accepting only low and high states, are also used to transmit sensor data. The information can be transmitted in the form of frequency, fill factor or communication protocols such as CAN, LIN or just SENT.

How SENT works

The Single Edge Nibble Transmission (SENT) protocol defined by SAE J2716 was created as a digital replacement for analog signals. Data is transmitted unidirectionally over a single line. Three wires lead to the sensor (ground, 5V power line and signal). In the sensor’s housing you will find a complete circuit that measures the parameter and encodes it as a standard.

Compared to analog sensors, ways of checking the correctness of the signal, reporting faults or transmitting more than one parameter through the sensor have been obtained. The SENT protocol is most commonly used for MAP pressure sensors, MAF flow meters, throttle position sensors or variable geometry sensors.

Correct signal waveform

If you connect an oscilloscope, you will see the waveform as shown in the illustration. Data is transmitted as pulses of different widths encoding 4 bits at a time. The last portion of the data is a CRC code calculated from the previous data, which makes the protocol resistant to single errors. A single longer synchronization pulse ensures recognition of the beginning of the frame.

Oscilloscope waveform of the SENT sensor.

Oscilloscope waveform of the SENT sensor.

The appendix to the SAE J2716 standard contains several suggested schemes for encoding values in the SENT standard allowing one or two values to be transmitted in each frame, typically with a resolution of 12 bits. This means that the sensor sends a specific number from 0 – 4095 (at 12 bits) to the controller, which is immediately used for conversions to, for example, pressure or position values.

To see what the sensor is sending you will need to have an oscilloscope with the SENT protocol decoding function. The mere presence of a valid frame does not prove correct operation – a faulty sensor can transmit erroneous data – the same data over and over again, for example.

We check the SENT sensor without an oscilloscope

Many mechanics are not comfortable with oscilloscope measurements, but use a multimeter to check the analog sensor signal supply. When testing a SENT sensor, we can do the same. Using the QST-5 sensor tester, we can easily check the sensor with the SENT protocol. What’s more, we don’t need to know its pinout or even that we are dealing with just such a sensor. With help comes the tester itself, which, thanks to its automatic detection mechanism, will do it for us. Just press OK to see if the sensor is working by observing the change in readings.

An example of a MAP test of a sensor with the SENT protocol thanks to the QST-5 tester.

An example of a MAP test of a sensor with the SENT protocol thanks to the QST-5 tester.

 

In order not to get bogged down in details, the tester allows you to simply select one of the popular coding variants and get one or two measured values depending on the variant. For example, when checking the MAP sensor, we can use a vacuum pump, paying attention to the changing value from the sensor depending on the pressure.

We test turbocharger controllers

Newer models of turbochargers often use sensors with the SENT protocol. To test a controller that contains such a sensor, we will need a tester that can properly handle such a sensor. Using the VNTT-PRO or TP-TACT testers, we will easily test new models of controllers equipped with new sensors. We will also check the stand-alone sensor (the so-called pear) in the case of vacuum-controlled variable geometry. If your tester does not support these sensors then download a free update that will.

TP-TACT and VNTT-PRO tester can handle SENT protocol without any problems

TP-TACT and VNTT-PRO tester can handle SENT protocol without any problems

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