Antilock Braking System (ABS)

DTC 28 Wheel Speed Sensor Circuit Frequency Malfunction:

CIRCUIT DESCRIPTION
The toothed wheel generates a voltage pulse as it moves past the sensor; each tooth-gap-tooth series on the wheel generates these pulses. The frequency of these pulses is used by the EBCM to determine wheel speed. The amount of voltage generated in each pulse depends on the air gap between the sensor and the toothed wheel, and on wheel speed.

DTC 28 WILL SET WHEN
The EBCM cannot specifically identify which wheel speed sensor is causing the frequency error malfunction. If it can define the specific speed sensor causing the malfunction, the sensor DTC associated with the sensor (21, 25, 31, 35) will be set instead of DTC 28.

ACTION TAKEN
Antilock braking is disabled. The ABS warning lamp will be "ON" for the remainder of the ignition cycle. If the malfunction is intermittent, the EBCM will enable the system at the next ignition cycle and a history DTC 28 will be present.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
The ABS warning lamp will be turned "ON" and ABS will be disabled.
1. Checks wiring and connections for malfunctions. It is critical that this step is performed thoroughly, as wiring/connector malfunctions are the most likely cause for this DTC.
2. Uses the Tech 1 to monitor for vehicle electrical/electronic system "noise" being picked up by the speed sensor circuits.
3. Uses 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. Ensures malfunction is not due to suspected wheel speed sensor.
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 (partial) short. A short with a resistance less than 2 megohms, though not a "hard" short, can still cause DTC 28 to set.
8. Ensures DTC was not set due to electromagnetic interference.

DIAGNOSTIC AIDS
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 malfunction.
A worn hub/bearing assembly may cause this malfunction in extreme cases; the bearing play allows the sensor-to-toothed-ring gap to change excessively.