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Discussion Starter · #1 · (Edited)
I have quite a lot to report on ABS sensors and wheel hubs but this gift is a simple method to visualize the presence of magnetic 'stripes' or segments on an ABS wheel hub. A magnetic ring is fixed to the rotating hub at the rear and a closely spaced wheel speed sensor picks up magnetic flux changes to produce pulses processed by the ABS controller.

The ABS controller carries out two tests on the integrity of the wheel speed sensing system. At key on the controller looks for a 'normal' voltage on each sensor which should be 5V -- not 12V, zero or anything else. When the wheels are moving the sensor outputs are compared in real time. If a wheel pulse count is missing or one less than others, the ABS controller will fault. You can deduce that static tests at key on can be o.k but if the magnetic sensor ring or the tip of the sensor is damaged, a fault will be produced lighting up the ABS, traction control and tire pressure warnings.

Diagnostics will tell you which wheel is faulty and you then have to determine whether the fault is due to the sensor tip (ground away?), damaged magnetic stripes or most likely both. After much huff and puff you have a wheel hub in your hands and want that final confirmation before buying parts? I have my own methods using test equipment to count the number of wheel pulses detected for 360 degree rotation of the wheel, with the hub on the car. Diagnostics can give a live readout of each wheel speed which should be identical, but won't tell you if 1 of 41 segments is missing to trigger an ABS fault.

But there's another way that doesn't use test equipment once the wheel hub is removed: Take a sheet of paper that covers the outer ABS magnetic ring and fix it to metal parts of the wheel hub using double sided Sellotape. Do NOT stick tape to the sensor ring!

Use some synthetic magnetite (Ferric Oxide) and lightly brush over the paper around the magnetic ring using a fine sable blusher makeup brush, just as if you were dusting for fingerprints! I bought 100gm of the stuff which will do millions of ABS ring checks.

The location of the magnetic stripes will now show up. Carefully check around the pattern looking for any stripes missing. If you find gaps, mark their location on the side of the ring under the paper. You can fix your 'artwork' with lacquer spray or take a photo to reject it if faulty. Now remove the paper and inspect the suspect area you marked with a magnifying glass. Once you have confirmation you can order the correct size replacement hub and bearing kit. Some car models use wheel bearings with mag stripe paint silk screened on their bearings and require huge flypress pressure to fit them on a one way trip. Once fitted if an ABS warning is found, the new bearing is destroyed when removed. Much huff & puff and grief.

When you receive a new replacement hub, check the magnetic pattern before fitting. Some cheapos will supply them loose in polybags without protection over the ABS ring, which can be damaged in shipping. Always buy new hubs boxed with protection caps over the ABS ring and NEVER lever against it to pry out a stuck ABS sensor! You will have much grief, huff and puff if you have to go back and replace a new wheel hub again when the ABS fault lights don't go out! PS: Iron filings aren't fine enough to work as well.


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