Code Details
| Display code | SPN 731 FMI 2 |
|---|---|
| SPN | 731 |
| FMI | 2 |
| OEM code | None listed |
| Manufacturer | Allison Transmission |
| System | Transmission �?gear selection |
| Component | Clutch pack / planetary gear set |
| Source address | Unknown or not applicable |
| Severity | high |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The Allison TCM continuously monitors the ratio between turbine speed (input) and output shaft speed to calculate the actual gear ratio. If the actual ratio is different from the commanded ratio, the TCM detects that the transmission is not holding the intended gear �?either slipping or stuck in a different ratio. This is a significant fault that can indicate significant internal wear.
SPN 731 is related to the Transmission Current Gear parameter. FMI 2 indicates an erratic or intermittent data condition �?the actual ratio data does not correspond to the selected gear. On Allison Gen 4 and Gen 5 controls, the TCM calculates actual gear ratio from turbine and output speed sensors. A ratio mismatch indicates clutch slippage in the intended gear or an unexpected engagement of a different clutch pack. Allison DOC provides gear ratio actual vs. expected and clutch pack slippage indicators.
Common Symptoms
- Slipping or delayed engagement when shifting into a specific gear
- Harsh or abrupt shifts
- The transmission may skip or refuse certain gear ranges
- Possible burning smell from overheated clutch packs in severe cases
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Worn or burnt clutch pack in the affected gear range
- Low transmission fluid level reducing clutch apply pressure
- Transmission control valve body fault affecting clutch apply
- Internal mechanical damage to the planetary gear set
First Checks
- Check transmission fluid level and condition �?low level or burned fluid is a strong indicator
- Connect Allison DOC and review the fault history for clutch-related codes alongside SPN 731 FMI 2
- Use DOC to compare actual vs. expected gear ratio in live data
- If clutch wear is suspected, a transmission fluid drain and inspection of the drain plug for metallic debris can provide additional evidence
Can I Keep Driving?
A gear ratio mismatch fault typically indicates significant internal transmission wear. Continued operation under load can accelerate damage. Plan for a transmission inspection soon.
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
Can the Allison 3000/4000 gear ratio error be caused by incorrect programming or calibration?
An incorrect axle ratio or tire size programmed in the TCM can cause the TCM to calculate an incorrect expected ratio, potentially generating a ratio error if the calibration is significantly wrong. However, this is less common than actual clutch wear. Verify the TCM calibration data in Allison DOC against the truck's actual axle ratio and tire size before concluding it is a mechanical fault.
Is a gear ratio fault on the Allison 3000 always a sign of a major overhaul being needed?
Not always. If the fault is isolated to one gear range, an early-stage clutch pack that is beginning to slip may still be addressable with a band adjustment or a targeted clutch pack replacement without a full overhaul. However, if multiple gear ranges are showing ratio errors or if the fault is accompanied by burnt fluid, a more comprehensive internal inspection is needed.
How do I check Allison transmission fluid level correctly on the 3000 or 4000 series?
Allison 3000 and 4000 series transmissions use a closed fill system that requires a specific fluid level check procedure: the transmission must be at operating temperature (approximately 80°C sump temperature, confirmed in DOC), in Park or Neutral, and on a level surface. The correct fill level is measured at the transmission dipstick port or fill plug. Following the Allison procedure precisely is important �?overfill and underfill both affect clutch operation.