A027

DTC A027 Left Rear Excessive Wheel Speed Variation (Part 1 Of 3):

DTC A027 Left Rear Excessive Wheel Speed Variation (Part 2 Of 3):

DTC A027 Left Rear Excessive Wheel Speed Variation (Part 3 Of 3):

DTC A027 Left 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 A027 can be set when the brake is off. The purpose of this test is to detect a situation in which the left rear wheel acceleration or deceleration is beyond specified limits.

Action Taken:
A DTC A027 is stored, ABS is disabled and the "ABS" indicator 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 test checks to see if excessive wheel bearing end play caused the DTC.
3. This step will identify a wheel speed sensor or circuitry that is damaged and visibly apparent.
4. This test checks wheel speed sensor for proper resistance values.
5. This test ensures the wheel speed sensor and sensor ring generate the proper voltage.
6. This step ensures the wheel speed sensor is not shorted to ground.
7. This step checks for proper voltages at speed sensor harness connector.
8. This test ensures that the wheel speed sensor circuitry is not internally shorted.
9. This checks for an open in CKT 884A or CKT 884B.
10. This checks for an open in CKT 885A or CKT 885B.
11. This test checks for a short to ground in both wheel speed signal circuits.
12. This step ensures that DTC AO27 was not set due to a poor connection between the EBCM connector C1 and the 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.

It the customer's comments reflect that the "ABS" indicator is on only during moist environmental changes (rain, snow, vehicle wash, etc.), all wheel speed sensor circuitry should be thoroughly inspected for signs of water intrusion. Use the following procedure:
1. Spray down the suspected area with 5% salt water solution (10 ml [2 teaspoons] of salt to 355ml [12 fl.oz.] of water).
2. Start vehicle and allow it to run for ten seconds.
3. If the DTC returns immediately, replace the suspected harness.

Any circuitry that is suspected of 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.

When measuring wheel speed sensor resistance, ensure vehicle is at room temperature (approximately 20°C [68°F]). 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 the wheel speed sensor harness.