PACCAR MX-13 SPN 191 FMI 9 - Speedometer erratic and shifting issues

Question

My 2018 Kenworth with a Paccar MX-13 is acting up under load. The speedometer is jumping all over the place and the Allison transmission is hunting for gears or going into a limp mode. Looking for help with the SPN 191 FMI 9 code that keeps popping up on my dash.

Sam Holloway · Accepted Answer

The issue was finally resolved by replacing the transmission output wire harness section (Part # K295-462-2). The internal wiring had suffered from heat soak and copper degradation about 6 inches back from the sensor plug. I also found the output yoke nut was slightly under torque, allowing the tone ring to vibrate; I retorqued the yoke nut to 600 lb-ft and the SPN 191 code hasn't returned in 10,000 miles.

Greg Walters · Answer

I've seen this happen when the charging system has a bad diode in the alternator. The AC ripple can interfere with the VSS signal. Try running the truck with the alternator belt off for a minute to see if the signal cleans up on your scan tool.

road_rambler · Answer

Check the wiring harness where it runs over the top of the bellhousing. Paccar trucks are notorious for the harness rubbing against the frame rails or zip ties being too tight, causing intermittent opens in the twisted pair communication lines.

Kevin Tran · Answer

Since you already swapped the sensor, you should check the air gap. If the tone ring is loose or if there is excessive play in the output shaft bearing, the sensor won't get a clean reading which triggers the FMI 9 abnormal update rate.

PACCAR MX-13 SPN 191 FMI 9 - Speedometer erratic and shifting issues

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