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ABS Wheel Speed Sensor Testing
 
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Pelican Technical Article:

ABS Wheel Speed Sensor Testing

Nick Czerula

Time:

2 hours2 hrs

Tab:

$150

Talent:

****

Tools:

DVOM, backprobes, lab scope, MINI Scan tool, floor jack, jack stands, wheel chocks, safety glasses, torque wrench

Applicable Models:

R56 MINI Cooper Hatchback (2007-11)
R56 MINI Cooper S Hatchback (2007-11)

Parts Required:

ABS wheel speed sensor, Molykote Longterm 2 grease

Hot Tip:

Check for fault codes before testing

Performance Gain:

Repair faulty ABS sensor

Complementary Modification:

Properly route-wiring and use new clips if needed

The ABS wheel speed sensor monitors road wheel movement and sends an electronic signal to the ABS / DSC module. This signal is used to calculate road wheel speed to help with ABS braking and stability control features. Sensors are bolted to each wheel hub and are therefore susceptible to damage from impacts, road grime and brake dust.

MINI R56 models utilize magneto resistive style wheel speed sensors. This style of sensor allows accurate wheel speed reporting as soon as the wheel moves, unlike older style inductive sensors that required a speed of about 7 MPH to start working. This allows greater dynamic system control at lower vehicle speeds. This sensor type creates a high to low signal. The low signal is just above 9 volts, and the high about 12 volts. It is important to use a lab scope when testing these sensors, as this is the best way to display the digital signal created by the sensor.

When an ABS wheel speed sensor fails, the ABS / DSC module looses road wheel speed for the corresponding wheel and sets a fault code for the ABS wheel speed signal. When a fault code is set, the ABS / DSC warning light will illuminate. You will have to scan the module and read the fault codes to determine which speed sensor is the problem. Keep in mind that other problems can illuminate the ABS / DSC warning light. Be sure to scan and read fault codes before replacing parts.

You can use a scan tool while monitoring wheel speed signals to check for a slower or faster wheel. This can help to pinpoint a faulty sensor. Compare all four-wheel speed signals together. Look for one that doesn't match the others.

In this tech article, I will go over how to test individual sensors and remedy the related fault codes.

Remember that your car may have been serviced before and had parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you're working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

Our vehicle may vary slightly from yours as models do change and evolve, as they grow older. If something seems different, let us know and share your info to help other users. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

Lift the axle of the vehicle you are replacing the ABS sensor on. See our tech article on lifting your vehicle safely.

A faulty wheel speed sensor can set multiple brake warning lights (green arrows).
Figure 1

A faulty wheel speed sensor can set multiple brake warning lights (green arrows). It is important to check for fault codes using a MINI scan tool to identify the problem sensor.

It is important to check for fault codes using a MINI scan tool to identify the problem sensor.
Figure 2

It is important to check for fault codes using a MINI scan tool to identify the problem sensor. My subject vehicle has a fault code for the right rear and left front sensors -- one fault code pertaining to the circuit and one to the signal. This helps to identify the problem sensors.

Using the scan tool I checked static wheel speed.
Figure 3

Using the scan tool I checked static wheel speed. The left front sensor was stuck at 255. This indicates an open circuit (red arrow). I gave the left rear wheel a spin. It read 5 km/h (green arrow). This told me my scan tool was reporting current and valid data.

The front wheel speed sensor is mounted in the spindle (red arrow), at the top.
Figure 4

The front wheel speed sensor is mounted in the spindle (red arrow), at the top. It reads a toothed wheel on the wheel hub. I will show you how to test this sensor. The other four are similar.

Front sensor: To test the sensor we will have to access the electrical connector.
Figure 5

Front sensor: To test the sensor we will have to access the electrical connector. Working in the front wheel well, remove the Phillips head plastic screw (red arrow). Then remove the expanding anchor (inset).

Front sensor: Move to the bottom of the wheel well liner.
Figure 6

Front sensor: Move to the bottom of the wheel well liner. Remove the Phillips head plastic screw (red arrow). Then remove the expanding anchor. The green arrow points to the previously removed plastic screw and anchor.

Front sensor: Fold the wheel well liner (red arrow) away from the wheel well to expose the sensor connector (green arrows).
Figure 7

Front sensor: Fold the wheel well liner (red arrow) away from the wheel well to expose the sensor connector (green arrows).

Pull the ABS sensor electrical connector out of the holder.
Figure 8

Pull the ABS sensor electrical connector out of the holder. The sensor side of the connector is sealed (red arrow). You will have to backprobe on the control module side of the harness. Be careful when doing so. If you damage it, the harness will have to be replaced or repaired.

Rear sensor: Follow the sensor wiring to the body in front of the rear wheel well.
Figure 9

Rear sensor: Follow the sensor wiring to the body in front of the rear wheel well. Pull down on the small plastic shield (red arrow) for easier access. Pull the connector (green arrow) out of the holder for testing.

To test the sensor, gain access to the electrical connector.
Figure 10

To test the sensor, gain access to the electrical connector. Then install backprobes into each terminal (red arrows). Connect your DVOM to the backprobes and set it to DC Volts.

Turn the key on with the engine off.
Figure 11

Turn the key on with the engine off. Earlier we saw our scan tool displayed 255 (red arrow) for the left front wheel. This indicates an open circuit. With my DVOM connected it displayed battery volts (yellow arrow). This is the reading you will get if the circuit is open. Note the high voltage on my battery due to a power supply being connected. The rest of the wheels are displaying 0 km/h (green arrow). Try clearing the fault code. If the sensor reading returns to 0 km/h, test the signal to see if it drops out. Otherwise, replace the sensor with an open circuit.

I cleared the fault code and my left front sensor returned to 0 km/h.
Figure 12

I cleared the fault code and my left front sensor returned to 0 km/h. This tells me it could be an intermittent sensor fault. To test signal output, use the DVOM as previously instructed to connect it. Then slowly rotate the wheel for the sensor you want to test. This sensor type creates a high to low signal. The low signal is just above 9 volts (green arrow), and the high about 12 volts (yellow arrow). The voltage on your DVOM will switch between these numbers while rotating the wheel. It is important to use a lab scope when testing these sensors, for accuracy. However, I understand everyone doesn't have access to one. The DVOM test should help you.

Disconnect the DVOM and place a lab scope across the sensor terminals.
Figure 13

Disconnect the DVOM and place a lab scope across the sensor terminals. Then slowly rotate the wheel for the sensor you want to test. This sensor type creates a high to low signal. The low signal is just above 9 volts, and the high about 12 volts (red arrow). Using a lab scope, you can see the digital signal the sensor creates. If you do not have this signal, replace the sensor. Monitor the sensor over a long period of time for intermittent faults, looking for low or high voltage spikes, indicating a short or faulty sensor.

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