DIAGNOSIS The speed selector ring is engaging, but the gear train for low speed (Speed 1) is damaged. A common failure point is the small pinion gear that engages when you slide the selector.
FIX Replace the transmission assembly. The gears for both speeds are housed together in one sealed unit.
- Remove battery and housing screws.
- Separate the motor housing from the gear housing.
- Swap the old transmission for a new one. The speed selector ring transfers over.
- Reassemble. The repair fixes both speeds.
IF THAT DIDN'T WORK If the noise persists with a new transmission, the issue is the motor itself. The armature might be damaged and only partially engaging under the high torque demand of Speed 1.
HEADS UP Forcing the drill to work in Speed 1 while it's grinding will destroy the transmission housing. You'll go from needing a $40 transmission to needing a whole new drill body.