Spin the wheel

Frank Massey looks at how you need to approach a problem when different types of sensor are involved

Published:  09 October, 2017

I have been asked several times about ABS wheel sensors. Like many other components, the technology is changing. The changes reflect the expansion in integrated chassis dynamics.

Just imagine how many functions require wheel speed and rotational differential data.

ABS, dynamic stability, hill start, audio volume, navigation, self park, all wheel drive, active steering assist, electronic handbrake etc. Sharing this data on a high speed can network ensures very accurate vehicle motion dynamics.

Older variable reluctance sensors (VRS) rely on a coil generating an alternating voltage when rotation occurs. The problem is they are not directional sensitive and cannot report motion at very low speed. Air gaps were critical as they affect signal amplitude. They are often referred to as passive sensors. So, the introduction of digital or active sensors was inevitable.


Principles
How do we tell them apart? Active sensors require a voltage supply from the ABS PCM, with a ground or signal return. They operate with different principles of signal generation; hall, and magneto resistive. Pure hall effect sensors will switch between the supply potential voltage and ground. Magneto resistive sensors operate on the principle of current and voltage change in response to a change in magnetic induction. This change can be introduced in several ways reflected in wheel bearing and sensor design. Smaller sensors with integrated magnetic field rings are now the norm. Developed by NTN at their Annecy facility they are called encoded bearings. A small ring mounted at one end of the bearing carries a series of north south poles. These have now been replaced by dual encoding, two sets of magnetic rings with a unique offset. This enables the abs module to determine direction of rotation.


Subtle differences
There are two very subtle differences in the digital outputs. They can be called pull up or pull down. The sensor supply voltage will be slightly lower than battery voltage this is due to the different internal resistance values. However, it will be around 10.5/11.5v.

The ground or return signal value will vary between 0v or 1.4/1.8v. You could have a sensor or circuit fault; let me try and explain the subtle differences, and how to prove which is which. Remember the golden rule if in doubt compare a wheel circuit that works normally.

First unplug the sensor and measure both circuits in the loom. With no load applied the supply voltage should jump up to NBV

Next check the ground circuit if its true ground then it’s a pull-down type and the signal will be on the power line, and may only be around 200mv

If a small voltage exists then it’s a pull up type and the signal will be on this wire not the supply. The digital signal will be very small when the wheel rotates. It could be small around 200/400mv, or as high as 0.5/1.8v, depending on the manufacturer variant

Common sense would dictate the serial route is easiest, however how would you determine an intermittent fault? It could be a faulty sensor, faulty encoder, or a circuit error. The only way is using a scope. Should we measure voltage or current though? Both change in the circuit. Unless you have a very special current clamp, go for voltage and select a AC coupling.

The specific question I am often asked is current measurement, well I can tell you in a pull-down circuit its around 7-15 ma with a 400mv voltage change. The pull up type will produce around 6/13ma with 0.2/0.35mv.     However, these voltage values can vary due to the value of the two parallel internal sensor resistors these are normally 1.4k ohms, with a much higher resistor in the meg ohm range, within the ABS pcm.

I hope this helps. The pico image was taken from a VW Golf 1.4 TSI. The easy bit is replacing the wheel sensors. Ever since metal housings were replaced with plastic they never corrode in the housings
do they…?

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    I’m pleased to be writing a second article and this time I’ve decided to look at the active wheel speed sensor.
    Originally electronic anti-lock braking systems (ABS) used magnetic inductive sensors to measure wheel speed. These are classed as ‘passive’ sensors and are actuated by a rotating toothed ring. The sensor contains a magnetic core w1 vith a fine coil of wire around it. As each tooth passes the sensor it generates an alternating current (AC) analogue signal. The faster the wheel goes, the greater the frequency and the higher the voltage. The wheel speed is determined by the frequency. The main disadvantage of this is the weakness of the signal at low speeds.  

  • Glowing, going, gone! 

    I decided to share this case study for my first article because what I expected to be a simple job turned into something a little more complex and gave me an opportunity to study a and learn about a system that until now I’d probably taken for granted.

    We were presented with a 2010 Skoda Fabia 1.6 TDi by a car dealer who had recently taken it in part exchange. The engine management was light illuminated, however with no other symptoms. The previous owner told the dealer that the MIL had been on for around a year and her local garage had failed to repair it. It had also recently been recalled for the ‘Dieselgate’ VAG emission software update. The dealer told the customer there were DTCs stored for the glow plugs and that they needed replacing to which she declined as she was sure they had previously been replaced. We already had a reasonable amount of vehicle history to start with, and were ready to take a look.

