Wheel Speed Sensor: Description and Operation
WHEEL SPEED SENSORSThe EBC 125 (RWAL) system uses only one speed sensor mounted in the rear axle for the rear wheels.
The EBC 325 (ABS) system uses one wheel speed sensor on each front wheel, and one mounted in the rear axle for the rear wheels.
The sensor measures the wheel speed by monitoring a rotating tone wheel. The signal generated by the sensor and tone wheel is transmitted to the CAB.
Each sensor has:
- A magnetic/coil pick-up (speed sensor) that is mounted to a fixed component
- An air gap between the tone wheel and the speed sensor assembly
As the teeth of the tone wheel move through the magnetic field of the sensor, an AC voltage is generated. This signal frequency increases or decreases proportionally to the speed of the wheel. The CAB monitors this signal to check for a sudden change in single or multiple wheel decelerations. If the deceleration of one or more wheels is not within a predetermined amount, the CAB takes control for antilock action through the HCU.
Diagnostically, the coils of the wheel speed sensors have different amounts of resistance based upon the location. When measured across the connector two terminals, the resistance should be:
Front sensor:
90 °C (194 °F) 2259 - 2761 Ohms
25 °C (77 °F) 1800 - 2200 Ohms
-40 °C (-40 °F) 1332 - 1628 Ohms
Rear sensor:
90 °C (194 °F) 2900 - 3600 Ohms
25 °C (77 °F) 1600 - 2300 Ohms
-40 °C (-40 °F) 1000 - 1400 Ohms
NOTE:For all resistance ranges add 30% to the value for extreme heat, subtract 30% for extreme cold.
On a EBC 325 (ABS) system each front wheel speed is monitored through the speed sensor mounted at the wheel end of the hub. On the EBC 125 (RWAL) and EBC 325 (ABS) systems the rear wheel speed is monitored through the speed sensor mounted in the rear axle assembly.
The CAB will disable antilock control, illuminate the amber ABS warning indicator circuit via the PCI BUS, and store diagnostic trouble codes if it detects a problem with any or all of the wheel speed sensors:
- Incorrect circuit resistance when checked with no vehicle movement
- Incorrect sensor output during vehicle movement
- Erratic sensor output during vehicle movement
DESCRIPTION
The ABS brake system uses 3 wheel speed sensors. A sensor is mounted to each front hub/bearings. The third sensor is mounted on top of the rear axle differential housing.
OPERATION
The Wheel Speed Sensor consists of a magnet surrounded by windings from a single strand of wire. The sensor sends a small AC signal to the CAB. This signal is generated by magnetic induction. The magnetic induction is created when a toothed sensor ring (exciter ring or tone wheel) passes the stationary magnetic WSS.
When the ring gear is rotated, the exciter ring passes the tip of the WSS. As the exciter ring tooth approaches the tip of the WSS, the magnetic lines of force expand, causing the magnetic field to cut across the sensor's windings. This, in turn causes current to flow through the WSS circuit in one direction. When the exciter ring tooth moves away from the sensor tip, the magnetic lines of force collapse cutting the winding in the opposite direction. This causes the current to flow in the opposite direction. Every time a tooth of the exciter ring passes the tip of the WSS, an AC signal is generated. Each AC signal (positive to negative signal or sinewave) is interpreted by the CAB. It then compares the frequency of the sinewave to a time value to calculate vehicle speed. The CAB continues to monitor the frequency to determine a deceleration rate that would indicate a possible wheel-locking tendency.
The signal strength of any magnetic induction sensor is directly affected by:
- Magnetic field strength; the stronger the magnetic field, the stronger the signal
- Number of windings in the sensor; more windings provide a stronger signal
- Exciter ring speed; the faster the exciter ring/ tone wheel rotates, the stronger the signal will be
- Distance between the exciter ring teeth and WSS; the closer the WSS is to the exciter ring/tone wheel, the stronger the signal will be
The rear WSS is not adjustable. A clearance specification has been established for manufacturing tolerances. If the clearance is not within these specifications, then either the WSS or other components may be damaged. The clearance between the WSS and the exciter ring is 0.005 - 0.050 inch.
The assembly plant performs a "Rolls Test" on every vehicle that leaves the assembly plant. One of the test performed is a test of the WSS. To properly test the sensor, the assembly plant connects test equipment to the Data Link Connector (DLC). This connector is located to the right of the steering column and attached to the lower portion of the instrument panel. The rolls test terminal is spliced to the WSS circuit. The vehicle is then driven on a set of rollers and the WSS output is monitored for proper operation.