my drill works on speed 2 but makes a horrible grinding noise and barely moves on speed 1

Diagnosis

The grinding noise and lack of torque on Speed 1 indicate a mechanical failure within the planetary gear system of the DCD771C2. Specifically, the low-speed gear train—which consists of a series of reduction gears designed to multiply torque—has likely suffered a "stripped" tooth or a fractured gear face. When you shift to Speed 2, the drill bypasses the primary reduction stage, which is why it continues to function, albeit with less power. The root cause is typically caused by "over-torquing" the tool or attempting to force a fastener that exceeded the drill's rated capacity, causing the hardened steel teeth of the Speed 1 pinion to shear off. You can confirm this by shifting to Speed 1 and pulling the trigger lightly; if you hear a high-pitched metallic rattling or "chattering" without the chuck rotating, the internal gear teeth are no longer meshing. If the chuck rotates but slips under the slightest pressure, the gear teeth are stripped.

How to Fix It

Because the gear assembly in the DCD771C2 is integrated, you cannot replace a single gear; you must replace the entire transmission/gearbox assembly.

1. Power Down: Remove the 20V Max battery pack immediately to prevent accidental trigger activation.
2. Housing Disassembly: Using a T15 Torx driver, remove the 6-8 screws securing the plastic clamshell housing. Be careful not to strip the plastic bosses.
3. Internal Extraction: Gently pry the two halves of the housing apart. Lift the motor assembly out of the chassis, ensuring you do not pinch the wires connecting the motor to the trigger switch.
4. Gearbox Removal: Slide the speed selector ring forward to the "neutral" position between 1 and 2. Unscrew the mounting bolts holding the gearbox to the motor frame.
5. Installation: Slide the new transmission assembly (Part #D25312-000 or equivalent gear kit) onto the motor armature. Ensure the splines are perfectly aligned before tightening the mounting bolts.
6. Reassembly: Seat the motor and trigger assembly back into the housing. Align the housing seams and tighten the T15 screws in a star pattern to ensure even pressure across the casing.

If That Didn't Work

• Inspect the Speed Selector Ring: If the new gears don't solve it, the sliding selector ring may be worn or cracked, preventing it from fully seating into the Speed 1 position. If the ring feels "loose" or doesn't click firmly into place, replace the selector sleeve.
• Check for Motor Armature Debris: Inspect the motor shaft for any metal shavings or "glitter" left over from the failed gears. If debris has entered the motor brushes or the commutator, it can cause erratic speed behavior. Clean with compressed air.
• Verify Chuck Alignment: In some rare cases, the grinding is actually coming from the chuck bearings rather than the gears. Try removing the chuck; if the gearbox rotates smoothly without the chuck attached, the issue is a failed chuck bearing.
• Test the Trigger Switch: If the drill "stutters" on Speed 1 but not Speed 2, the variable speed trigger may have a dead spot in the low-voltage range. Test with a multimeter to ensure consistent voltage delivery to the motor.

Heads Up

• Avoid "Force-Shifting": Never move the speed selector ring while the trigger is being pulled. Shifting under load is the primary cause of stripped gear teeth in the DCD771 series.
• Grease Maintenance: When installing a new gearbox, ensure it is packed with high-temperature lithium grease. If the unit arrives dry, apply a small amount of molybdenum disulfide grease to the gear faces.
• Torque Management: If you frequently find yourself needing more torque than Speed 1 provides, switch to a dedicated 20V Max Impact Driver to avoid burning out the planetary gears of your drill.