the scanner on my brother mfc j4335dw is making a loud grinding noise and won't scan

Diagnosis

The loud grinding noise emanating from your Brother MFC-J4335DW is the sound of the stepper motor continuing to rotate while the scanner carriage (the CIS unit) is physically immobilized. This happens when the carriage cannot complete its "homing" sequence—the process where the scanner head moves to one side to calibrate its position upon startup. If the head cannot reach the home position sensor, the motor will force the gears, creating a harsh, mechanical grinding sound. This failure is typically caused by one of three things: a foreign object (like a staple or a scrap of paper) wedged in the carriage path, a buildup of grime/dust on the drive belt that causes the carriage to slip, or a "jumped" belt where the rubber timing belt has slipped off the drive gear. You can confirm this by observing the scanner head; if it vibrates rapidly in place or doesn't move at all while the noise persists, the drive system is disconnected or obstructed.

How to Fix It

To resolve the grinding and restore scanning functionality, follow these precise steps to clear the path and reset the carriage:

1. Power down the printer completely using the power button and unplug the AC power cord from the wall outlet. This is critical to prevent electrical shorts when manipulating the scanner head.
2. Lift the scanner cover. Open the Automatic Document Feeder (ADF) cover by lifting the top plastic assembly to expose the internal glass and the white plastic underside.
3. Inspect the entire length of the scanner glass for any small debris, such as staples or paper clips, that may have fallen into the gaps between the glass and the plastic frame.
4. Gently place your hand on the scanner carriage (the rectangular bar with the light) and attempt to slide it slowly from left to right. If you feel a hard stop or a "bump," there is a physical obstruction. Use a pair of tweezers to remove any debris.
5. If the carriage moves freely but the grinding persists upon startup, check the timing belt. Look for a thin, black ribbed rubber belt. If it has slipped off the small gear on the motor end, gently hook it back onto the gear teeth using a small flat-head screwdriver.
6. Once cleared, plug the printer back in and power it on. The carriage should glide smoothly to the home position without noise.

If That Didn't Work

• Hard Reset the Logic Board: Sometimes the motor driver enters a "fault state" and continues to send power to the motor even after an obstruction is cleared. Unplug the power cord and hold the power button down for 30 seconds to discharge the internal capacitors. Leave it unplugged for 10 minutes before restarting.
• Clean the Encoder Strip: Look for the thin, clear plastic strip that runs horizontally across the scanner bed. If this strip is smudged with ink or grease, the scanner doesn't know where the carriage is and will "hunt" violently, causing a grinding sound. Carefully wipe this strip with a lint-free cloth dampened with a tiny amount of isopropyl alcohol (70% or lower).
• Lubricate the Carriage Rail: Over time, the factory grease on the metal guide rail can dry out or collect dust, increasing friction. Apply a tiny amount of white lithium grease or a silicone-based lubricant to the metal rail the carriage slides on. Do not use WD-40, as it can degrade the plastic components.
• Replace the CIS Unit: If the grinding is accompanied by a "Scanner Error" message on the LCD screen and the belt is intact, the internal motor or the CIS (Contact Image Sensor) assembly has failed mechanically. This requires a full replacement of the scanner unit assembly.

Heads Up

• Never force the scanner carriage if you feel significant resistance; you risk snapping the timing belt or stripping the plastic gear teeth, which turns a simple fix into a hardware replacement.
• Avoid using glass cleaners with ammonia on the scanner bed, as the fumes can degrade the plastic seals that keep dust out of the carriage track.
• Always ensure the ADF is completely closed and latched before powering on the device, as a partially open feeder can sometimes interfere with the initial calibration sequence.
• Keep the printer in a low-dust environment; the MFC-J4335DW uses a precision rail that is highly sensitive to airborne particulates.