Bobcat M4109 (41-09) Fault Code: Alternator Low Output

What does Bobcat error code M4109 mean?

M4109 is a charging-system code on Bobcat S-series and T-series loaders. It is an M-series code, which means the machine's main (Gateway) controller reported it, and the same fault shows on the panel in the two-part form 41-09. The controller watches system voltage while the engine runs and sets M4109 when that voltage stays below 11.9 volts for a continuous 30 seconds with the engine turning above 1500 rpm.

That combination tells the controller the alternator is not keeping the battery charged under load. The machine may still run for a while on stored battery power, but voltage keeps falling until electrical systems act up or the engine stalls. Because the trigger is a sustained low reading and not a momentary dip, a one-off M4109 after a jump start or a very low battery can clear on its own once charging recovers.

What triggers a Bobcat M4109 code?

The controller sets M4109 when all of these are true at once: the key switch is on (run/enter), the engine is running above 1500 rpm, and system voltage stays below 11.9 volts continuously for 30 seconds. A brief drop does not set the code, which is why the fault usually points to a charging problem under load rather than a simple dead battery at startup.

Common causes of M4109

• Loose, glazed, or worn alternator belt (the most common cause).
• Loose or corroded battery or alternator cable connections, or a bad ground on wire 2015, causing a voltage drop even when the alternator is healthy.
• Blown or corroded charging-circuit fuse. Depending on the machine this is Fuse 7 or FRC1 fuse 12.
• Open excitation wire 1800 to the alternator.
• Cracked or broken alternator bracket letting the belt slip.
• A failing alternator that no longer holds output voltage.
• Moisture, corrosion, or pushed-back pins in the controller, alternator, or fuse connectors, or in the engine or mainframe harness.
• A faulty main (Gateway) controller (rare, checked only after the wiring and alternator test good).

How to troubleshoot Bobcat M4109: first checks

1. 1With the engine running above idle, read charging voltage at the battery. A healthy 12-volt system charges about 13.5 to 14.5 volts; M4109 sets when it holds below 11.9 volts. A fully charged battery at rest reads about 12.6 volts.
2. 2Inspect the alternator belt for looseness, glazing, or cracks, and check the alternator bracket. A slipping belt is the number one cause.
3. 3Check the battery terminals, alternator cables, and grounds for looseness or corrosion, including ground wire 2015. Clean and tighten as needed.
4. 4Check the charging-circuit fuse (Fuse 7 or FRC1 fuse 12, depending on the machine) and its connector, then check excitation wire 1800 for an open.
5. 5If the alternator reads about 14 volts at its own output post but the battery still reads low with the engine running, the loss is a voltage drop in the cables or grounds between them, not the alternator.
6. 6If the belt, connections, fuse, and wiring all test good, have the alternator load-tested (a parts store can bench-test it). If the alternator is good, the main controller is the last suspect.

How the code clears

M4109 is self-resetting. Once charging voltage recovers and stays above the threshold, the code clears on its own. It does not need a scan tool to reset, but if it returns after clearing, the underlying charging fault is still present.

Affected models and serial ranges

M4109 appears in our records across 16 Bobcat models. Match your machine by model and serial number.