i dropped my drill and now the trigger is all loose and sometimes it just starts running on its own, can i fix this?

Diagnosis

The impact from the drop has likely caused a mechanical failure within the trigger switch assembly or the electronic control module (ECM) interface. In the Milwaukee M18 FUEL 2904-20, the trigger is not just a simple switch; it is a variable-speed potentiometer linked to a physical spring-return mechanism. When the unit hits a hard surface, the plastic mounting tabs that hold the trigger assembly in alignment often shear off, or the return spring becomes dislodged from its seat. The "self-starting" behavior is the most critical symptom. This happens because the trigger is no longer returning to the "neutral" or "off" position, leaving the internal contact points closed or the potentiometer signal active. This creates a dangerous condition where the tool can engage without user input. If the trigger feels "mushy" or lacks the usual tactile snap, the internal plastic housing of the switch assembly is likely cracked, preventing the spring from applying the necessary counter-pressure to keep the motor off.

How to Fix It

To resolve this, you must inspect the internal trigger assembly and, if necessary, replace the switch module. Since the 2904-20 is a high-torque FUEL model, precision in reassembly is vital to prevent electronic shorts.

1. Power Down: Remove the M18 battery pack immediately. Do not attempt to open the housing with the battery inserted, as you risk shorting the control board.
2. Housing Disassembly: Using a T10 Torx driver, remove the screws securing the clamshell housing. Pay close attention to the screws near the base of the handle and the ones hidden under the grip stickers.
3. Clamshell Separation: Carefully pry the two halves of the housing apart. Be cautious not to snap the plastic alignment pins. Gently lift the right-hand side to expose the trigger assembly and the wiring harness.
4. Trigger Inspection: Locate the trigger switch assembly. Check for hairline cracks in the black plastic housing of the switch. If the return spring is simply loose, use a pair of needle-nose pliers to reseat it into its guide notch.
5. Switch Replacement: If the plastic is cracked or the trigger continues to self-activate, the entire switch assembly must be replaced (Part #45-01-0000 or equivalent for the 2904-20). Unplug the wiring harness from the main PCB, unscrew the switch mounting bolts, and swap in the new assembly.
6. Testing and Closing: Before fully tightening the housing, insert the battery and test the trigger for a crisp "snap" and ensure it does not engage on its own. Once confirmed, align the clamshell and tighten the T10 screws in a cross-pattern to ensure even pressure across the housing.

If That Didn't Work

• Check the Variable Speed Dial: Inspect the speed selector ring at the top of the drill. If the drop shifted the internal gear of the speed dial, it can sometimes put pressure on the trigger mechanism, preventing it from resetting fully. Ensure the dial rotates freely and clicks into place.
• Inspect the PCB for Solder Bridges: Look closely at the main control board for any loose solder joints or components that may have shifted during the impact. A loose wire touching the trigger input pin on the PCB can mimic a trigger pull. Use a can of compressed air to clear any debris and a small amount of isopropyl alcohol to clean the board.
• Verify Housing Alignment: If the outer plastic shell is warped or cracked, it may be pinching the trigger from the outside. Check the "seam" of the drill; if the two halves are not flushing perfectly, the plastic may be pressing against the trigger arm, keeping it engaged. You may need to trim a small piece of distorted plastic from the interior of the shell.
• Full Trigger Module Swap: If you only replaced the spring and the tool still glitches, the internal potentiometer in the trigger assembly is likely damaged. Replace the entire electronic trigger module rather than attempting to repair the internal circuitry.

Heads Up

• Safety Warning: Never store the tool with a battery inserted if the trigger is malfunctioning. A "ghost" trigger pull can lead to accidental drilling or tool runaway.
• Screwdriver Choice: Only use high-quality Torx T10 bits. The screws in the M18 FUEL series are soft; using a generic or worn bit will strip the heads, making it impossible to open the tool without drilling the screws out.
• Debris Control: When the tool is open, ensure no metal shavings or construction dust have entered the motor housing. Use a vacuum to clean the area around the brushes and the PCB to prevent future electrical failures.
• Avoid Over-tightening: When reassembling the clamshell, do not over-torque the screws. Over-tightening can crack the plastic housing or put undue pressure on the trigger assembly, recreating the original problem.