38 Wheel Speed Mismatch

NAME OF CODE:
38 Wheel Speed Mismatch


SET CONDITION:
The following criteria must be met for this trouble code to be detected:

1. Rotational wheel speed differing from the nominal for the vehicle for ANY of the following cases:
a. One mismatched speed more than double or less than half the other two.
b. All three speeds differing from each other by more than 8 percent (3 significantly different tire sizes).
2. Nominal speed greater than 12 MPH.
3. ABS inactive.
4. No extended rear sensor input acceleration exceeding 1.0g since the last time all speeds were approximately equal.
5. Criteria 1-4 satisfied for 24 consecutive controller checks spaced 0.5 second apart.


POSSIBLE CAUSES:
- One or more significantly different size tires installed on the vehicle
- Damaged or wrong number of teeth on tone wheel
- Disturbances that might momentarily mimic mismatched tire size include:
a. Driven wheel(s) spinning faster than vehicle speed
b. Differences in wheel speeds while turning
c. Rapid increase in apparent sensor frequency due to sensor/tone wheel vibration or EMI