Code Details
| Display code | SPN 3509 FMI 3 |
|---|---|
| SPN | 3509 |
| FMI | 3 |
| OEM code | None listed |
| Manufacturer | Allison Transmission |
| System | Transmission �?electrical |
| Component | TCM power supply / battery / charging system |
| 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 monitors the voltage of its power supply from the truck's electrical system. When the voltage is consistently above the normal operating range �?typically above about 16 to 16.5 volts on a 12-volt system �?this fault sets. Overvoltage can damage electronic components over time and can cause erratic TCM behavior.
SPN 3509 is Electrical Supply Voltage 1 in J1939. FMI 3 indicates the measured voltage is above the normal operating range. On Allison Gen 4 and Gen 5 controls, the TCM monitors its main battery and switched ignition supply voltages. A faulty alternator voltage regulator that allows overcharge above 14.4 to 14.8 volts on a 12-volt system is the primary cause. The Allison DOC diagnostic tool can display live TCM supply voltage alongside the fault history.
Common Symptoms
- Transmission caution indicator with an electrical fault code
- Possible erratic transmission behavior if overvoltage is severe
- Battery warning light on the instrument cluster if the alternator is overcharging
- Other electrical system faults from overvoltage affecting multiple vehicle modules
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Faulty alternator voltage regulator allowing overcharge
- Failed alternator with no regulation producing excessive voltage
- Loose battery connections causing voltage spikes
- External battery charger connected to the vehicle at too high a setting
First Checks
- Measure battery voltage with the engine running at 1500 to 2000 RPM �?should be between 13.8 and 14.8 volts for a 12-volt system
- If voltage is above 15 volts at any engine speed, the alternator or regulator is likely faulty
- Connect Allison DOC and verify the TCM reported voltage matches the measured value
- Check that no external charger or boost pack is still connected to the electrical system
- Inspect battery terminal connections for corrosion or looseness that could cause spikes
Can I Keep Driving?
A persistent overvoltage condition can damage the TCM and other vehicle electronics. While the transmission may operate normally initially, prolonged overvoltage risks component damage. Diagnose and repair the charging system promptly.
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 voltage should the Allison 3000/4000 TCM see on a healthy truck?
The Allison TCM is designed for 12-volt systems with a normal operating range of approximately 11 to 15 volts. At idle with the alternator charging, a healthy system typically shows 13.8 to 14.4 volts. Above about 15 to 16 volts, the TCM will log an overvoltage fault. The Allison DOC tool can monitor live supply voltage during a drive to confirm the fault occurs under specific conditions.
Can the Allison 3000/4000 TCM be permanently damaged by SPN 3509 FMI 3 overvoltage?
Yes. Sustained overvoltage above the TCM's rated maximum can degrade internal components. While the TCM is protected against brief spikes, persistent overcharging at high voltage levels can cause premature failure. Resolving the charging system fault promptly is important to prevent TCM damage.
Will SPN 3509 FMI 3 go away on its own when the alternator is replaced on an Allison 3000?
After a new alternator is installed and confirmed to be regulating voltage correctly, SPN 3509 FMI 3 should become inactive after a key cycle. If the voltage is confirmed within range and the fault persists, it can be cleared using the Allison DOC tool. A drive cycle to verify the fault does not return will confirm the repair.