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Diagnostics sent into orbit – By James Dillon of Technical Topics

By autotech-nath on July 31, 2018

The advancement of technology in the motor trade is, and has been for many years, relentless. The changes over the past five years have seen the widespread proliferation of the electrification of motor transport, significant changes to diesel engine emission control systems, a shift towards engine downsizing, major advancement in direct petrol engine technology, internal engine friction reduction measures, the ‘Connected Car’ and Over-The- Air-Updates, autonomous technology and so forth. It’s a very exciting time to be involved.

Many of these changes will lead us, as technicians, having to change the way we work. We will have to adopt different working processes to repair and maintain vehicles. We will have to adopt new techniques and learn to use different tools, or similar tools in a new way, to effect diagnosis and repair. It appears that the only constant in the world of the motor vehicle technician is change itself.

One significant change in the nuts and bolts of vehicle control systems, which will definitely impact on the future day-to-day of the diagnostic technician, is the implementation, within control systems, of sensing devices that use the SENT protocol. The implementation of this technology will change the way we work without doubt, particularly during control system diagnosis, as SENT protocol sensors encode the data they gather from the vehicle (such as coolant temperature or exhaust pressure). The goals of the SENT implementation are low cost, high data rate, high accuracy sensors. It is a unidirectional (from sensor to ECU), point-to-point (no Bus) protocol. The big question is how will this technology effect the diagnostic processes of the technician?

The potential impact of this method of sensor information encoding may be better understood when imagining a typical system malfunction diagnosis. As our candidate vehicle, let’s select a ‘cooking model’ VW Golf. Let’s say it has the 1.6 Diesel engine fitted and is a 2018 Model Year. This vehicle has dropped into limp mode and the engine management warning light is on. The system has restricted performance and upon interrogation with the scan tool, the system believes that the DPF has significantly elevated levels of pressure prior to the DPF.

Figure 1: Diagnostic Process Loop

Typically, the traditional diagnostic process would involve them observing the system behaviour to confirm the fault. They would then develop a hypothesis, or theory, based on the observation experience, expertise, and resultant ‘gut-feel’ on what could be causing the observed behaviour. This hypothesis would identify suitable candidate components which require data gathering tests, such as system functions, sensors and actuators. The technician would perform these tests then analyse the data, using their expertise, in order to either prove or disprove the malfunction of the candidate components. This process repeats until the root cause is identified, see Figure 1: Diagnostic Process Loop.

In practice, the process would run the following course… The technician would validate the symptom that the customer described, perhaps through a test drive. They would reach
for the serial data stream to retrieve a diagnostic trouble code (observe system behaviour). This would help them form a hypothesis. The technician would call up relevant data PIDs (candidate selection), such as exhaust pressure actual and desired, air flow, EGR, exhaust temperature, DPF regen data, boost command and feedback and such like to gather supporting data. Any anomalies in the serial data stream would usually lead the technician towards carrying out a more detailed component test with a scope, multimeter or pressure gauge (test specification pass or fail). Analysis of this test data should lead to the problem being solved.

But what if your multimeter or oscilloscope couldn’t be used to verify the data output from a suspect sensor and you were only able to see a signal similar to a LIN data signal? How would this make you feel? And more importantly, would it change or affect the way you work? A key aspect of the diagnostic process is to test the output of the suspect component to validate its operation. Typically, this would involve both the serial data stream and the raw signal voltage. This dual measurement approach is required as many component failures can lead the controlling computer (the ECU) to set a default value in the datastream (the PID) to enable necessary calculations to be carried out so the system can continue in limited operation mode (Limp).

Figure 2: SENT component internals

The big question is that if the PID cannot be validated against a raw signal voltage comparator, how can differentiation be made upon the root cause of the issue which prompted the component validation in the first place? It is no longer possible, with components using the SENT protocol, to probe and compare. Also, it is no longer possible to use a signal simulator (or decade box) to substitute the signal and observe expected PID/data variation, as the raw signal never leaves the internals of the sensing component, so voltage simulation will not work. See Figure 2: SENT component internals.

The SENT component on the VW Golf looks a little like a normal 3-wire sensor, fitted with a 5V supply, a ground and a ‘signal’ wire (importantly, there are also 2-wire SENT devices). Upon inspection, the signal wire can be seen outputting a regular train of pulses and a typical data transfer profile. The data channel has a logic low state at < 0.5 V and a logic high state at > 4.1 V.

Figure 3: SENT waveform

The image in Figure 3 shows the data capture on the oscilloscope. I have turned on the decoding tab to look at the data contained within the SENT waveform. The image shows a protocol ID, a diagnostic message (slow channel) and the sensor data packets (fast channel). The data packets values change in line with the physical pressure changing. Detailed analysis of this data may offer an opportunity for further diagnostics.

Keeping pace with modern technology is critical to remaining competitive in fixing broken motor cars. Technicians have to keep pace with the changes and continually develop their professional competencies. The ever-changing scene of vehicle technology makes this work challenging, but also highly rewarding.




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