DTC 28

Fig. 119 Code 28: Wheel Speed Sensor Frequency Error.:

DTC 28: Wheel Speed Sensor Frequency Error:

TEST CONDITION(S)
1. Tested continuously.

FAILURE CONDITION(S)
1. While Driving: One wheel is detected at least 30 km/h or 19 mph faster than the remaining wheels for an extended period of time.
OR
2. One wheel is at least 6 km/h or 4 mph less than or greater than the remaining wheels or an extended period of time.

ACTION TAKEN
1. Code set.
2. ABS/TCS switched "OFF."
3. "ANTILOCK" indicator "ON."
4. "TRACTION DISABLED" message displayed.

Code 28 is set if the EBTCM cannot specifically identify which wheel speed sensor is causing the frequency error problem. If it can define the specific speed sensor causing the problem, the Sensor Fault code associated with the sensor (21, 25, 31, 35) will be set instead of Code 28.

TEST DESCRIPTION
Test numbers refer to circled numbers on the diagnostic chart.
1. Checks wiring and connections for problems. It is critical that this step is performed thoroughly, as wiring/connector problems are the most likely cause for this code.
2. Use the TECH 1 to monitor for vehicle electrical/electronic system "noise" being picked up by the speed sensor circuits.
3. Use the TECH 1 to monitor the wheel speed sensors while the vehicle is operating. If the TECH 1 Auto-Trigger Snapshot mode triggers, the speed sensor or the wiring and/or connectors are going intermittent during the road test.
4. Checks for proper resistance in the sensor itself.
5a.Replace rear wheel speed sensor as the likely cause of the triggering intermittent.
5b.Replaces front wheel speed sensor as the likely cause of the triggering intermittent.
6. Use the TECH 1 to monitor the wheel speed sensors while the vehicle is operating. If the TECH 1 Auto-Snapshot mode triggers, the EBTCM must be concluding there is a problem with wheel speed sensor frequencies when there is not.
7. Checks for a short to ground in the speed sensor input circuit wires. The circuit to be tested depends on which speed sensor triggered the TECH 1. The ground may be either a "hard" short to ground, or a resistive short. A short with a resistance less that 2 megohms, though not a hard short, can still cause Code 28 to set.

DIAGNOSTIC AIDS
Be sure the speed sensor wiring is properly routed and retained. This will help prevent false signals due to electrical noise being picked up by the wiring.

It is very important that a thorough inspection of the wiring and connectors be performed. Failure to carefully and fully inspect wiring and connectors may result in misdiagnosis, causing part replacement with reappearance of the fault.

Front Speed Sensors Only:
Check the toothed wheel for any large grooves, gouges, marks, etc. that might influence the tooth's signal at the wheel speed sensor. Also check for a buildup of foreign material in the gaps between teeth in the toothed wheel; this material may cause this fault.

A worn hub/bearing assembly may cause this fault in extreme cases; the bearing play allows the sensor-to-toothed-ring gap to change excessively. Never reuse a wheel seed sensor once it has been separated from the front hub/bearing assembly .