The wheel speed sensors are magnetic pickup assemblies. When a gap between two teeth on the tone ring passes by the sensor, it disturbs the magnetic field which creates a pulse of voltage that is sent to the computer. All four sensors are expected to produce the same number of pulses per wheel revolution. When the brakes are applied, the computer starts watching the four wheel speeds very closely. If one is going slower than the other three, the computer interrupts the flow of brake fluid to that wheel until it picks up speed again. The pressure to the brake on any wheel can be pulsed on and off up to 30 times per second.
On most Chrysler products, the front tone rings are on the outer cv joints next to the wheel and can be seen if you crawl underneath the car or have the wheel removed. You have to rotate the axle by hand to inspect the ring all the way around for a crack. When one is cracked, the sensor picks it up as an extra pulse per tire revolution. During braking, that makes it look to the computer as though the other three wheels are going slower and are about to lock up and skid. The computer will reduce the braking force to those three wheels in a misguided attempt to speed them up to match the one that appears to be turning faster.
Sometimes water gets under the tone ring, freezes and expands, causing the ring to break. Sensors can also develop problems but most of the time that will be detected when the computer performs a system check right after engine startup. Other things are monitored while driving, and still other things are monitored only when the brake pedal is pressed. When the computer detects an electrical problem, a loss of one wheel speed signal, or it doesn't see the expected response after actuating a valve, it will memorize the appropriate fault code, disable the system, and turn on the warning light. The codes will point the mechanic to the circuit or system with the problem that needs further testing.
Wednesday, September 8th, 2010 AT 8:15 PM