The Eaton 3 pedal Auto shift transmission in a Volvo truck refused to go into gear, requiring a tow to the shop. Initially, the service light was on, accompanied by shift rail error codes. The old transmission wiring harness was replaced with an updated Eaton version. While this was a necessary update, it didn’t resolve the shifting problem.
Next, the XY shifter was replaced with a new, genuine Eaton part, using the correct part number. This resulted in the display changing from dashes to a solid “N” for neutral. However, the keypad display flashed in “D” mode while the main display remained a solid “N” without any up or down arrows. Despite multiple calibrations of the new XY shifter following Eaton’s instructions, the problem persisted.
To further diagnose the issue, the XY shifter was removed and tested. It cycled correctly – side to side, forward and back, and then to center. Reinstallation and recalibration didn’t change the symptoms. No new codes were present, and all previous codes showed as inactive.
The input speed shaft sensor was tested and initially showed a reading of 2k ohms. However, the input shaft itself wasn’t turning when the clutch pedal was depressed. Suspecting a weak magnet, the input speed shaft sensor was replaced, but the solid “N” with no arrows persisted. The brake lights are functioning correctly, and according to Volvo, this particular truck model only utilizes one brake light sensor. Consultation with Eaton provided minimal assistance.
This troubleshooting process has eliminated the wiring harness, the XY shifter itself, and the input speed shaft sensor as the root cause. The focus now shifts to other potential culprits, such as issues within the transmission’s internal components, the valve body, or the transmission control module (TCM). Further diagnostics, potentially involving specialized tools and software, will be necessary to pinpoint the exact problem within the Eaton transmission.
