C1221

Circuit Description
As a toothed ring passes by the wheel speed sensor, changes in the electromagnetic field cause the wheel speed sensor to produce an AC voltage signal. The frequency of the AC voltage signal is proportional to the wheel speed. The amplitude of the AC voltage signal is directly related to wheel speed and the proximity of the wheel speed sensor to the toothed ring. The proximity of the wheel speed sensor to the toothed ring is also referred to as the air gap.

Conditions for Setting the DTC
DTC C1221 can set when the vehicle is not in an ABS stop.
A malfunction exists if both of the following conditions occur:
- The left front wheel speed sensor input signal equals zero.
- The vehicle's reference speed is greater than 8 km/h (5 mph)

Action Taken When the DTC Sets
- A malfunction DTC stores.
- The ABS disables.
- The ABS warning indicator turns ON.

Conditions for Clearing the DTC
- The condition responsible for setting the DTC no longer exists and the Scan Tool Clear DTCs function is used.
- 100 drive cycles pass with no DTCs detected. A drive cycle consists of starting the vehicle, driving the vehicle over 16 km/h (10 mph), stopping and then turning the ignition OFF.

Diagnostic Aids
The following conditions may cause a malfunction:
- A poor connection
- Rubbed-through wire insulation
- A broken wire inside the insulation
- Wheel speed sensor physically damaged
- Wheel speed sensor input circuits shorted together
- Missing speed sensor ring
- Excessive wheel speed sensor gap
Use the enhanced diagnostic function of the Scan Tool in order to measure the frequency of the malfunction.
If the customer's comments reflect that the amber ABS warning indicator is ON only during moist environmental changes (rain, snow, vehicle wash), inspect all the wheel speed sensor circuitry for signs of water intrusion.
Thoroughly inspect any circuitry that may be causing the intermittent complaint for the following conditions:
- Backed out terminals
- Improper mating
- Broken locks
- Improperly formed or damaged terminals
- Poor terminal-to-wiring connections
- Physical damage to the wiring harness
Resistance of the wheel speed sensor will increase with an increase in sensor temperature.
Use the following procedure when replacing a wheel speed sensor or harness:
1. Inspect the wheel speed sensor terminals and harness connector for corrosion and/or water intrusion.
2. Replace the wheel speed sensor and jumper harness if evidence of corrosion or water intrusion exists.

Test Description
4. This test checks the wheel speed sensor for the proper resistance value.
5. This test ensures that the wheel speed sensor generates the proper voltage.
6. This test checks for a short between the wheel speed sensor high and low circuits.
10. This test determines if a wheel speed sensor circuit could be shorted when the EBCM is connected.