Your dryer uses of a combination of heat and airflow to dry your clothes. The heat generated in your dryer is produced by a heating element controlled by a series of thermostats. If any of your dryer's thermostats are defective, the results can be disastrous.
Safety Warning: Before performing any tests or repairs on your dryer disconnect the power source to eliminate the risk of electric shock. You can do this by unplugging the dryer, removing the related fuse from the fuse box, or flipping the appropriate switch on the breaker panel.
A dryer thermostat is usually oval shaped and about an inch and a half in length. The thermostat contains a bimetal that opens and closes a pair of contacts depending on the temperature inside the dryer.
The bimetal inside the thermostat is designed to bend at specific temperatures. When the bimetal bends, it pushes an actuator. The actuator then pushes on a contact, opening the electrical circuit and breaking the electrical connection to the related component. When the thermostat cools, the bimetal returns to its original shape and the contacts close, allowing the current to flow through the circuit.
Here's how it works: The dryer is turned on and heat begins to radiate from the heating element into the dryer's drum. The air in the drum passes by a thermostat. As the thermostat reaches its maximum temperature, the bimetal bends, cutting the power to the heating element. The circuit stays open until the bimetal cools. Because the heating element isn't providing additional heat, the dryer's temperature gradually falls. The bimetal returns to its original shape and the electric current flows to start the heating element again. This process happens many times throughout your dryer's cycle.
There are at least two thermostats in your dryer: cycling (operating)thermostat and hi-limit safety)thermostat. The difference between these two thermostats is their opening and closing temperatures.
The cycling thermostat is typically found in the path of the air leaving the drum. A cycling thermostat is usually found on the fan housing or just under the lint filter area, on the blower wheel housing or inside the venting/exhaust system.
Some dryers may have as many as five thermostats - one for each of the different heat cycles. The temperature setting or cycle selected determines which thermostat is used to control the heat. If your dryer is malfunctioning on the low heat setting, the thermostat for that particular setting is probably defective.
The thermostats for the high and medium temperature selections are not the problem. However, if the dryer isn't working properly for most of the heat settings, it's probably a problem with your vent rather than the thermostats.
The hi-limit thermostat protects your dryer from overheating. This thermostat is usually found on the heating element, housing, or cage assembly. If the airflow in the dryer becomes obstructed by a plugged or improperly installed vent, bad drum seals, or a defective blower, the high-limit thermostat cuts power to the heating element. This means there are other problems with your dryer.
In conjunction with thermostats, dryers use thermal fuses as a safety device. Some dryer models may have two thermal fuses to detect extreme heat. If the hi-limit thermostat fails to cut power to the heating element and the element gets too hot, the thermal fuse blows and cuts all power to the dryer. This could mean that a thermostat is defective or something is wrong with your venting duct, filters, seals, or blower. You cannot reset thermal fuses so once they blow they must be replaced. Get the proper replacement fuse for your model and replace the hi-limit thermostat as well. Never bypass a thermal fuse.
Have a look at your dryer's manual for the locations of its thermostats. It's unusual for a dryer's thermostat to continue operating at a different temperature than originally intended and the only way to test for this would be by checking the temperature of the exhaust. You can do this by placing a pocket thermometer inside the exhaust vent. This test is done with the dryer running, so be extremely careful.
Checking continuity is another way to test your thermostat. There's a wire leading to each of the thermostat's terminals. The wires are connected by metal slip-on connectors. Label the wires before you remove them so that you're able to correctly reconnect them later. To remove the wires use needle nose pliers to pull on the connectors - don't pull on the wires themselves.
Set your multimeter to the RX1 setting. With the thermostat at room temperature, touch one meter probe to one terminal and touch the other meter probe to the other terminal. You should receive a reading of zero. If a thermostat is tested when it's heated to its limit, a reading of infinity should be produced. You should replace your thermostat if it fails either of these tests.
The thermostat is attached to the dryer with two screws. Remove both screws and discard the faulty thermostat. Install a new thermostat, securing it in place with two screws. Reconnect the two wires, put your dryer back together, and restore power to the dryer. Run your dryer through a cycle to make sure it's working properly.
Protect your family and protect yourself - maintain your dryer regularly. But if you can't do it yourself, make sure you contact a service technician to do it for you.
