the mode selector ring is stuck and i can't switch from drill to hammer.

Diagnosis

The mode selector ring on the Milwaukee M18 FUEL Hammer Drill (2904-20) typically jams due to the accumulation of fine construction dust, drywall mud, or metallic shavings that migrate behind the plastic collar. These contaminants mix with the factory lubricant to create a thick, abrasive paste that physically blocks the selector ring from rotating across the detent positions. In more severe cases, the internal spring-loaded detent ball—which provides the "click" feel when switching modes—has either slipped out of its race or the spring has fatigued/snapped. When this happens, the ring may feel "mushy" or completely locked because the ball is wedged between the selector sleeve and the gear housing. You can confirm this by attempting to rotate the ring with a pair of slip-joint pliers; if the ring moves but doesn't "lock" into a mode, the detent mechanism is failed. If it won't budge even with pliers, the issue is external debris or a catastrophic internal gear shear.

How to Fix It

Follow these steps to clear the jam and restore functionality without fully dismantling the motor housing:

1. Remove the M18 battery pack immediately to prevent accidental trigger engagement during the cleaning process.
2. Use a high-pressure air compressor nozzle or a can of compressed air. Aim the nozzle directly into the narrow gap between the rotating selector ring and the main tool body. Blast air in a 360-degree circle while simultaneously trying to wiggle the ring.
3. Apply a small amount of electronic-grade contact cleaner or a non-corrosive penetrating oil (like WD-40 Specialist) into the gap. Let it sit for 5-10 minutes to break down the hardened debris.
4. Using a stiff nylon brush or a dental pick, gently scrape away any visible crust or buildup from the seam of the selector ring.
5. Grip the ring firmly and apply a quick, sharp "snap" motion toward the desired mode. Avoid using excessive force that could crack the plastic collar.
6. Once the ring moves, rotate it through all three settings (Drill, Hammer, and the intermediate settings) five to ten times to ensure the detent ball is seated correctly in its track.

If That Didn't Work

• Partial Disassembly: Remove the screws holding the outer gear housing. Carefully slide the selector ring off the shaft. Inspect the groove for a flattened detent spring or a missing ball bearing. If the spring is broken, replace it with a heavy-duty spring of the same gauge to restore the locking tension.
• Degreasing the Sleeve: If the ring is stuck due to "gunked" lubricant, use a lint-free cloth soaked in isopropyl alcohol to wipe the internal sleeve. Re-lubricate with a very thin layer of lithium grease; over-greasing will actually attract more dust and cause the ring to jam again in the future.
• Mechanical Alignment Check: Check for any warping in the plastic ring. If the tool was dropped while in a specific mode, the ring may have shifted axially. Gently tap the ring toward the motor housing with a rubber mallet to reseat it on the selector shaft.
• Professional Component Replacement: If the internal selector cam is sheared, the entire gear housing assembly must be replaced. This is a common failure if the tool was forced into a mode while the drill was under heavy load.

Heads Up

• Never force the mode selector while the trigger is pulled; this can strip the internal locking pins and render the tool permanently stuck in one mode.
• Avoid using heavy-duty lubricants like motor oil on the selector ring, as these attract concrete dust which turns into an abrasive grinding paste.
• Periodically blow out the selector ring with compressed air every 20-30 hours of use, especially when working in dusty environments like masonry or drywall installation.
• If you notice the "click" becoming softer over time, it is a sign that the detent spring is wearing out; address it before the ring becomes completely loose.