Antilock Braking System (ABS)

DTC 21 RF Wheel Speed Sensor Circuit Malfunction:

CIRCUIT DESCRIPTION
As each wheel turns, the speed sensor for that wheel creates an AC voltage as the toothed ring rotates by the stationary sensor. The sensor consists of a permanent magnet and coil generating voltage by magnetic induction. The frequency and voltage of the induced signal is proportional to wheel speed.
The EBCM uses the speed sensor signal to calculate vehicle reference speed, individual speed, acceleration and slip values for each wheel which determine when antilock control is required.

DTC 21 WILL SET WHEN
There is a short to voltage or ground in CKT 872 or 833, or a malfunctioning speed sensor. The testing for this malfunction occurs when the vehicle is in motion; it will not set with the ignition "ON" and the vehicle at rest.

ACTION TAKEN
If a malfunction is detected which causes a speed sensor DTC 21 to set, antilock braking is disabled and the EBCM turns "ON" the ABS warning lamp 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 21 will be present.

IMPORTANT
^ The RF wheel speed sensor is located in the RF steering knuckle. In order to prevent electromagnetic interference from disturbing the speed sensor signal, the sensor wires are twisted together 9 times for every foot of wire.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Checks for a short to battery in either of the speed sensor circuit wires.
2. Checks for a short between the speed sensor circuit wires.
3. Checks for a short to ground in the speed sensor input circuit wire. 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 21 to set.
4. Checks if the DTC resets during a road test. If so, since tests 1-4 have validated that the circuitry and components are good, intermittent malfunctions are suspected.
5. Checks wiring and connectors for intermittents.
6. Replace the wheel bearing/speed sensor assembly, as it is likely the cause of a DTC reset experienced during the road test, since other portions of the circuit have checked out OK. If the DTC resets after sensor replacement, the EBCM must be concluding there is a malfunction present in the speed sensor circuit when there is not.
7. Ensures DTC was not set due to electromagnetic interference.

DIAGNOSTIC AIDS
Setting of speed sensor DTCs may be caused by improperly mounted sensors or improper wire routing. Verify that the sensor is properly mounted and free of foreign material such as metallic particles, dirt or grease. Verify that sensor wires are properly routed and secure. Improper RF wheel speed sensor wire routing may cause DTC 25 and/or DTC 35 to set. To aid in troubleshooting intermittent conditions the Tech 1 can be used while test driving the vehicle however, antilock braking will be disabled. Refer to the Tech 1 Brake Cartridge Operators Manual for additional information. If this does not identify the intermittent, wet the speed sensor harness on the underside of the vehicle and road test monitoring wheel speeds with the Tech 1.