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.

Reply to
Bob
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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
Reply to
Michael Pardee

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

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Robert Dietz

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John Robertson

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