Code Details
| Display code | SPN 161 FMI 3 |
|---|---|
| SPN | 161 |
| FMI | 3 |
| OEM code | None listed |
| Manufacturer | Allison Transmission |
| System | Transmission �?speed sensing |
| Component | Turbine speed sensor / wiring |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The turbine speed sensor reports the rotational speed of the torque converter turbine to the Allison TCM. When the sensor signal voltage is higher than the normal range, the TCM cannot accurately calculate turbine speed for shift control and converter lockup management. This type of fault is usually from a wiring issue rather than mechanical wear. On trucks with Allison 3000 or 4000 Series transmissions, the turbine speed sensor provides input shaft speed to the TCM for torque converter lockup and shift timing calculations. Allison DOC can run a turbine speed sensor circuit test and display the raw sensor signal at various input shaft speeds. A sensor that fails only at specific temperatures — from thermal expansion affecting the air gap between the sensor and the reluctor wheel — may pass a bench resistance test but fail in service. Allison DOC data logging during a test drive replicates the in-service temperature condition and is more reliable than a static bench test for intermittent sensor faults.
SPN 161 is Transmission Input Shaft Speed. FMI 3 indicates voltage above normal in the sensor circuit. On Allison Gen 4 and Gen 5 controls, the turbine speed sensor provides a frequency signal. FMI 3 can result from a short to a power supply in the signal wire, an open in the sensor ground circuit causing the signal to float high, or a damaged sensor. Allison DOC wiring diagrams identify the circuit routing for the specific vehicle application.
Common Symptoms
- Shift quality issues �?harsh or erratic shifts
- Converter lockup may be inconsistent
- Active fault in Allison DOC
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Short to power in the turbine speed sensor signal wire
- Open in the sensor ground circuit
- Damaged sensor producing an out-of-range voltage
- Connector corrosion causing a high-resistance ground path
First Checks
- Connect Allison DOC and monitor the turbine speed sensor voltage in live data
- Inspect the turbine speed sensor connector and harness for damage or corrosion
- Check the sensor ground circuit continuity from the connector to chassis ground
- Verify there is no short to a power supply in the signal circuit
- Connect Allison DOC and perform the turbine speed sensor circuit test during a drive cycle at normal operating temperature
- Inspect the turbine speed sensor air gap against specification — excessive gap from reluctor wear is a common intermittent fault cause
Can I Keep Driving?
Shift quality may be degraded. Diagnose and repair to restore correct turbine speed feedback and smooth shift control.
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
What is the difference between FMI 3 and FMI 8 for SPN 161 on the Allison 3000?
FMI 3 indicates the sensor circuit voltage is above the normal range �?typically an electrical fault in the wiring such as a short to power or an open ground. FMI 8 indicates an erratic or abnormal frequency signal �?typically from a damaged tone wheel, a mispositioned sensor, or metallic debris on the sensor face. The diagnostic approach differs: FMI 3 requires wiring circuit testing; FMI 8 requires mechanical inspection of the sensor and tone wheel.
Can I replace just the turbine speed sensor on the Allison 3000 or does the transmission need to come out?
The turbine speed sensor on the Allison 3000 and 4000 series is typically accessible externally on the transmission case without removing the transmission from the vehicle. Access varies by chassis and mounting, but the sensor threads into a port on the transmission housing and can usually be reached with hand tools.
Will the Allison 3000 TCM store the fault history after the wiring fault is repaired?
Yes. Allison DOC stores inactive fault codes with occurrence counters and timestamps. After the wiring fault is repaired and the active code clears, the code will remain in the inactive fault history. This history is useful for documenting the repair and confirming the fault does not recur. Inactive codes do not affect transmission operation.