How's the speedometer behave, particularly below 20MPH? If it's also
erratic, the most likely cause is a failed rear wheel speed sensor. It's on
top of the rear differential, and comes out with a single bolt. It will
cost under $40 for a new sensor, and should take about 5 minutes to change
If the speedo. isn't erratic, it's still a posibility that the RWSS is
bad/performing poorly. It could also be a failed front wheel sensor, low
brake fluid, air in the ABS pump, an ABS pump/controller failure, etc. (all
this is assuming you have 4-wheel ABS). Unfortunately, the ABS system is
proprietary, and most shops don't have the equipment to diagnose it
(directly, anyway). That's something only a dealer can do.
On a '98, it's possible to disconnect two of the three wheel speed sensors
at a time, and use the speedometer to see if they're good. I've done this
before. The rear sensor disconnects on top of the rear differential. The
front sensors disconnect at a gray connector on top of each front inner
fender panel (follow the wire next to the brake lines coming off the front
calipers). Drive the truck with one sensor at a time hooked up. If one of
them fails to allow the speedometer to register, you've found the bad
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