CARspec recently had a 2007 Toyota Highlander (front wheel drive) in the shop, the owner concerned over the ABS, TRAC, and VSC lights coming on intermittently. The light were on when the vehicle arrived and CARspec set to diagnosing the concern. The codes out of the VSC/ABS module pointed to malfunctions out of both rear wheel speed sensors. The technician cleared the malfunction codes and took the vehicle out for a drive while monitoring the data out of the suspect rear sensor plotted against those of the front. During the drive all the wheel speed sensors performed flawlessly.
Unable to confirm the problem, the technician performed a visual inspection of the wheels, suspension, and general condition of the rear ABS system. Upon removing the rear wheel speed sensor harness connector, the corrosion on the rear speed sensor terminals was apparent. More common on the Scion xB than the Highlander, the technician removed both rear harness connectors for a full view of the terminals:
Above are the terminals removed from the plastic connectors. Much like the Statue of Liberty, the copper terminal connectors were green from corrosion. The corrosion is conductive, growing like mold out of the terminals and intermittently causing both rear ABS signals to short. Further inspection showed the corrosion had infiltrated the left rear sensor terminals itself (not just the harness as picture above). As such, both rear wheel speed sensor harnesses required replacement in addition to the left rear wheel bearing/hub/sensor assembly. While the rear wheel speed sensor is available as a separate part, replacement requires you remove the entire wheel bearing assembly to gain access to the ABS sensor. Here in MN, rust gets into the rear suspension to where removing the wheel bearing/hub/sensor destroys the wheel bearing. Any auto mechanic will tell you the same thing – rust makes everything worse. After much use of the torch, completely removing the rear parking brake, and heavy use of an air hammer the bearing looked like this:
You can see in the above the rust that lines the collar of the bearing just inside of the mounting flange. Not an easy removal. After everything was replaced, the technician confirmed the rear sensors were fully functional and the vehicle was returned to service.