the variable speed trigger on my saw isn't working right, it's either full blast or nothing, no in-between control

Diagnosis

The issue you're experiencing with the Makita XRJ05Z is a failure of the potentiometer integrated into the trigger switch assembly. In a variable speed tool, the trigger isn't just a simple on/off switch; it's a variable resistor that tells the motor controller how much voltage to send to the motor. When this resistor fails or the internal contact points wear down, the circuit loses its ability to modulate speed. This "binary" behavior—where the tool only operates at 100% power or stays completely off—usually happens because the internal wiper of the potentiometer has snapped or become disconnected. Alternatively, fine sawdust and metallic shavings from cutting can migrate past the housing seals and lodge themselves inside the trigger mechanism, creating a physical or electrical block that prevents the trigger from engaging the low-voltage stages of the circuit. You can confirm this by observing if the tool "stutters" or jumps to full speed only after the trigger is pulled past a certain depth; if there is no smooth ramp-up, the switch assembly is electronically compromised.

How to Fix It

Because the variable speed controller is integrated directly into the switch housing, the only permanent solution is to replace the entire trigger switch assembly. For the XRJ05Z, you need part number 413038-6.

1. Remove the 18V LXT battery from the tool to prevent any accidental activation.
2. Using a T10 Torx driver, remove the screws securing the plastic side housing. Be careful not to lose the small screws, as they vary slightly in length between the grip and the motor housing.
3. Carefully pry the two halves of the plastic casing apart. Note the position of the rubber grip; it may shift and need to be reseated during reassembly.
4. Locate the wire harness connecting the trigger switch to the motor. Disconnect the wires by gently pulling the connector terminals apart. Do not pull on the wires themselves.
5. Unscrew the mounting screws holding the switch assembly to the chassis. Lift the old switch (part 413038-6) out of the housing.
6. Insert the new switch assembly, ensuring the trigger arm is properly aligned with the external trigger button.
7. Reconnect the wire harness, ensuring the click-lock engages fully.
8. Reassemble the housing, tightening the T10 screws in a crisscross pattern to ensure an even seal against dust ingress.

If That Didn't Work

• Inspect the Battery Terminals: If the tool still behaves erratically, check the battery contact pins on the saw. If the pins are recessed or charred, they may cause intermittent voltage drops that mimic a failing trigger. Clean them with a cotton swab and 90% isopropyl alcohol.
• Check for Debris in the Trigger Gap: Use a can of compressed air to blow out the gap between the trigger and the tool body. Sometimes a large piece of debris can prevent the trigger from reaching the "low speed" travel distance, forcing it to only engage the "full speed" contact point.
• Test with a Different Battery: A failing cell in an 18V LXT battery can sometimes cause the motor controller to behave unpredictably. Test the saw with a known-good, fully charged 5.0Ah battery to rule out power delivery issues.
• Examine the Motor Brushes: While the XRJ05Z is an LXT model, if the brushes are severely worn or pitted, the motor may struggle to start at low voltages, making it feel like the variable speed isn't working when it's actually just a motor startup failure.

Heads Up

• Avoid Over-Tightening: When reassembling the plastic housing, do not over-torque the T10 screws. The plastic threads in the casing strip easily, which will compromise the tool's dust seal.
• Dust Management: To prevent this from happening again, periodically blow out the trigger area with compressed air after heavy use in dry-cut environments.
• Trigger Discipline: Avoid "feathering" the trigger rapidly during high-torque cuts, as this creates heat spikes in the potentiometer and accelerates wear on the internal contacts.
• Compatibility: Ensure you are using genuine Makita LXT batteries; third-party clones often have inconsistent voltage regulation that can damage the sensitive variable speed circuitry in the switch.