Truck Fault Codes

Eaton UltraShift SPN 191 FMI 8 �?Output Shaft Speed Sensor Erratic

SPN 191 FMI 8 on an Eaton UltraShift means the output shaft speed sensor is producing an erratic or abnormal frequency signal. Check for a damaged sensor tone wheel, a sensor that has moved out of position, or wiring harness damage. Use Eaton ServiceRanger to monitor the sensor output in real time.

Code Details

Structured details for SPN 191 FMI 8
Display codeSPN 191 FMI 8
SPN191
FMI8
OEM codeNone listed
ManufacturerEaton
SystemTransmission �?speed sensing
ComponentOutput shaft speed sensor
Source addressUnknown or not applicable
Severitymedium
Review statussource backed
Source confidencemedium
Last reviewed2026-06-11

Plain-English Meaning

The output shaft speed sensor tells the Eaton UltraShift transmission how fast the output shaft �?and therefore the vehicle wheels �?are rotating. This information is used to calculate vehicle speed, determine shift points, and manage clutch engagement. When the sensor produces a noisy, erratic, or interrupted signal, the TCM cannot calculate speed accurately and may make incorrect shift decisions.

SPN 191 is the Transmission Output Shaft Speed parameter in J1939. FMI 8 indicates an abnormal frequency (erratic, too-high frequency, or noisy signal). On Eaton UltraShift transmissions, the output shaft speed sensor is a magnetic or Hall-effect sensor reading a tone wheel on the output shaft. A sensor that has shifted position, a damaged tone wheel with missing or cracked teeth, or wiring damage causing noise in the signal can all produce FMI 8. ServiceRanger displays the actual output speed value, which can help distinguish signal noise from a sensor positioning issue.

Common Symptoms

  • Erratic or unexpected shifts in the UltraShift
  • Transmission may not shift out of a gear at expected vehicle speed
  • Speedometer reading may be erratic if it sources from the transmission
  • Active fault in Eaton ServiceRanger

Possible Causes

Possible causes may include the items below. The list is not a parts diagnosis.

  • Output shaft speed sensor physically moved or loose in its mounting location
  • Damaged tone wheel with missing, cracked, or corroded teeth
  • Sensor wiring harness damaged causing signal noise
  • Metallic debris on the sensor face from a bearing or gear failure
  • Internal sensor failure

First Checks

  • Connect Eaton ServiceRanger and monitor output shaft speed in live data while rolling the truck at a known speed �?erratic readings confirm the fault
  • Inspect the output shaft speed sensor mounting and air gap �?reposition or reseat the sensor if it has moved
  • Inspect the tone wheel for damaged, missing, or corroded teeth through the sensor port
  • Inspect the sensor harness for chafing, damage, or connector corrosion
  • Check for metallic debris on the sensor face that may indicate a bearing or gear issue

Can I Keep Driving?

An erratic output speed signal can cause the UltraShift to make incorrect shift decisions �?unexpected upshifts, downshifts, or shift delays. The transmission may also enter a reduced-function mode if it cannot determine vehicle speed accurately. Diagnose promptly to restore normal shift behavior.

SPN 1231 FMI 9Transmission �?communication · J1939 CAN network / Eaton transmission control moduleSPN 191 FMI 8Transmission �?speed sensing · Output shaft speed sensor

Related Lookup Pages

Sources

  • Eaton Roadranger Technical Resources — UltraShift and Advantage AMT Service Documentation Eaton Corporation (Roadranger) · oem · accessed 2026-06-11 · confidence medium

    Source: Eaton Corporation (Roadranger), Eaton Roadranger Technical Resources — UltraShift and Advantage AMT Service Documentation. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source
  • Eaton UltraShift PLUS Troubleshooting Guide TRTS0940 Eaton · oem · accessed 2026-05-05 · confidence medium

    Source: Eaton, Eaton UltraShift PLUS Troubleshooting Guide TRTS0940. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source
  • SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium

    Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source

FAQ

Is the output shaft speed sensor on the Eaton UltraShift Plus the same as on the older Fuller automated transmissions?

The UltraShift Plus and the earlier Fuller AutoShift use similar speed sensor architectures �?a magnetic pickup or Hall-effect sensor reading a tone wheel �?but the specific sensor part numbers, tone wheel configurations, and mounting details may differ between transmission generations. ServiceRanger part look-up or the Roadranger service portal can confirm the correct part for the specific transmission model.

Can a worn transmission output seal affect the tone wheel and cause SPN 191 FMI 8 on the Eaton UltraShift?

A worn output seal can allow lubricant to contaminate the sensor area, which may foul the sensor face and produce erratic readings. Inspecting the sensor for oil contamination alongside checking the tone wheel condition is worthwhile during diagnosis.

How does the Eaton UltraShift use output shaft speed data compared to vehicle speed from the ABS?

The Eaton UltraShift TCM uses its own output shaft speed sensor for primary shift control. It may also receive vehicle speed data from the ABS via J1939 as a reference. When both sources are available and agree, shift control is optimal. When the transmission output sensor is faulted, the TCM may attempt to use the J1939 vehicle speed as a fallback, but this depends on the specific transmission generation and calibration.