My Harbor Freight Multifunction Tool wouldn't start the other day. It came with a spare set of motor brushes, so I thought that brush wear be the problem. Took the three screws off the back cover and found that the brushes were in good shape. I sprayed the commutator with some contact spray and buttoned it back up. It then started. A few days later, it again refused to start. This time I removed the cover and removed one brush holder. I cut off a piece of 220 grit silicone carbide abrasive paper about 1-1/2 square and held it against the commutator while directing compressed air from a blowgun at the motor cooling fins. The commutator whirled around and the abrasive paper cleaned it up in about 30 seconds. After reinstalling the brush holder and the cover, the tool started right up.