15 Apr 2024

A month in the life of a vehicle technician

Part 12
A “Superb” opportunity for Frank to cover the importance of accurate voltage and current measurement recently presented itself

By Frank Massey


In this issue I intend to discuss recent challenges in the workshop, focusing on the function of the systems under investigation and the options of test technique.
Antilock braking systems have been with us for many years. Following on from electronic ignition and fuel delivery, it was the third most common system to be fitted.

Relatively reliable and with little or no prospect of repair, the diagnosis is the primary challenge as replacement parts are expensive. As I write this piece, we have an Audi A6 in the shop requiring a replacement ABS control module with the OEM price of £2,200! The most common system failures are the wheel speed sensors. They share a much more integrated responsibility in current chassis systems. The functional output is wheel rotation frequency. This is achieved by inductive or active sensor technology.
The output is shared for multiple chassis systems; Braking, tyre pressure monitoring, vehicle speed indication, traction control, hill start, Stop/Start, navigation, and dynamic stability.

Passive sensors are comprised of a coil around a permanent magnet. The principles of induction are met by ferrous tone or phonic wheel attached to the rotating part of the road wheel system. Voltage is generated proportional to the magnetic field strength, air gap, rotation speed and rotor shape. They do not require a voltage supply. The output is based on a modified sine wave (AC).

Sensor performance
The rotor condition is vital to the sensor performance. Physical damage, corrosion, and air gap changes are system-critical. BMW 1 Series drive shaft corrosion is example.
Active wheel speed sensors fall into two types; Hall effect or magneto resistive principles. The Hall effect diverts the hall IC voltage when influenced by a ferrous or magnetic encoded rotor. This produces a digital waveform image.

Magneto resistive sensors use magnetically encoded rotors. These may have duel offset segments allowing for not only frequency but a directionally sensitive output. The magnetic segments cause a change in resistance when passing the semiconductor. More accurate and with range improvement, both offer a more reliable performance not easily affected by external influences. They both require a power supply; Many vehicles suspend the voltage supply when a fault or disconnection event takes place and will not reinstate it until the DTC is cleared.

There are several options for testing wheel frequency irregularities. I deliberately avoided the word failure as my actual workshop study will demonstrate.

Diagnostic assessment
A Skoda Superb was already on my ramp awaiting a diagnostic assessment with faults in PCM, ABS, nav, EPS, and TFA. I focused on ABS as that was the prime fault request. Two DTCs were logged; 00287 right rear wheel sensor, 01325 tyre pressure monitoring.
As the car was on the ramp the most obvious test was to look at wheel speed live data. I would normally do this on the road to compare all 4-wheel speeds. The output was erratic, dropping in and out as the wheel was rotated smoothly. This convinced me that the fault was the bearing and encoder disc not sensor or wiring. I set up a training exercise for the workshop staff. Using my Pico scope, I made connection with break out patch leads.

Extensive rusting
Please refer to Fig.1 and Fig.2. Attention should be drawn to extensive rusting of the general area. The next step was scope set-up; The ABS output is found on the 12v power supply. However, the amplitude is discrete, measured in millivolts, approximately 400mv peak to peak. This would normally be difficult to observe even with the zoom feature. So, I set the input coupling to AC, and 1v range. This removed the 12v DC value showing the sensor AC value. Please refer to Fig.3. Note AC coupling, voltage range and sweep time.
You will observe the output was only present for approximately 50% of the rotation. This is proof positive of a mechanical fault. Stripping out the hub I could now show the magnetic encoder disc worn away by mechanical conflict with the bearing dust cover. I’m hoping those who don’t use a scope can see from this example the true power and accuracy it provides in a diagnostic assessment. The waveform matches exactly the mechanical condition of the encoder disc, before the decision to strip out was made. Please refer to Fig,4 and Fig.5.

I was able to remove the wheel sensor with a round punch. As it is often the case, that corrosion had also gripped the sensor in the hub assembly. A new hub and bearing assembly were fitted with a new wheel sensor. Tyre pressures correctly adjusted and set. The remaining system faults were cleared.

As a further training opportunity, I demonstrated a comparison between voltage and current measurement. This can only be done with a special low range hall effect current clamp 0-500ma. The range conversion is 10mv=1ma.Please refer to Fig.6. The DC voltage = 185mv DC. The current 0.95ma.


Fig 2
Fig 3
Fig 4
Fig 5
Fig 6