DTC 28

DTC's 25, 26, 27, & 28 Excessive Wheel Speed Variation:

CIRCUIT DESCRIPTION
As a toothed ring passes by the wheel speed sensor, changes in the electromagnetic field cause the wheel speed sensor to produce an AC voltage signal whose frequency is proportional to wheel speed. The magnitude of this sig0nal is directly related to wheel speed and the proximity of the wheel speed sensor to the toothed ring, often referred to as the air gap.

DTC WILL SET WHEN
DTC 25, 26, 27, or 28 can be set when the brake is "OFF. The purpose of this test is to detect a situation in which one wheel acceleration or deceleration is beyond specified limits.

ACTION TAKEN
A malfunction DTC is stored, ABS/TCS is disabled and the "ANTILOCK" and "TRACTION OFF" Indicator lamps are turned "ON."

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This step identifies a wheel speed sensor or circuitry that is damaged and visibly apparent.
2. Checks to see that the wheel speed sensor coil is within proper resistance values.
3. Verifies that the wheel speed sensor can generate an appropriate output signal.
4. The J 39700 Pinout Box with the J 39700-25 adapter cable must be connected before this step is perfo0rmed. This check ensures that the wheel speed circuitry is not shorted together.
5. Ensures that the malfunction was not due to physical damage.

DIAGNOSTIC AIDS
Possible causes:
^ Wheel speed sensor has low or no output.
^ Excessive air gap, due to worn or damaged wheel speed sensor/bearing assembly.
^ Wheel speed sensor circuit has intermittent open.
^ Damaged tooth or teeth on wheel speed sensor ring.

An intermittent malfunction may be caused by a poor connection, rubbed through wire insulation, or a wire that is broken inside the insulation.

If the customer's comments reflect that the ABS or TCS warning lamps are "ON" only during moist environmental changes (rain, snow, vehicle wash), all wheel speed sensor circuitry should be thoroughly inspected for signs of water intrusion. If DTC is not current, clear DTCs and simulate the effects of water intrusion. Use the following procedure. Spray the suspected area with a 5% salt water solution (two teaspoons of salt to 12 oz. of water). Test drive vehicle over various road surfaces (bumps, turns, etc.) above 24km/h (15 mph) for at least 30 seconds. If DTC returns, replace suspected harness. If evidence of corrosion or water intrusion exists, replace wheel speed sensor jumper harness. Likewise, if replacing a wheel speed sensor jumper harness, inspect sensor terminals. If evidence of corrosion or water intrusion exists, replace wheel speed sensor.

The frequency of the malfunction can be checked by using the enhanced diagnostic function of the Tech 1.

Resistance of the wheel speed sensor will increase with an increase in sensor temperature.