Does your electric lawn mower keep stopping for no obvious reason? Well, it could be down to the thermal cut-off switch doing its job, or it might be a fault with your lawn mower. So, instead of struggling with your mower and cutting your lawn in short bursts, let’s look at the thermal cut-off switch and find out why your electric lawn mower starts then stops.
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Why Your Electric Lawn Mower Keeps Stopping (Overview)
Thermal switches are small inline electronic devices that protect your lawn mower from overheating-related issues. Several different issues, including the following items, can cause an electric lawn mower to overheat:
- Overworking the Lawn Mower
- Poor Motor Ventilation
- Electric Motor Issues
- Repeated Start/Stop of Lawn Mower
- Voltage Rectifier Producing Incorrect Voltage
- Defective Thermal Cut-Off Switch
What is a Thermal Cut-Off Switch?
A thermal cut-off switch is a device that senses the ambient temperature. These devices can be found in all kinds of items, such as household appliances, electric weed whackers, and lawnmowers. The cut-off switch is specifically designed to break the electrical circuit to the electric motor once the motor reaches a level that is considered potentially dangerous or damaging. For example, a Kobolt 21″ corded lawn mower has a cutoff switch that breaks the circuit once the motor reaches 130 C / 266 F. So if this electric lawn mower reaches this specified temperature, it will cut out and stop working.
How a Lawn Mower Thermal Cut-Off Switch Works
Once a thermal switch reaches a specific operating temperature, the switch disconnects and breaks the circuit to the motor. To do this, the contacts inside the switch move apart as they heat up. The connection is lost once the thermostatic contacts reach the specific heat rating. Then, as the contacts cool down, they return to their original position and reestablish continuity to complete the circuit. This is why your lawn mower can take ten or fifteen minutes to work again.
Electric Lawn Mower Starts Then Dies (A Closer Look)
Now that you know more about thermal switches and how they work, let’s go over what causes them to engage and some of the issues they can face.
Overworking Your Lawn Mower
The most common reason a lawn mower thermal switch kicks in is that the mower is overworked. Usually, this happens when the lawn mower tries to cut grass longer and thicker than it can handle. Without getting too technical, an increased load on an electrical mower needs a greater draw of electrical current, which means a greater pull of amps. As a result, more amps equals more heat. So, if your electrical lawn mower is working hard, even for a short time, then it’s likely that the thermal switch will engage and cut power to the motor.
Poor Motor Ventilation
If you look around your electric lawn mower, you’ll see several vents that allow cool air into your mower’s motor and hot air out. Now, most, if not all, homeowner electric lawnmowers are air-cooled and depend on a clear path of airflow in and out of the motor. If the air vents are clogged with grass clippings, the motor will get hotter and hotter as it has no way of releasing heat. As a result, your lawn mower will quickly exceed the thermal switch’s limit, and the motor will stop working.
Electric Motor Issues
There are a few different issues that a lawn mower motor can have that can cause a thermal switch to exceed its cut-off point. These can include problems such as dragging motor bearings that increase the load on the motor or the reduction in motor winding insulation due to excessive overheating. So, whatever is causing the motor to overheat, the thermal switch will react.
Repeated Start/Stop of Lawn Mower
Again, without getting too technical, an electric motor needs a power boost to start turning. So, just like overworking your lawn mower, the start-up process pulls additional amps, generating more heat. So, if you start and stop your electric lawn mower a lot, you’re likely overheating the motor and causing the thermal switch to break the circuit.
Voltage Rectifier Producing Incorrect Voltage
First of all, what is a voltage rectifier? Well, it’s an electronic device that converts AC power to DC power on your corded electric lawn mower. You need one because the motor in your lawn mower is DC, and the power from your outlet is AC. So this little device converts the power so your lawn mower can operate.
Unfortunately, like everything else, they can go bad. Typically, you will find that the rectifier will produce too much or too little DC power. As a result, the incorrect input power to the motor will cause overheating, and the cut-off switch will react. Again, this is the switch telling us that there is an issue that needs to be taken care of.
Defective Thermal Cut-Off Switch
As I mentioned, most things go wrong, including the thermal cut-off switch. The simple fact that you can buy a replacement switch kind of proves this is the case and that manufacturers expect this to happen. So, if your lawn mower does, in fact, start up again once it’s cooled down, then it’s an indication that the switch is okay. But, if it doesn’t or engages very quickly after start-up, you might need to replace the switch.
You can test a thermal cut-off switch with an affordable infrared temperature sensor to see when the cut-off switch engages. Also, you can test for continuity when the switch is cool using an inexpensive electrical multimeter. These tests will tell you if the switch is doing what it should in both positions. Now, if you find that there is power in and no power out when the switch is cool, it’s defective, and you’ll need to install a replacement.
How to Replace a Thermal Cut-Off Switch
Replacing a thermal switch will differ from one lawn mower to the next. But usually, the switch is simply connected to the motor with push connectors. So, it’s just a case of disconnecting the old switch and installing the replacement, then ensuring it’s mounted to your lawn mower correctly.
Now, there are a few things you need to get right to protect both yourself and your lawn mower:
- Make sure your lawn mower is disconnected from the power source.
- Ensure you install a thermal switch with the same rating as the original.
- Connect the new switch the same way as the original.
Electric Lawn Mower Won’t Start (More Reasons)
Now, if you have tested your thermal switch and everything seems okay, but your electric lawn mower starts then dies, you’ll need to look elsewhere.
Here are the other items I would check when diagnosing why my electric lawn mower won’t run.
- Incoming Power
- Lawn Mower Cables & Extensions
- Power Source Connection
- Electrical Connections
- Power Rectifier
- Motor Brushes
- Power Switch
- Power Lever
If you want to learn how to solve other potential electric lawn mower problems, check out my blog on 8 Common Electrical Problems on Electric Lawn Mowers
