Wheel-Speed and Output-Speed Mismatch on a Retarder-Equipped Bus
A 60-foot articulated bus with a retarder-equipped automatic transmission reports intermittent ABS/traction warning lamps and a driveline shudder during low-speed turns. The transmission controller logs a ratio error between engine speed and output speed that appears only below 10 mph. Wheel-speed sensors were replaced on the drive axle last month after a corrosion complaint. Output shaft speed sensor resistance is in spec. The retarder was serviced two months ago. Tire size on the drive axle was changed from 275/70R22.5 to 305/70R22.5 during the same period, and the axle ratio parameter in the transmission controller still shows the original value. The shudder occurs when the inter-axle differential is unlocked.
1. Explain how the tire change and axle-ratio parameter interact to produce a ratio error and a low-speed shudder, and identify which is the root cause.
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The chain of events:
- The 305/70R22.5 tire is larger in diameter than the 275/70R22.5. That changes the effective rolling radius, so for a given output-shaft speed the vehicle now travels slightly faster (or the shaft turns slightly slower per unit distance).
- The transmission controller calculates ratio from engine speed vs. output-shaft speed and compares it to the programmed axle ratio and tire size. If the axle-ratio parameter still reflects the old tires/ratio, the calculated ratio will not match the expected ratio - especially at low speed where resolution is coarse - and the controller flags a ratio error.
- The low-speed shudder during turns with the inter-axle differential unlocked is consistent with driveline wind-up from mismatched effective tire diameters between axles (if only the drive axle was changed) combined with the controller's confused speed calculations and retarder interaction.
Root cause: the configuration parameter was not updated when the tires changed. The ratio error is a configuration/calibration fault, not a sensor fault. The shudder is a secondary effect of mismatched tire circumference across the vehicle plus controller misbehavior.
Judgement: sensors and wiring were already replaced and are in spec - stop chasing hardware. Verify tire circumference on all axles and update the controller's tire-size and axle-ratio parameters to match the actual hardware. If tires of different sizes remain on different axles, that must be corrected because it causes real wind-up and differential wear.
2. What would you check to confirm the shudder is driveline wind-up rather than a transmission internal fault?
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Confirm wind-up:
- Measure loaded rolling circumference on each axle's tires (not just sidewall size). A difference of more than about 1/4 inch in circumference between axles is enough to cause wind-up on a locked inter-axle differential.
- Road test: does the shudder disappear or change when the inter-axle differential is locked vs. unlocked? Wind-up typically appears when locked and a true internal fault persists in both.
- Check differential oil for heat/discoloration after a loaded run - wind-up generates heat.
- Verify the inter-axle differential lock is actually disengaging (air supply, solenoid, switch) - a partially engaged lock mimics wind-up.
Rule out transmission internal fault:
- Compare the ratio error with the vehicle stationary vs. rolling; an internal clutch/gear fault usually produces a ratio error at all speeds, not only below 10 mph.
- Check for metal in transmission fluid and inspect the retarder circuit, since it was recently serviced.
- If ratio error persists after parameters are corrected and tire circumferences are matched, then look inside the transmission.
Judgement: match tires and correct configuration first; only open the transmission if the error survives a correct baseline.