The most vulnerable part of a wheel bearing hub unit with integrated wheel speed sensor is the connector. Regardless of the design, the integrity of the circuit comes down to the seal in the connector. If the seal is not replaced and properly seated, water and corrosion can cause an intermittent problem with the wheel speed sensor signal.
The most common failure is the foam or rubber seal falling out of the female connector during installation.
SKF has released a tech tip on the subject in their August Pole Position e-newsletter.
Preventing ABS sensor failure in pigtail style hub assemblies
“Make sure the weather pack has not fallen out of the harness. Replace if missing. Without the weather pack, water intrusion will occur damaging both the ABS sensor on the hub and the harness side wiring and connector.”
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BMW in has released its own Technical Service Bulletin (TSB) on the topic:
SEAL ON 2- AND 3-PIN PLUG CONNECTORS
When connecting or disconnecting 2- or 3-pin plug connectors on any BMW model, it is possible for the rectangular seal to fall out of the plug housing. If this should occur, the loss of the seal can easily go unnoticed. See Fig. 5.
In addition to preventing moisture from seeping into the plug, the seal also ensures that the two halves of the plug are properly tensioned so that the contacts do not move within the plug. If such movement should occur, the contacts may develop corrosion.
Corroded and/or loose contacts can cause problems with the proper functioning of the vehicle, and may cause difficulty with diagnosis due to the possible intermittent nature of such problems.
Service Procedure:
It is essential that you check that the seal is in place before connecting a 2- or 3-pin plug connector. If the seal is missing, it must be replaced.