Code Details
| Display code | SPN 168 FMI 4 |
|---|---|
| SPN | 168 |
| FMI | 4 |
| OEM code | None listed |
| Manufacturer | Allison Transmission |
| System | Transmission �?electrical / power supply |
| Component | Battery / charging system / TCM power supply wiring |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-12 |
Plain-English Meaning
The Allison 3000 and 4000 series TCM requires a minimum battery voltage to function reliably. When voltage drops below the operational minimum �?from weak batteries, a failing alternator, high electrical system loads, or a wiring issue in the TCM supply circuit �?SPN 168 FMI 4 is set. In vocational trucks (buses, refuse, utility) with high electrical accessory loads, battery voltage issues are a common source of transmission-related faults. On Freightliner, Kenworth, Peterbilt, and Volvo trucks with Allison 3000 or 4000 Series transmissions, battery voltage is shared between the engine ECM and TCM. A low battery voltage event that triggers SPN 168 FMI 4 may also generate companion fault codes in the engine ECM, ABS module, or body controller. Allison DOC can record the exact voltage level and duration during the fault event, which helps distinguish a one-time voltage dip from a chronic charging system problem. On vocational trucks with high electrical accessory loads — cranes, refrigeration units, PTO-driven hydraulics — charging system sizing should be evaluated as part of the diagnosis.
SPN 168 FMI 4 is Battery Potential/Power Input below normal or shorted low. Allison DOC displays battery voltage as a live parameter and records the voltage at the time of fault in the event data. The fault event voltage reading is a direct indicator of whether the issue is a momentary voltage dip or a systemic low-voltage problem.
Common Symptoms
- Transmission warning indicator
- Possible shift inhibits or gear range limitations
- May be accompanied by other electrical system fault codes
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Weak or failing batteries
- Failing alternator not maintaining charge voltage
- High parasitic draw depleting batteries
- Corroded or loose TCM power supply or ground connections
First Checks
- Load-test the truck batteries �?a battery that passes open-circuit voltage may fail under load
- Connect Allison DOC and monitor battery voltage at the TCM
- Check alternator output voltage and current under load
- Inspect TCM power supply harness connectors and ground straps for corrosion
- Connect Allison DOC and review the voltage fault event data — check minimum recorded voltage and duration
- Measure battery resting voltage and charging system output voltage at rated RPM under load
Can I Keep Driving?
The transmission may restrict gear selection under low voltage. Address the charging system or battery issue to prevent transmission TCM faults.
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 a weak truck battery cause Allison 3000 / 4000 transmission faults?
Yes. The Allison TCM is sensitive to supply voltage. A battery that appears functional at rest but cannot maintain voltage under the combined load of engine cranking and accessory operation can cause momentary voltage drops that trigger SPN 168 FMI 4. Load-testing the batteries under realistic electrical system load is the definitive test.
Does Allison DOC show the battery voltage at the time of the SPN 168 FMI 4 fault?
Yes. Allison DOC stores fault event records that include the battery voltage reading at the time the fault was set. Reviewing this data helps distinguish a momentary voltage dip (e.g., during engine cranking) from a sustained low-voltage condition that indicates a systemic charging system problem.
Is SPN 168 FMI 4 on the Allison transmission the same as a low voltage fault on the engine ECM?
SPN 168 is the J1939 parameter for battery voltage, so SPN 168 FMI 4 from the Allison TCM and a similar code from the engine ECM both indicate low battery voltage, but each module monitors its own supply circuit. Both faults can appear simultaneously if the underlying cause is a system-wide low voltage condition. Identifying whether the issue is system-wide or isolated to one module's supply circuit guides the repair.