240 ABS Light Came On (Again)

My 93 240 wagon (90K miles) ABS dash light came on again for the second
time. First time, two months ago, it was "fixed" with 90K servicing and new
brake pads. Service guy said when light comes on, the car has full brake
functionality but no ABS. I'm ok with this for the short term but want my
ABS back. Curious if anyone 1) has any comments on service guy's opinion
and 2) any thoughts on what causes ABS light to come on. Thanks.

 

2) IIRC dirty sensor wheels (behind each wheel) are a common cause of ABS
lights. Maybe somebody who actually knows what they're talking about can
expand on that <8^)

Mike

 

When you turn on the key the ABS control unit performs checks.
1. Measures the resistance of each wheel sensor
2. Measures the feed back voltage from the modulator pump
3. Measures the resistance of each of the control solenoids in the ABS
hydraulic unit.
4. If these pass it turns out the light
5. When the vehicle reaches a very low speed it operates the pump motor
for a few hundred milliseconds.
6. Once the vehicle speed exceeds a low number 3-5 miles per hour it
meausures the voltages from each wheel sensors and tests them for
accuracy. Among the three sensor signals there is a small tolerance
allowed for speed difference due to turning.
7. If the light has been on for some reason (spinning the wheels on ice,
running the car on a lift with wheels off the ground) and the speed
discrepancy disappears after 20 or so mph has been attained the the
light is extinguished.

The system relies on signal strength. All the cabling is twisted pair to
reduce attenuation. The sensors are simply a permanent magnet surrounded
by a coil fixed in proximity to a castellated ring attached to the stub
axle on the front wheels. Since the rear axle acts as essentially one
wheel the signal from the rear is simply the vehicle speed signal for
the speedometer modified to match the same frequency signal generated by
the front wheels. The wheel sensors generate an analog signal that is
clipped in the control unit to form an approximation of a square wave
digital signal that can be accurately counted.

When the brake pedal is applied and braking force rises to the point
where wheel slip is detected; one sensor stops reporting rotation
(lockup) or rotation slows proportionally quicker on one wheel
(predicting impending lockup) then the control unit rapidly fires the
solenoid for the offending hydraulic circuit which reduces the pressure
in that circuit to allow the wheel to speed back up and regain traction.
This rapidfire deapplication and reapplication of pressure is modulation
and it what trained drivers have done at high speed for years by rolling
pressure on and off the brake pedal based on feedback from the steering,
body motion in the seat and sound of the car slowing. A detectable
pumping is undetected since the leg motion required simply takes too
long.

As the brakes wear, iron filings collect in the wheel sensor.
Suspensions, body flex and gravity pull harnesses down into positions
different from installed locations. So we can see intuitively that
wiring can be abraded, signal strength can be degraded, and RFI can be
induced on the signal. On the mechanical side, failure to maintain
uncontaminated brake fluid in the system can cause solenoid valves to
stick due to inernal corrosion from the electrolysis of dissimilar
metals.

If the light stays on at startup, first check the surge protector fuse,
obtain the wiring diagram and then measure the resistance of each
circuit to determine which component is setting the light.

Hope this is useful.

Bob