How to Replace a Brushless Motor on a Cordless Strimmer (Weed Eater) & The Battery Trap

As an engineer, I see firsthand the continuous battle against built-in obsolescence in domestic and garden appliances. It’s ludicrous that a well-maintained 18-month-old cordless strimmer, a Makita in this case, is simply deemed “disposable” when it stops working.

This guide will walk you through the diagnosis and replacement of a brushless strimmer motor, which is a highly common failure point in these tools. Furthermore, we will address the critical design flaw that forces consumers into a single brand: the proprietary battery bank system.

The Critical Baseline Check: Is the Battery Faulty?

Before you order any parts, the first step in any cordless tool repair is establishing a correct baseline voltage. Even if your battery works fine in another tool (like a cordless drill), an internal fault can bring the whole system down.

The strimmer in question uses an 18V battery bank. This is typically constructed of five 3.6V cells wired in series to achieve an 18-volt nominal charge.

Diagnosis Procedure:

1. Check Open-Circuit Voltage: Use a multimeter set to the DC 20V range. Place the probes across the positive and negative terminals of the battery pack. (If you don’t have a multimeter, you can get one via our Amazon affiliate link.)
2. Expected Voltage: A fully charged 18V pack will read approximately 19.5V to 20V. A pack requiring a charge might read lower, such as 17.6V.
3. Identifying Dead Cells: If the reading is significantly low, for example, 15.4V, this typically indicates that one 3.6V cell in the series is dead or failing. A low-voltage cutoff mechanism within the tool will prevent the motor from engaging, even if the remaining cells are healthy.

Note: Specialized equipment like a balanced charger can be used to re-balance or replace individual cells to restore a dead battery pack.

Diagnosing the Cordless Strimmer Motor Failure

Once you’ve confirmed the battery is delivering a healthy charge, the fault lies within the strimmer unit itself.

In the case of this repair, the motor was intermittent:

1. When the battery was inserted and the trigger pressed, nothing happened.
2. A slight tap or touch to the motor head caused it to spring to life briefly, indicating a likely mechanical or electrical fault within the motor windings or bearings.

Final Motor Check:

1. Disassemble the Motor Cover: Follow the steps in the replacement section below to expose the motor terminals.
2. Test for Power Input: Connect the multimeter to the two motor terminals.
3. Attempt Operation: Insert the good battery and press the trigger.
4. Confirm Failure: If the multimeter reads the full battery voltage (19.8V) at the motor terminals, but the motor fails to turn, the motor assembly is confirmed faulty and requires replacement.

Step-by-Step Brushless Cordless Strimmer Motor Replacement

The strimmer motor is often replaced as a complete assembly. The replacement motor, in this case, was a relatively inexpensive brushless motor unit. (Here is the Amazon affiliate link for the brushless motor.)

Safety First: Always disconnect the battery before commencing any disassembly or repair work.

1. Remove the Spool and Guard: First of all, press the two buttons on the sides of the strimmer head to remove the spool. Then, you should use a sturdy screwdriver to undo both screws securing the safety guard. Lift the guard and set it aside.
2. Access the Motor Assembly: Locate the five screws positioned around the main motor cover on the side of the unit. After that, you will undo all five screws and carefully lift the plastic cover away. Be cautious of any small nuts or fittings that may drop out from mounting locations.
3. Remove the Old Motor: Now, you need to disconnect the two wiring terminals attached to the motor. Gently pull the motor assembly out of its housing. Note the presence of rubber inserts at the top and bottom of the bearing area; these provide shock absorption and insulation.
4. Check Bearings and Clean Housing: After removing the old motor, you should inspect the bearings on the shaft connecting to the strimmer head. Ensure they are running smoothly. Clear any built-up grass, plastic, or debris from the housing. This debris can cause overheating.
5. Install the New Motor: Now, you can connect the wiring terminals to the new motor assembly. Apply a light touch of grease to the steel cog on the motor shaft to aid lubrication against the plastic cogs of the drive system.
6. Reassembly: Finally, you can replace the motor cover, ensuring all components align. Replace and tighten the five mounting screws. Snug hand-tight plus a quarter turn is sufficient; over-tightening can strip the plastic threads.

The Greater Problem: A Call for Standardization

This repair highlights the core problem in the cordless tool industry. The consumer (like my neighbor, Jerry) is unknowingly trapped within one brand because their investment in compatible batteries is substantial. He bought the same strimmer despite being unhappy with the original, purely for battery compatibility.

Manufacturers use slight variations in pin configuration to lock you into their ecosystem, creating immense electronic waste with incompatible battery banks across the industry.

We need governing bodies to step in and mandate a standard, similar to how Apple was eventually forced to adopt the USB-C charging standard globally. Standardizing battery interfaces would:

• Reduce electronic waste.
• Increase competition among tool manufacturers.
• Save consumers significant costs.

If you believe in the right to repair and industry standardization, please share this guide and the video to help amplify the message. Acting as a community is the only way to challenge the false economies propped up by high consumption today.

We hope this guide has given you the confidence to tackle a repair on your cordless strimmer. For a full visual walkthrough and dismantling process, make sure to check out the video on our YouTube channel.

If you have any questions about your appliances, please provide your brand, full model number which is written on the identification label, and a detailed description of the issue. Please visit our Contact Us page and make sure you fill out the request for what your problem is. A video shoot of the problem would be perfect as well.

If you found this guide helpful, consider supporting us by clicking on the Buy Paul a Beer link to help us continue making repair videos and guides.

You can use these Amazon affiliate links below to help support our channel:

• Multimeter
• Cordless Strimmer Replacement Motor
• Cordless Strimmer Replacement Gear Shaft
• Cordless Strimmer Replacement Line