The rear wheel speed sensor is part of the ABS system. It's actually the
ABS computer that provides vehicle speed data (as read from the rear wheel
sensor) to the PCM, which in turn drives the speedometer gauge. So, when
your RWSS starts flaking out, you not only get erratic speedometer function,
but the ABS computer sees a differential between the rear wheels and the
fronts, and sets a fault (which illuminates both lights).
I just fixed my 99 Dodge pickup of this aliment. The red brake is on all the
time with the yellow ABS light. The speedometer starts working when the
truck goes at least 15 mph. It was the sensor on top the the front housing
on the rear end. A $30 part, one nut holding it on, changed it out and poof,
all is well!
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