i put in a fresh battery and pull the trigger but the chain just jerks a little and won’t spin, makes a humming noise

Diagnosis

The humming noise you hear is the sound of the brushless motor attempting to rotate while facing an immovable mechanical load, creating a "stalled" state. Because the M18 Fuel 2727-20 uses a high-torque electronic controller, it will try to push through the resistance until the internal thermal protection or the battery's BMS (Battery Management System) cuts power to prevent the windings from melting. The "jerk" is the motor attempting to overcome static friction before the electronic safety trip engages. This is rarely an electrical failure of the battery or motor itself. Instead, it is typically caused by one of three things: an over-tensioned chain that has locked the drive sprocket, a foreign object (like a small stone or thick wood chip) wedged between the sprocket and the side plate, or a seized drive sprocket bearing. You can confirm this by removing the battery and attempting to pull the chain backward and forward by hand (while wearing gloves); if there is zero play or the chain feels "welded" to the bar, the issue is mechanical binding.

How to Fix It

Follow these steps in order to clear the jam and restore rotation:

1. Power Down: Remove the M18 battery immediately. Attempting to "force" the motor to spin while it is humming can permanently damage the motor controller or trip a permanent fault in the battery.
2. Release Tension: Locate the bar and chain tensioning knob. Turn it counter-clockwise to loosen the chain. If the chain was overtightened, it may have compressed the drive links against the sprocket, creating enough friction to stall the motor.
3. Inspect the Bar Nuts: Use a socket wrench to loosen the two bar nuts on the side cover. Remove the side cover entirely to expose the drive sprocket and the clutch mechanism.
4. Clear the Sprocket: Check the area around the drive sprocket for debris. Use a compressed air canister or a stiff brush to remove sawdust and pitch buildup. Ensure no small pebbles or metal shards are lodged in the sprocket teeth.
5. Manual Rotation Test: With the side cover off, manually rotate the drive sprocket. It should spin freely. If it feels "crunchy" or resists movement, check for a bent drive link on the chain.
6. Reset and Test: Reinstall the side cover, tighten the bar nuts firmly, and set the chain tension so there is approximately 1/8" to 1/4" of slack. Reinsert the battery and test the trigger in short bursts.

If That Didn't Work

• Chain Brake Reset: Ensure the chain brake handle (the plastic guard in front of the top handle) is pushed all the way forward. If the brake has partially engaged or the internal brake band has snapped and wedged itself, the motor will hum but the chain won't move. Pull the brake lever back and push it forward firmly until it clicks.
• Drive Sprocket Replacement: If the sprocket is worn down or the internal bearing has seized, the motor will struggle to turn the assembly. Inspect the sprocket for "hooking" (where the teeth are worn into a sharp point). If the bearing is seized, the sprocket assembly must be replaced.
• Battery Compatibility Check: Ensure you are using a High Output (HO) or Forge battery. While standard M18 batteries fit, the 2727-20 draws massive current. A degraded or low-capacity battery may provide enough voltage to trigger the electronics but not enough amperage to break the static friction of the chain, resulting in a hum and a jerk.
• Internal Gearbox Failure: If the sprocket spins freely by hand but the motor still hums when powered, the internal planetary gears may have stripped or jammed. This requires a full teardown of the motor housing to inspect for sheared gear teeth.

Heads Up

• Tensioning Warning: Never tighten the chain while the bar is clamped tight against the saw body; always leave a tiny gap to allow the chain to settle as it heats up during use.
• Lubrication: Check your bar oil reservoir daily. A dry chain increases friction exponentially, which can lead to motor stalling and premature wear on the drive sprocket.
• Battery Heat: If the battery feels hot to the touch after the "humming" incident, let it cool for 15 minutes before attempting another start to avoid triggering the thermal shutdown.
• Cleaning: Every 5-10 loads of wood, remove the side cover and clean the sprocket area with a solvent to prevent pitch buildup from causing mechanical resistance.