Code Details
| Display code | SPN 191 FMI 8 |
|---|---|
| SPN | 191 |
| FMI | 8 |
| OEM code | None listed |
| Manufacturer | ZF |
| 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
ZF automated transmissions rely on precise output shaft speed data for correct shift timing and clutch management. An erratic signal prevents the TCM from accurately calculating vehicle speed, potentially causing incorrect or unexpected shifts. On Freightliner, Kenworth, and Volvo trucks with ZF Astronic or Traxon transmissions, the output shaft speed sensor provides road speed to the TCM for automated shift calculations. ZF ETAS diagnostic tool monitors output shaft sensor signal quality in real time and can display the raw sensor waveform to identify intermittent signal anomalies. A damaged sensor tone ring — from impact with road debris — produces irregular signal intervals that trigger SPN 191 FMI 8 under specific shaft speed conditions.
SPN 191 is Transmission Output Shaft Speed in J1939. FMI 8 indicates abnormal frequency �?erratic, noisy, or interrupted signal. ZF AS-Tronic and TraXon transmissions use a tone wheel and magnetic or Hall-effect sensor for output speed measurement. ZF Testman (for AS-Tronic) or WABCO Toolbox (where applicable) can display the live output speed signal to characterize the fault during low-speed maneuvering.
Common Symptoms
- Erratic or unexpected gear changes
- Possible shift hesitation at expected vehicle speed transitions
- Fault active in ZF Testman or WABCO Toolbox
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Loose or shifted output shaft speed sensor
- Damaged tone wheel teeth
- Wiring harness damage near the sensor causing noise
- Metallic debris on the sensor face from internal wear
First Checks
- Connect ZF Testman or WABCO Toolbox and monitor output shaft speed live data at low vehicle speed to characterize the signal
- Inspect sensor mounting and position relative to the tone wheel �?correct air gap is critical
- Inspect tone wheel for damaged or missing teeth
- Inspect harness for chafing or connector corrosion near the sensor
- Inspect the output shaft speed sensor tone ring for damage or debris contamination
- Connect ZF ETAS and monitor the output shaft sensor signal waveform at various vehicle speeds
Can I Keep Driving?
The transmission may make incorrect shift decisions with an erratic output speed signal. This can result in unexpected gear changes. Diagnose and repair to restore proper shift control.
Related Codes
Related Lookup Pages
Sources
- TOOLBOX PLUS Diagnostic Software ZF Commercial Vehicle Solutions · oem · accessed 2026-05-05 · confidence medium
Source: ZF Commercial Vehicle Solutions, TOOLBOX PLUS Diagnostic Software. 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
What is the difference between the ZF AS-Tronic and ZF TraXon in terms of speed sensor design?
The ZF TraXon is the current-generation automated transmission platform that superseded the AS-Tronic for new applications. Both use output shaft speed sensors for shift control, but the sensor types, connector formats, and tone wheel designs differ between generations. ZF Testman covers both platforms with specific procedures for each model.
Can the ZF TraXon transmission enter a safe mode if SPN 191 FMI 8 is active?
Yes. If the TCM cannot determine vehicle speed accurately from the output sensor and no alternative speed reference is available, the TraXon may restrict certain operating modes or gear ranges as a protective measure. The specific response depends on the severity of the signal fault and the transmission calibration.
Is ZF Testman or WABCO Toolbox required to diagnose SPN 191 FMI 8 on the ZF AS-Tronic?
While the fault code itself is readable with any J1939-capable scan tool, ZF Testman provides the live data parameters �?output shaft speed, sensor signal quality, and bidirectional tests �?needed to effectively characterize the fault. Generic scan tools typically cannot access these specific ZF live data channels.