DTC 28 - Wheel Speed Sensor Frequency Error: Notes

Fig 1: Wheel Speed Sensor System Schematic

Courtesy of GENERAL MOTORS CORP.

Toothed wheel generates a voltage pulse as it moves past sensor. EBTCM uses these pulses to determine wheel speed. EBTCM uses wheel speed sensor signal to calculate vehicle reference speeds and speed, acceleration and slip values for each wheel. These values are used to determine when anti-lock or traction control is needed.

DTC 28 will set when one wheel is detected at least 19 MPH faster than the remaining wheels for an extended period of time.

DTC 28 will set when one wheel is at least 6 MPH less than or greater than the remaining wheels for an extended period of time. DTC 28 is set if the EBTCM cannot specifically identify which wheel speed sensor is causing the frequency error problem.

Fig 2: DTC 28 Diagnostic Flow Chart

Courtesy of GENERAL MOTORS CORP.

1. Checks wiring and connection for problems.
2. Use Tech 1 to monitor for vehicle electrical system noise pick-up by speed sensor circuits.
3. Use Tech 1 to monitor wheel speed sensors while vehicle is operating.
4. Checks for correct resistance in sensor. 5a) Replaces front wheel speed sensor as probable cause of intermittent. 5b) Replaces rear wheel speed sensor as probable cause of intermittent.
5. Checks if DTC resets during test drive.
6. Checks for short to ground in speed sensor input circuit wires. Circuit to be tested depends on which speed sensor triggered Tech 1. A short with a resistance of less than 2 megohms, though not a hard short, can still cause DTC 28 to set.