can you feel the wheel?

Top Technician 2017 winner Karl Weaver is back, and looking at the active wheel speed sensor

Published:  25 May, 2018

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.  

As technology developed and the requirements for safer and more precise anti-lock braking systems vehicle manufactures started to use ‘active’ wheel speed sensors. Although its purpose is the same as its predecessor,  it is able to give an accurate signal at much slower speeds. The sensor reads from a ring of magnets, which are usually incorporated into the seal of the hub bearing. The sensor is supplied with a voltage and returns a direct current (DC) digital signal where speed is determined by the frequency of the switching of the circuit. There are several variations in the way they work and what to expect to see when testing them.

Interpretation
The workshop was presented with a 2010 Seat Ibiza with an intermittent ABS fault. The customer’s complaint was that occasionally the ABS light comes on when driving and doesn’t go out until the car is restarted. At this point it’s important to gather as much information from the customer as possible about when the fault occurs. In this case the customer noted that it seemed to happen more after rain. Without the series of questions we asked the customer they would never have mentioned this and in many cases it can prove to be that vital clue that makes for a faster and successful diagnosis.

After inspecting the vehicle we carried out a full diagnostic scan which revealed one DTC stored in the ABS controller: “00283 – ABS wheel speed sensor; Front left (G47) 012 – Electrical fault in circuit.”

A fault code is seldom a definitive answer on where the fault is. It is the control unit’s best interpretation of the fault which can sometimes be a symptom of something completely separate. It should always be treated as a clue. As technology has progressed, DTCs have become better. Once upon a time the DTC may have just been ‘front left speed sensor’ but now we have sub-codes which enable a better and more useful description: “Front left speed sensor- Circuit open, plausibility, coherence, short to positive, short to ground.” This tells us much more about the nature of the fault. In this case it was referring to the circuit so we know that it’s unlikely to be caused by the magnetic ring or a mechanical fault. It would suggest the cause could be any of the components that make up the circuit including the sensor, the control unit or any of the wiring between these.

Tool of choice
Our typical British weather conditions gave the perfect test conditions and after a 10 minute road test we were able to replicate the fault and the same DTC was stored. Live data confirmed no speed signal for that wheel so we put the vehicle back onto the lift for inspection and this time we were unable to clear the code. The fault was at this point permanent which meant the perfect opportunity for testing. Our tool of choice for this was the oscilloscope. With an intermittent fault that is present it is best to conduct the testing whilst disturbing as little as possible. When touching wiring, particularly with an intermittent fault that only happens occasionally you may easily make the connection good again and mask the fault. So the least intrusive way was to back-probe at the sensor connector.  

From previous experience we had a good idea what to expect. However, as we knew the remaining three wheel speed sensors were fully operational, this gave us the ultimate test reference data. Disturbing this circuit wasn’t an issue so we opted for a break-out lead. When testing the front right speed sensor with the ignition on, we noted battery voltage on one wire and just below 200mV the other. Rotating the wheel gave us a square wave signal on the low voltage wire, switching roughly between 180 and 350mV (See Figure 1).

Now back to the faulty side; Our test showed no power. At this point we now know that either the control unit’s supply to that sensor has failed, the circuit is open or is being shorted to ground. We repeated the test at the control unit plug which showed the same reading so that eliminated an open circuit. Next we disconnected the sensor from the harness to see if it is shorting the supply to ground but still no power. It did however reveal a corroded connection which could well be the fault but why no power?

The connection was cleaned and reconnected, the ignition was cycled and the DTC was cleared. The power returned and the sensor and circuit’s function was confirmed. Due to the condition of the connector and sensor terminals it would be necessary to replace the sensor and splice in a harness repair section. As this sensor was relatively close to the controller we opted to join the section at the ECU harness connector.

Reliable diagnosis
We have to ask though, at this point is it a 100% reliable diagnosis? Just because we have seen a corroded connection and the system is now functioning again. Can we guarantee to the customer that the same fault won’t return? It would have been very easy to carry out the repair, test it and leave it at that but by spending an extra 20 minutes carrying out some additional tests were able illuminate the other components and make some interesting observations about how the control unit monitors the circuit and when it turns the power off. If the fault wasn’t so visually obvious and let’s say there was a semi-broken wire within the harness or a fault within the sensor we could have easily wrongly condemned the control unit due to its loss of sensor power supply. By disconnecting the wiring at either end it is easy to load test it and check for any short to ground.

We carried out some more tests on the other fully functional right-hand side. Within just over a quarter of a second of the sensor being unplugged the power is turned off (See Figure 2) and if the circuit is open or incorrect at the point the ignition is turned on it gives two brief pulses of voltage on the supply side whilst it monitors the current flow and the voltage on the signal wire (See Figure 3).

There are several different ways of testing this type of sensor/circuit arrangement and it often comes down to personal preference or the tools available. The circuit signal could be measured with a very low reading amps clamp. In this case it would need to be able to accurately measure current below 20mA as the signal switches between 7 and 14mA. There are some good ABS sensor simulators on the market that can be connected in place of the sensor to transmit a signal that can be observed via means of live data. If you just wanted to test the circuit without monitoring the signal you could use a decade box provided you know what resistance you require. Or if you have suitable test leads and connectors you could connect the front left wiring to the front right sensor – the ultimate sensor simulator.

Information and test data is easily available from several sources for cars like this but that is not always the case. Measuring and observing duplicate circuits on a vehicle to gather reliable test comparison data can be vital not only for a successful diagnosis but to learn how circuits behave so you’re fully prepared for the next one. I believe they call it ‘CPD.’ Every day really is as school day in this trade!


 

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