Cordless Drill: Motor Spins but Chuck Doesn't

See Transmission.

See Transmissions.

The speed selector allows you to select which speed you want to operate your drill at, for optimal performance, for whatever job you are working on. If the speed selector is in-between gears, it will cause the transmission not to spin when the motor does. Make sure the selector is locked into the proper gear before further diagnostics. Over time, the switch itself can wear out with heavy use and will eventually need to be replaced.
To access the speed selector switch, start with removing the end cap then separate the case from the housing and remove the two transmission screws on the same side the cover is located on. Remove the cover. Lift all of the components out of the casing, including the motor, gear assembly, trigger assembly and chuck. Inspect the plastic switch for damage and where it locks onto the steel wire (also part of the switch). If just the plastic switch is broken, not allowing it to connect to the steel wire, then all you have to do is replace the plastic switch. If not, then continue by using tweezers and grab the metal leg on either side of the switch and lift it outwards from the case. Remove the other leg manually – it should easily come out.
Clean out any debris inside the holes where the metal leg inserts into. Install the new switch and reinstall the motor, gear assembly, trigger assembly and chuck back in the housing and reinstall the cover, securing it tightly. Reinstall the two transmission screws and the end cap.

The transmission transmits power from the motor to the drill chuck and contains gears that allow you to choose different speeds. If the motor works fine, but the drill chuck is not moving, this indicates a bad transmission. Usually, the problem is stripped gears inside the transmission.
To access the transmission, remove the end cap and make sure the transmission is in the first gear. Insert a screwdriver into the drill chuck and snug the chuck up against the screwdriver. Remove the screw – remember this screw is reversed thread so turn it clockwise. Next, insert a large allen wrench inside the chuck and tighten the chuck down on it. Hit the allen wrench with a hammer to loosen. You can also place a pipe over the allen wrench for leverage. Next, remove all the screws on the cover securing it to the housing as well as the screws securing the transmission clutch assembly. Take the cover off and then the transmission clutch assembly. Pull the gear selector switch out.
Remove the motor from the transmission and insert the motor into the new transmission. Reinstall the gear case and the motor back in the housing, aligning it up properly to lock them into the housing. Reinstall the gear selector switch and then the cover back onto the case, making sure all wires are tucked into the proper wire channels inside the housing to prevent any pinching of wires and causing more issues. Reinstall the transmission clutch assembly to the case and reinstall the rear cover. Reinstall the drill chuck hand tight. Install the screw back inside the chuck, remembering it is reverse thread so turn counter-clockwise to tighten.

The pinion gear is located on the armature shaft. If it has become worn, or damaged, it can jump teeth on the gear and cause it not to transmit power into the transmission.
To check and repair this gear, remove the end cap on the drill then remove all the screws securing the cover onto the housing. Remove the two screws located on the same side as the cover, securing the transmission assembly to the drill housing. Leave the remaining two screws on the transmission, attached to the housing. Remove the cover and lift the motor and transmission out and separate the motor from the transmission.
Depending on your drill, you may have a sealed motor or a motor where you can pull the armature out of the field. Remove the armature from the field, if you can. You can either use a small gear puller to try and remove the gear or you can place the motor, or armature, carefully in a vice or on top of something hollow. Heat the gear up, then tap on the shaft in the center with a small punch and hammer to get the gear off. Once off, allow the shaft to cool. Place the armature, or motor, into a freezer for a while in order to get the shaft to contract, then heat up the new gear to get it to expand.
Place the armature, or motor, into a vice (or on top of something hallow) and gently tap the gear onto the shaft. Marry the motor back together with the transmission and install back into the drill housing. Tuck any wires back into their channels and reinstall the cover as well as the two transmission screws, securing everything tight. Reinstall the end cap.