    Voltage and current
    A code read revealed DTCs for all four glow plugs being open circuit and a glow plug module communication fault. A quick inspection of the engine revealed that the glow plugs were not that old and also there was a new glow plug module fitted, plus an old one found in the boot.

    While checking the resistance of the glow plugs may tell us something, measuring the voltage and current with an amps clamp paints a much clearer picture. The oscilloscope was connected and the ignition was cycled. The screen capture revealed a healthy 12 volts for around 10 seconds then pulsed at random, however there was zero amps flowing (on all glow plugs). It was clear the plugs had gone open circuit for some reason so they were removed for inspection. It was then we noticed that the heater plugs fitted were rated at 4.4 volts, so now we know why they burnt out! Could they be the wrong glow plugs? Could it be the wrong control module? We checked and found the part numbers were correct.

    At this point it was crucial that we understood exactly how the system is wired and how it should operate. By studying a wiring diagram we were able to plan how we were going to test the system (see image 1). Starting with the power supplies and ground, it is always best to test a circuit in its normal environment which means we really need the current load of working heater plugs. If we were to fit new heater plugs at this point there was a high risk of them being damaged which is expensive so we substituted four headlamp bulbs instead. The fuse rating for the circuit was 50A so with a quick bit of maths we calculated the current required for four bulbs was safe. The main live feed, ground and ignition switched live were all good so we moved on to the two communication wires that link directly to the PCM.  

    If the PCM can log individual codes for each glow plug then we know that it must have a two-way communication system. Scoping both wires with the module connected and disconnected showed us that there was clearly a command signal from the PCM and although it was random and rather messy (see image 2), the glow module responded directly by activating the glow plugs at the same rhythm.

    The second wire had totally different digital signal which had to be the feedback to the PCM. The noise and irregularity of the command signal was clearly an issue so we checked the wiring back to the PCM and with the aid of the good old-fashioned wriggle test the fault was identified as a poor connection in the PCM harness connector. The connection was cleaned and the system retested which revealed a much healthier scope pattern and the communication DTC was cleared (see image 3).

    Reliable repair
    At this point we could have fitted new glow plugs but to save unnecessary expense we wanted to make sure it was a reliable repair so we decided to monitor the system with the faulty glow plugs still installed and the leads connected to the bulbs. We started by monitoring all four glow plug voltages on the oscilloscope. Using the scan tool to activate the glow plugs showed us that the 4.4 volts is achieved by pulse width modulation at a duty cycle of around 13% with a frequency of around three times per second. What was more interesting was that all four plugs were individually triggered in a sequence (see image 4) so there is never more than one glow plug energised at any one time. The logic behind this is that it makes a substantial reduction in power consumption.

    Our next test was to observe the control strategy of the PCM from a cold start and warm-up phase. The objective here was to ensure that there was no software related issues. From the point of key on there is a 1.5 second supply phase to heat the plug as fast as possible then temperature is maintained by the 13% duty control.

    Decade box
    Of course, after a period of time, once the engine starts to warm up the system turns off and the communication wires go quiet. If you want to test it more than once then you’d have to wait for the engine to cool so to save time we connected a decade box in place of the engine coolant temperature sensor and by observing the coolant temperature in serial data on the scan tool we were able to select a variety of resistances that would represent low temperatures and fool the PCM into commanding glow plug activation.

    The decade box has proved to be an extremely useful tool really is a must in any diagnostic technician’s tool box. It is great for substituting in place of certain sensors and components to check the integrity of a circuit or to observe an ECU responding to a variation in signal (resistance).

    The final test was an observation of voltage over current on one glow plug. The other interesting thing we noticed was the simplicity of the digital feedback signal. By unplugging each glow in turn you could see the pattern in the signal change and when all were connected and working it was a regular pattern.

    Summing up
    Clearly more time was spent on this job than necessary and the labour charge remained fair. In a busy workshop it is hard to find spare time for these situations but my point is that sometimes sacrificing a lunch hour or staying behind for half an hour gives an opportunity to learn so much which can only aid you in speeding up diagnostic time and process on future jobs.

    Winning the Top Technician 2017 competition was unexpected. It has not only introduced me to some very inspiring, like-minded people, but has also taught me you can never have too much training, whether it’s self-training like in this instance or on a professional training course. There are some fantastic training companies offering a variety of courses available now. Also, some of the best and most respected all regularly write for Aftermarket!  





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