Code 25/26/27/28 - Excessive Wheel Speed Variation: Notes

Code 25 - Left Front Wheel
Code 26 - Right Front Wheel
Code 27 - Left Rear Wheel
Code 28 - Right Rear Wheel

As toothed ring passes wheel speed sensor, changes in magnetic field causes wheel speed sensor to produce an AC voltage signal whose frequency is proportional to wheel speed. The magnitude of the signal is directly related to wheel speed and proximity of wheel speed sensor to toothed ring, referred to as air gap. EBCM uses this signal to compare individual wheel speed data and to detect wheel lock-up.

Code can set when brake is off. This test detects a situation in which wheel acceleration or deceleration is beyond specified limits. Code will be stored, ABS will be disabled and ABS warning light will illuminate.

NOTE: Test numbers refer to numbers on diagnostic chart. Use only 10-megohm (minimum) DVOM for all voltage and resistance tests. For identification of circuits and terminals referenced in charts and text, see WIRING DIAGRAM .

Fig 1: Code 25/26/27/28 Diagnostic Flow Chart - 1 Of 3

Courtesy of GENERAL MOTORS CORP.

Checks if wheel speed sensor resistance is correct.
Ensures wheel speed sensor is not shorted to ground.
Ensures wheel speed sensor is not shorted to voltage.
Ensures code was not set due to poor connection at EBCM or physical damage to circuit.
Check for proper voltage at wheel signal low speed sensor harness connector.
Checks for proper voltage at wheel signal high speed sensor harness connector.