drill seems weak, bogs down way easier than it used to with a fresh battery.

Diagnosis

The loss of torque and premature bogging on a Milwaukee M18 FUEL 2904-20 typically stems from a failure in the electronic load-sensing circuit or physical debris infiltration in the planetary gear set. Because this is a brushless model, the tool uses a sophisticated electronic controller to manage current; if the internal thermal sensors are malfunctioning or if there is a partial short in the winding of the brushless motor, the tool will enter a "protection mode" prematurely, cutting power before the motor actually reaches its physical limit. Another common cause is the accumulation of fine construction dust (drywall or concrete) inside the housing, which interferes with the cooling vents. When the brushless controller detects an overheat condition—even if the tool feels cool to the touch—it throttles the RPMs and torque to prevent the magnets from demagnetizing. You can confirm this by checking if the tool performs normally for 30 seconds and then suddenly loses power, which indicates a thermal throttling issue rather than a mechanical failure. If the drill makes a grinding noise while bogging, the issue is likely a worn planetary gear or a failed clutch spring in the transmission.

How to Fix It

To resolve power loss and restore the full torque capacity of your 2904-20, follow these steps to clear the electronics and mechanical paths:

1. Perform a hard reset of the electronic controller by removing the battery and holding the trigger depressed for 30 full seconds. This discharges the capacitors in the brushless control board and clears temporary error states.
2. Inspect the air intake vents located near the base of the motor housing. Use a can of compressed air or an air compressor set to 30 PSI to blow out the motor vents. Ensure you blow from the outside in to push dust away from the controller board.
3. Check the chuck for internal debris. Unscrew the chuck from the spindle (if you have the tools to do so) or simply clear the jaws with a wire brush. A jammed chuck can create parasitic drag that makes the motor feel weak.
4. Verify the battery contact points. Use a cotton swab with 90% isopropyl alcohol to clean the gold-plated terminals on both the tool and the M18 battery pack. High resistance at the contact point causes a voltage drop, which the tool interprets as a low battery, triggering the power-save mode.
5. Switch the torque collar to the highest setting (usually 15-20 depending on the ring) and test the drill in "Drill Mode" rather than "Hammer Mode" to determine if the power loss is specific to the percussion mechanism.

If That Didn't Work

• Inspect for Brushless Controller Failure: If the tool still bogs down despite a full charge and clean vents, the MOSFETs on the internal control board may be failing. This requires a replacement of the electronic trigger assembly. Check for a "stuttering" sensation in the trigger pull, which is a hallmark of a failing controller.
• Gearbox Lubrication: The planetary gears in the 2904-20 can lose lubrication over time, especially if used heavily in high-heat environments. Open the casing and apply a small amount of high-temp lithium grease to the gear train. If you see metallic flakes in the old grease, the planetary gears are stripped and the gear housing must be replaced.
• Battery Impedance Test: Even if the battery shows "full" on the fuel gauge, the internal cells may have high impedance. Test the tool with a brand new M18 High Output (HDP) battery. If the tool performs normally with a new pack, your old battery has developed a "voltage sag" and cannot provide the peak current required for high-torque applications.
• Check the Clutch Spring: The adjustable torque ring relies on a heavy-duty spring. If this spring has snapped or fatigued, the clutch will slip at much lower torque levels than intended, giving the illusion that the motor is weak when the issue is actually mechanical slippage.

Heads Up

• Avoid using the drill at the absolute limit of its torque for extended periods; the 2904-20 is powerful, but sustained "stalling" generates immense heat in the brushless windings that can permanently degrade the magnets.
• Never use WD-40 to clean the motor vents, as the solvent can degrade the plastic housing and attract more dust, leading to faster overheating.
• Always ensure the battery is fully seated with a distinct "click." A loose battery connection can cause intermittent power drops that mimic a failing motor.
• Keep the tool away from magnetic sources during storage to prevent interference with the Hall effect sensors that tell the brushless controller the position of the rotor.