Antilock Braking System (ABS)

Fig. 353 Code 27: Left Rear Excessive Wheel Speed Variation (Part 1 Of 3):

Fig. 353 Code 27: Left Rear Excessive Wheel Speed Variation (Part 2 Of 3):

Fig. 353 Code 27: Left Rear Excessive Wheel Speed Variation (Part 3 Of 3):

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.

DTC Will Set When:
DTC 27 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 malfunction DTC is stored, ABS is disabled and the "ABS" warning lamp is turned "ON."

DTC Chart Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Checks wheel speed sensor for proper resistance value.
2. Ensures the wheel speed sensor is not shorted to ground.
3. Ensures the wheel speed sensor is not shorted to voltage.
4. Ensures that DTC 27 was not set due to a poor connection at the EBCM or physical damage of the circuitry.
5. Checks for proper voltage at the wheel signal low speed sensor harness connector.
6. Checks for proper voltage at the wheel signal high speed sensor harness connector.
7. Tests for a short between the wheel signal high and wheel signal low circuits.
8. Checks for an open in the wheel signal low circuit.
9. Checks for an open in the wheel signal high circuit.
10. Checks for proper resistance of the wheel speed sensor.
11. Isolates wheel signal high short to ground between rear ABS axle harness and dash/body harnesses.
12. Isolates wheel signal low short to ground between rear ABS axle harness and dash/body harnesses.
13. Ensures that the wheel speed sensor harness is not shorted to ground.
14. Ensures that the wheel speed sensor is not shorted to ground.

Diagnostic Aids:
An "intermittent" malfunction is most likely 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 warning 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). Drive vehicle above 24 km/h (15 mph) for at least 30 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,

Resistance of the wheel speed sensor will increase with an increase m sensor 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.

If wheel speed sensor harness shows evidence of water intrusion, inspect wheel speed sensor for water intrusion. If water intrusion is evident, replace wheel speed sensor.