Eaton UltraShift Plus SPN 1624 FMI 20 - Grinding while shifting

Question

My 2018 Peterbilt is throwing a code for the Input Shaft Speed Sensor calibration drift while I am under heavy load. The transmission seems to struggle finding the gear when downshifting, and I hear a slight gear clashing sound coming from the bell housing. It usually happens after the transmission has reached full operating temperature in stop-and-go traffic.

Megan Rivera · Accepted Answer

The issue was the Input Shaft Speed Sensor (Part # 5586100) and a worn pigtail harness. Replacing the sensor cleared the drift error, but the root cause was the pilot bearing beginning to seize, which caused slight input shaft oscillation. Replaced the sensor, the pigtail, and the pilot bearing during a clutch swap. Total parts cost was around $1,100 including the new ECA clutch kit, and the truck is shifting smooth at 1100 RPM again.

Carlos Diaz · Answer

Check the resistance of the input shaft speed sensor itself. If the sensor is failing internally due to heat soak, it will report incorrect data that the TECU sees as a calibration drift rather than a total circuit failure.

obd2_owl · Answer

Verify your battery voltage under load. These Eaton TECUs are extremely sensitive to low voltage; if your alternator or batteries are weak, the computer can't process the high-speed frequency signals from the speed sensors accurately.

dieselmike88 · Answer

You need to pull the sensor and look for metal fuzz on the magnetic tip. If you have significant debris, it can distort the sine wave signal and trigger that FMI 20 code because the timing pulses don't match the engine RPM logic.

Eaton UltraShift Plus SPN 1624 FMI 20 - Grinding while shifting

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