Hello, all.
I recently experienced an unusual diagnostic problem, this time on an ABS
system. This was on a Workhorse route truck of an account my employer services.
The vehicle runs the Meritor Wabco type D ABS, which in my experience is a
reliable system. Usually problems with sensor signals are due to bad sensors or
incorrect bearing adjustment. This vehicle had been in numerous times and we
had been challenged with a repeated appearance of the light. Codes kept
resetting for the rear sensor data improper or the circuits shorted or open.
The tone rings were badly corroded (no real surprise there), but changing
hub/tone ring assemblies and proper bearing adjustment failed to provide the solution.
The sensor voltages were checked by spinning the wheels in turn, and the
sensors were providing an adequate signal. On a road test, the left rear sensor
had a varying and intermittent rpm signal which never matched that of the front
signal, and the right rear sensor always read at 0 rpm. . Complicating the
problem was the fact that the truck was only reliably available on Saturdays.
Finally,
the truck came in with the right rear sensor always reading 71 rpm, no matter
the operating conditions. When the sensors were reconnected left for right, the
right sensor still read 71 rpm. Even with the sensor unplugged, the sensor
output 71 rpm with the truck stationary. Obviously the module had failed, so a new one
was ordered for the next weekend.
When the
part arrived, the tech first attempted to eliminate the possibility of a
multiple problem by monitoring sensor voltages at the module. The rear of the
truck was jacked up one side at a time so it could be run at idle. The green
plug to the module was disconnected and voltage was taken across pins 3 and 10,
then pins 6 and 7. The voltages were found to be steady and approximately the
same side-to-side, with each side arriving at the proper set of pins.
The module
was changed and the codes cleared, but a code immediately reset for a short or
open in the rear sensors. The vehicle was driven to check the rear signals
against the fronts, but the rear sensors both read at 0 rpm. At least the right
side was no longer reading as 71 rpm, so some progress was made. Again the green
module connector was unplugged and each sensor circuit was checked. The left
side was fine, but both pins for the right side were shorted to ground.
However, the resistance was several hundred Kohms, so we obviously had an
unusual problem. The whole wiring harness was visually checked, but all looked
fine.
When the
miships connector was reassembled, the short reappeared, so the problem was
clearly at the connector, somehow. The wires for that sensor came from the
module as red and black, then changed to brown and black at the midships plug
which made sense, as the sensors both have brown and black wires. The red wire
from the module connected to the black sensor wire, whereas on the left side,
the yellow wire from the module connected to the brown wire to the sensor. (see
the image)
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| ABS wiring for rear speed sensors on Workhorse truck |
Eureka! The right side wires were removed from the rear part
of the midships plug and their positions exchanged. Everything was buttoned
back up and the codes cleared. No code was reset and the vehicle was driven. At
10mph, the light went out and stayed out for a short test drive.
Apparently
the harness was assembled incorrectly in manufacturing. I don’t know why we
were sometimes able, at first, to get the light to go away for a short period
of time, or why the vehicle could have passed manufacturing inspection without
a light being set. The problem must have somehow fused that circuit in the
module, causing the indicated rpm to freeze at 71, unless someone somehow
applied a voltage to it that did the damage.
Thanks for viewing Bruce
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