How to detect a Dead Milwaukee M18 Hackzall: MOSFET Repair & Blown MCU Teardown

Picked up this Milwaukee M18 Hackzall on Facebook Marketplace for twenty quid. It emitted a faint relay click and a brief LED flash, but the blade didn’t move. Fixing cordless brushless tools requires cutting through rubberized potting resin, inspecting surface-mount power MOSFETs with a microscope, and tracing high-voltage gate failures.

Testing and Results

Initial Inspection & Clamshell Disassembly

I started by testing the tool with a full 18V battery. The LED activated and the trigger generated an internal click, but the motor didn't spin. I verified the blade shaft manually to ensure the transmission wasn't seized. It rotated freely. Mechanical failure was ruled out.

Extracting Internal Electronics & Motor Assembly

I removed the drivetrain and control module from the housing. Milwaukee pots these modules in black plastic to dampen vibration and prevent moisture damage. The rear PCB connects to the stator and a ribbon cable carrying Hall effect sensor signals. The MOSFETs were tucked away.

Disassembling the Brushless Motor Mechanism

I separated the motor from the metal gearbox to reach the sensor PCB. A larger Torx bit loosened the motor bolts. Once open, I checked the rotor shaft and front bearing. The bearing showed slight wear, but it wasn't enough to stop the motor from turning.

Checking Hall Effect Sensors & Mechanical Clearance

I cleaned the dust from the stator and inspected the sensors. Misaligned rotors often strike these sensors. The three sensors appeared intact with no visible damage. Just a lot of debris.

Digging Out Potted Circuit Boards

I pried the module from its housing. I used a dental pick and 99% isopropyl alcohol to remove the silicone. It's slow work. The solvent helps the silicone peel away. I realized the controller is actually a dual-board assembly held together by potting.

Evaluating Replacement Cost vs Repair Feasibility

Milwaukee doesn't offer individual boards. You have to buy the entire potted assembly, which costs nearly as much as a new tool. Component repair is the only logical path.

Desoldering Sandwiched Control Boards

I used a vacuum desoldering tool at 380°C to clear the interconnect pins. Once clear, I separated the boards. The lower board contains the DRV91670 controller, which combines the MCU and pre-driver in one chip.

Uncovering the MOSFET Driver Array

I removed the heatsink screws. The board uses six surface-mount MOSFETs for the H-bridge. Lifting the heatsink showed carbon residue around one of the transistors.

Microscopic Inspection of Power MOSFETs

Under the microscope, I saw the MOSFET had failed completely. The epoxy package was split with clear cratering near the gate trace. These failures often occur when high current shorts through the gate.

Hot Air Rework & Desoldering Damaged Transistors

I removed the rubber seal to prevent melting. I used flux and 360°C hot air to heat the substrate. Once the solder flowed, I lifted the transistor with tweezers. I used copper braid to clean the residual solder.

Multimeter Diode & Resistance Testing

I tested the MOSFET. It showed a permanent 8-ohm resistance instead of proper gate isolation. The transistor was dead.

In-Circuit Testing & Selecting Replacement MOSFETs

I tested the remaining transistors to ensure they were safe. High-current failures often stress adjacent components. I opted to replace both MOSFETs on that phase leg to be safe.

Surface-Mount Soldering & PCB Sandwich Reassembly

I preheated the board, tinned the pads, and placed the new MOSFETs. I reflowed the solder with hot air. I reattached the heatsink, joined the two boards, and soldered the header pins.

Rehousing Internal Components & Dry Run

I organized the internal wiring and battery terminals back into the frame. I closed the shell and inserted the battery. The trigger activated the LED, but the motor remained dead.

Thermal Diagnosis of Catastrophic MCU Failure

I opened it up and used a thermal camera on the logic board. The controller IC was overheating. When the MOSFET failed, it dumped 18V into the microcontroller, destroying the internal logic. Without access to the proprietary firmware, the unit is non-repairable.

Summary & Tips

MOSFET replacement is feasible with basic rework tools. However, a gate-shorted MOSFET often destroys the MCU, and without factory firmware, that board is scrap.

Disclaimer: I bought these tools myself. This post contains affiliate links; I may earn a commission if you make a purchase.