the variable speed trigger doesn't work right, it's either full power or nothing, no in-between control

Diagnosis

The loss of variable speed control on the Milwaukee M18 Fuel Reciprocating Saw (2821-20) is almost always caused by a failure of the internal potentiometer located within the trigger switch assembly. The potentiometer is a variable resistor that tells the electronic speed controller (ESC) exactly how far the trigger is depressed. When the resistive track inside this component wears down or becomes contaminated with fine sawdust and metallic debris, the signal becomes "binary." Instead of a smooth voltage ramp, the controller sees a gap in the signal, resulting in a "dead zone" followed by a sudden jump to 100% power once the trigger reaches a specific physical point. To confirm this is a trigger failure and not a logic board issue, check if the tool behaves consistently across different batteries. If the "all-or-nothing" behavior persists regardless of the M18 battery capacity or age, the fault lies within the physical switch mechanism. You can also check for "stiction," where the trigger doesn't snap back instantly; if the trigger feels physically mushy but the power jumps instantly, the internal spring is fine, but the electrical contact strip has failed.

How to Fix It

Because the potentiometer is integrated into the trigger housing, you must replace the entire switch assembly (Part No. 45-20-0012 or 02-06-2821 depending on the production run). Follow these steps for a professional installation:

1. Remove the M18 battery and all saw blades to ensure the tool is completely inert.
2. Using a T15 Torx driver, remove the screws securing the plastic clamshell housing of the handle. Be careful not to strip the heads, as the plastic bosses can crack if over-torqued.
3. Carefully pry the two halves of the handle apart. Avoid using a metal screwdriver to pry; use a plastic trim tool to prevent gouging the casing.
4. Locate the trigger switch assembly. Disconnect the wiring harness leading to the main control board. Do not pull by the wires; grip the plastic connector firmly and pull straight back.
5. Remove the failed trigger switch from its molded seat in the handle.
6. Insert the new switch assembly, ensuring the trigger arm is properly aligned with the external trigger paddle.
7. Reconnect the wiring harness, ensuring the locking tab clicks into place to prevent vibration-induced disconnection.
8. Reassemble the housing, tightening the T15 screws in a cross-pattern to ensure an even seal against dust ingress.

If That Didn't Work

• Check the Control Board (ESC) for burnt components. If the trigger was replaced but the tool still jumps to full power, the MOSFETs on the main circuit board may have suffered a partial short, causing the tool to bypass the speed regulation logic. Look for charred areas or "bulging" capacitors on the board.
• Inspect the wiring harness for frays or pinches. A partially shorted signal wire between the trigger and the board can create a voltage drop that mimics a full-throttle signal, tricking the tool into running at max RPM immediately.
• Verify the battery contact pins. If the pins on the tool's battery terminal are recessed or heavily oxidized, the tool may experience voltage drops that cause the electronic controller to reset or glitch, leading to erratic power delivery. Clean the pins with an electronic contact cleaner and a cotton swab.
• Test with a different M18 High Output battery. In rare cases, a failing cell in a standard M18 battery can cause voltage fluctuations that interfere with the variable speed logic of the Fuel brushless motor.

Heads Up

• Avoid using compressed air to "blow out" the trigger while the tool is assembled; this often pushes fine conductive metal dust deeper into the potentiometer, accelerating wear.
• When reassembling the handle, ensure the rubber overmold is seated perfectly. If there is a gap, sawdust will enter the housing and contaminate the new switch within weeks.
• Avoid "feathering" the trigger excessively when starting a cut in heavy material; this puts maximum stress on the potentiometer and the ESC.
• Periodically check the T15 screws on the handle. The high vibration of the reciprocating action can loosen these screws, allowing moisture and debris to enter the electronics.