Code Details
| Display code | SPN 191 FMI 8 |
|---|---|
| SPN | 191 |
| FMI | 8 |
| OEM code | None listed |
| Manufacturer | Allison Transmission |
| System | Transmission �?speed sensing |
| Component | Output shaft speed sensor |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The Allison TCM uses the output shaft speed sensor to calculate vehicle speed, monitor gear ratios, and control shift timing. When this sensor produces an erratic or intermittent signal, the TCM cannot accurately determine vehicle speed or confirm the correct gear is engaged. This can cause erratic shifting, incorrect gear selection, or the transmission to default to a fail-safe mode.
SPN 191 is Transmission Output Shaft Speed in J1939. FMI 8 indicates the signal pattern is abnormal �?the frequency or amplitude of pulses is inconsistent with expected values for the current operating conditions. On Allison 3000 and 4000 Series, the output shaft speed sensor is a magnetic pickup or active Hall-effect sensor that reads teeth on a tonewheel. Metal shavings on the sensor tip, an out-of-spec air gap, a cracked tonewheel, or wiring damage between the sensor and TCM can all cause FMI 8.
Common Symptoms
- Check transmission lamp or erratic shifting behavior
- Speedometer may read incorrectly or fluctuate
- Transmission may select incorrect gears or shift at wrong points
- Possible default to a limp-home gear range
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Metallic debris accumulated on the speed sensor tip, interfering with signal generation
- Excessive air gap between the sensor tip and tonewheel teeth
- Damaged or cracked tonewheel
- Sensor wiring harness chafed, shorted, or broken
- Sensor connector with corroded or loose pins
First Checks
- Remove and inspect the output shaft speed sensor �?clean any metallic debris from the tip and check the sensor face for damage
- Check the air gap between the sensor tip and the tonewheel using a feeler gauge and compare to Allison specification
- Inspect the tonewheel for missing, chipped, or damaged teeth
- Check the sensor wiring harness from the sensor to the TCM connector for chafing, damage, or connector corrosion
- Use Allison diagnostic software to monitor the output shaft speed signal live while rotating the driveshaft �?an erratic signal pattern confirms the sensor or tonewheel issue
Can I Keep Driving?
A faulty output shaft speed sensor can cause the transmission to shift erratically or default to a limited gear range. The vehicle may be driveable but with degraded performance. If the transmission is stuck in a single gear or fails to shift, do not operate until the sensor issue is diagnosed.
Related Codes
Related Lookup Pages
Sources
- Allison On-Highway Automatic Transmissions Service Support Allison Transmission Inc. · oem · accessed 2026-06-11 · confidence medium
Source: Allison Transmission Inc., Allison On-Highway Automatic Transmissions Service Support. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - Allison Transmission Service and Support Allison Transmission · oem · accessed 2026-05-05 · confidence medium
Source: Allison Transmission, Allison Transmission Service and Support. 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
How do I clean the Allison output shaft speed sensor?
Remove the sensor from the transmission housing (the process varies by model �?some sensors thread in, others are retained by a clip). Clean the sensor tip using a clean cloth or lint-free wipe to remove metallic particles. Inspect the sensor face visually for cracks or damage. Reinstall and check the air gap per the Allison specification before testing.
Can SPN 191 FMI 8 on an Allison transmission cause the vehicle speedometer to malfunction?
Yes. If the vehicle speedometer receives its speed signal from the Allison TCM via J1939 (as many modern trucks do), an erratic output shaft speed sensor can cause the speedometer to fluctuate or read incorrectly. This is an additional symptom that supports the diagnosis but does not change the repair approach.
Is the output shaft speed sensor on the Allison 3000 and 4000 Series the same part?
The 3000 and 4000 Series share many components, but the specific sensor part number depends on the transmission serial number, model, and production date. Always verify the correct part number using the Allison parts catalog or diagnostic software for the specific transmission before ordering.