1996 Lexus ES 300 Anti-lock brakes activates everytime befo

Tiny
VCHAHAL
  • MEMBER
  • 1996 LEXUS ES 300
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 193,000 MILES
Whenever the car comes to a complete stop, just about at the slowest spped (about 5 - 10 mph), the anti-lock brakes engage. I have showed it to the mechanic and he has checked all four brake sensors. He said the problem is somewhere else.
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Thursday, April 15th, 2010 AT 10:46 AM

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Tiny
CARADIODOC
  • EXPERT
Hi vchahal. Welcome to the forum. Check the tone rings for cracks, in particular on the front.

Caradiodoc
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Thursday, April 15th, 2010 AT 12:17 PM
Tiny
VCHAHAL
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Please explain what are tone rings and how do these look like?
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Thursday, April 15th, 2010 AT 1:23 PM
Tiny
CARADIODOC
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Tone rings are the toothed wheels around the cv joint, hub, brake rotor, or drum that rotates with the wheel and tire. The sensor is a coil of wire wound around a magnet. Every time a tooth on the tone ring passes by the magnet, it disturbs the magnetic field. That "induces" a pulse of voltage into the coil of wire. It's those pulses that are counted by the Antilock Brake Computer and compared to the other three wheels to determine when one wheel is slowing down too quickly. When a tone ring cracks, it creates an extra pulse per tire revolution. That makes it look like the other three wheels are turning slower so the computer kicks in to reduce braking to them.

Some of the clues to help lead you to the problem include: If the ABS light comes on right after starting the engine, and before moving the car, suspect a sensor electrical problem or hydraulic actuator problem that was detected during the system self-test. If the warning light comes on only after the car has been moving, suspect a sensor mechanical problem causing it to read a wheel speed different than the other three wheels. This can include metal filings on the sensor's magnet, a cracked tone ring, or a completely cracked off and missing tone ring. These usually happen when water gets under them and freezes in winter causing them to crack. (Or in the case of one's mother getting her Grand Caravan stuck in the snow and spinning the wheels until the already-cracked tone ring breaks into a bazillion pieces)!

I've welded tone rings on my vehicles in the past, but only to prove to my students it is possible to weld cast iron. I've also replaced them, but you can't buy new ones. I remove them from half shafts in the salvage yards, then glue them in place with silicone RTV gasket sealer to keep the water out.

When the light does not come on, the computer accepts the readings it is getting from the sensors but is responding to extra pulses from one of them. The cracked tone ring, again, is the most common cause. If the brake pulsation continues after the vehicle is stopped, something is causing the pulses to continue. Misrouted spark plug wires can induce pulses magnetically into ABS sensor wires if they are too close together. That will make the computer think one wheel is still turning and the other three should be also. This is somewhat common on Ford trucks. Haven't really heard of it happening in other vehicles.

The ABS system should only be activating when you apply the brake pedal. If you feel it activate when the pedal is not pressed, suspect a computer problem or the pump is rebuilding used up hydraulic pressure. To determine if the pump is what you're hearing, turn the engine off, pump the brake pedal until it feels hard, then turn the ignition switch on. The pump should run for up to 20 seconds and the power assist should come back.

Caradiodoc
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Thursday, April 15th, 2010 AT 2:01 PM
Tiny
VCHAHAL
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My mechanic had checked all four wheels for cracks in tone rings as he had hoisted up the car and systematically examined near the wheels but could not determine the real cause.

The pulsation due to anti-lock braking at slow speeds occurs only when the brake pedal is pushed.
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Thursday, April 15th, 2010 AT 2:24 PM
Tiny
CARADIODOC
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The next thing would be to connect a hand-held computer called a scanner to the car and view the sensors in real time. That might require finding a dealership or a different shop with a scanner that can access the ABS computer. By federal law all manufacturers must make their proprietary information available to aftermarket test equipment manufacturers only if it is emissions-related. These manufacturers have to figure out the other systems over time. For that reason, not all equipment can do everything on your car that the dealer's equipment can. Most independent repair shops can't afford to buy the same equipment the dealers use because unlike the dealer that needs to cover one brand of car, independent shops work on up to 30 or more different brands. They have to spend their money carefully on equipment that will work on as many brands as possible, but the trade-off is it won't do everything.

On the other hand, if your mechanic was already using a scanner that was able to access your ABS computer, he should be able to watch the sensors too. I don't mean to imply he doesn't know what he is doing, but one other common thing that can be overlooked is if there is one tire on the car that is a different size than the other three. That will make it spin at a different rate and cause the ABS to kick in. That would show up on the scanner readings.

Caradiodoc
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Thursday, April 15th, 2010 AT 3:01 PM

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