Paccar MX-13 SPN 191 FMI 9 - Transmission Output Shaft Speed Signal Lost

Question

My 2018 Peterbilt 579 is throwing a derate warning and the speedo just dropped to zero while I was cruising at 65. The Allison 4000 series transmission is shifting very hard into gear and it feels like it's stuck in a limp mode. Checked the dash and I'm seeing an active SPN 191 FMI 9 code on my DAVIE4 scan tool.

obd2_owl · Accepted Answer

Found the issue after back-probing the harness. There was a corroded wire in the P94 connector near the frame rail on the driver's side. Cut out the green crusty section, soldered in a new 18-gauge lead with heat shrink, and replaced the Output Shaft Speed Sensor (Part #29543432) just to be safe. Torque for that sensor is tiny, only 18-22 lb-ft. Reset the codes and the speedo came back instantly.

fleet_chief_42 · Answer

You need to ohm out the output speed sensor itself. It should show between 200 and 500 ohms; if it's open, the sensor is toast. Also, make sure there isn't metal debris on the magnetic tip of the sensor from a failing bearing.

Sarah Thompson · Answer

Check the wiring harness where it runs over the top of the bellhousing. Those Paccar looms are notorious for rubbing through against the fire wall or the transmission air lines, which will drop the CAN signal to the TCU.

shoptech_carlos · Answer

Since you have FMI 9, it means the transmission controller is expecting an update that isn't arriving. If the wiring checks out, check the gap on the tone ring. If that ring is loose on the shaft, you'll get erratic signals before it fails completely.

Paccar MX-13 SPN 191 FMI 9 - Transmission Output Shaft Speed Signal Lost

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