there's a fine black powder blowing out of the vents near the motor, is my grinder burning up?

Diagnosis

The fine black powder emanating from the motor vents of your Makita XAG04Z is carbon dust, a byproduct of the friction between the carbon brushes and the rotating armature commutator. In a brushed motor, these carbon blocks conduct electricity to the spinning part of the motor. As they rub against the copper segments of the commutator, they microscopically wear down, creating a conductive soot that the internal cooling fan then blows out through the vents to prevent buildup. While a small amount of dust is normal, a noticeable "cloud" or heavy accumulation of black powder usually indicates that the brushes are wearing down prematurely or are sparking excessively (arcing). This happens most often if the grinder is used heavily for grinding hard materials like stainless steel or masonry, where the motor is pushed to its thermal limit, or if the tool has been exposed to high levels of conductive metallic dust which creates a "bridge" and accelerates brush erosion. If you see blue sparks inside the motor housing accompanying the powder, the brushes are likely near the end of their service life.

How to Fix It

To resolve this, you must inspect the length of the carbon brushes and replace them if they have worn past the critical threshold. Follow these exact steps:

1. Remove the 18V LXT battery pack immediately to ensure the tool cannot be accidentally triggered.
2. Locate the two circular plastic brush caps on opposite sides of the motor housing. Use a flat-head screwdriver to rotate the caps counter-clockwise until they unscrew and pull them out.
3. Using a pair of needle-nose pliers or a small screwdriver, gently compress the brush spring and slide the carbon brush assembly out of the housing.
4. Measure the remaining length of the carbon block. If the carbon piece is shorter than 1/4 inch (approx. 6mm), it can no longer maintain consistent pressure against the commutator, leading to the excessive dusting and arcing you are seeing.
5. Clean the brush holders with a can of compressed air to remove any accumulated soot before inserting new brushes.
6. Install a new set of Makita carbon brushes (Part No. 903371-1). Ensure the brush slides smoothly in the holder without binding.
7. Reinsert the spring, seat the brush firmly, and screw the plastic caps back into place until tight.

If That Didn't Work

• Check for Commutator Scoring: If new brushes still produce excessive powder, inspect the copper commutator (the rotating part the brushes touch). If you see deep grooves, burnt black spots, or "pitting," the commutator is damaged. You can attempt to lightly clean the copper bars with a fine-grit emery cloth, but severe scoring usually requires a professional armature replacement.
• Verify Vent Clearance: Ensure the intake vents are not clogged with sawdust or metallic filings. If the motor cannot breathe, it runs hotter, which causes the carbon brushes to degrade and "flake" much faster than normal. Use compressed air to blow out the entire motor housing from the front to the back.
• Inspect for Bearing Failure: If the motor shaft has any "play" or wobble, the brushes will hit the commutator unevenly, causing rapid, asymmetric wear and heavy powder output. Shake the spindle; if there is lateral movement, the motor bearings are shot and the tool needs a rebuild.
• Switch to a Higher Amp Battery: If you are using a low-capacity battery, the tool may be struggling under heavy load, causing the motor to run inefficiently and overheat. Switching to a 5.0Ah or 6.0Ah LXT battery provides more consistent current and can reduce thermal stress on the brushes.

Heads Up

• Never attempt to "stretch" the life of brushes by jamming springs or using non-OEM carbon replacements; this can permanently weld the brushes to the commutator and destroy the motor.
• Always replace brushes in pairs. Even if only one looks worn, the other has experienced the same amount of operational hours and will likely fail shortly.
• Avoid using the grinder at low RPMs for extended periods. The internal fan is tied to motor speed; running too slow prevents heat from escaping, which accelerates carbon degradation.
• Periodically blow out the motor vents with compressed air every 20 hours of use to prevent the black powder from insulating the motor and causing a thermal shutdown.