Antilock Braking System (ABS)

DTC 28 Right Rear Excessive Wheel Speed Variation (Part 1 Of 2):

DTC 28 Right Rear Excessive Wheel Speed Variation (Part 2 Of 2):

DTC 28 Right Rear 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 a sinusoidal (AC) voltage signal whose frequency is proportional to wheel speed. The magnitude of this signal 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.

FAILURE CONDITION
DTC A028 can be set when the brake is "OFF." The purpose of this test is to detect a situation in which the right rear wheel acceleration or deceleration is beyond specified limits.

ACTION TAKEN
A malfunction DTC is stored, Antilock Braking System (ABS) is disabled and the ABS indicator lamp is turned "ON."

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This test verifies whether the malfunction is currently present.
2. This step will identify a wheel speed sensor or circuitry that is damaged and visibly apparent.
3. This test checks wheel speed sensor for proper resistance values.
4. This test ensures the wheel speed sensor and sensor ring generate the proper voltage.
5. This step ensures the wheel speed sensor is not shorted to ground.
6. This step checks for proper voltages at body to ABS and fuel sender harness connector.
7. This test ensures that the wheel speed sensor circuitry is not internally shorted.
8. This checks for an open in the wheel signal high circuit.
9. This checks for an open in the wheel signal low circuit.
10. This test checks for a short to ground in both high and low wheel signal circuits.
11. This step ensures that DTC A028 was not set due to a poor connection between the world connector and Electronic Brake Control Module (EBCM).

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

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

If the customer's comments reflect that the ABS amber indicator lamp is "ON" only during moist environmental changes, (rain, snow, vehicle wash) all wheel speed sensor circuitry should be thoroughly inspected for water intrusion. Use the following procedure. Spray down the suspected area with 5% salt water solution (two teaspoons of salt to 12 oz. of water). Start vehicle, allow vehicle to run for 10 seconds. If DTC returns immediately, replace suspected harness.

Any circuitry, that is suspected as causing the intermittent complaint, should be thoroughly checked for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wiring connections or physical damage to the wiring harness.

Excessive end play can cause this DTC to set. Check integral bearing for excessive axial end play,

When measuring wheel speed sensor resistance, ensure vehicle is at room temperature. Wheel speed sensor resistance will vary with temperature.

When replacing a wheel speed sensor, inspect the sensor terminals and harness connector for corrosion and/or water intrusion. If evidence of corrosion or water intrusion exists, replace wheel speed sensor harness.