Proper Operation
Learn how your appliance is supposed to operate so you can determine if it is malfunctioning.


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Replacing the Heater Element

Thermostat

The temperature of the air inside the drum is regulated using thermostats. Thermostats open and close an internal switch with respect to the temperature of their environment.

There are two different thermostats in your dryer: the operating thermostat(s) and the limit thermostat. The only difference in these thermostats is their opening and closing temperatures.

Operating Thermostats

Operating thermostats control the temperature of the air inside the drum. You may have one or more operating thermostats mounted on the exhaust side of the blower housing. One may be used for the "Normal" drying cycle and will open at the highest temperature. The thermostat for "Permanent Press" or "Gentle" will open at a lower temperature. The operating thermostats are in series with the heater element. Operating thermostats normally open from 120 to 170 degrees and are always closed at room temperature.

Limit Thermostats

The limit thermostat is located on the side of the heater chimney.

Limit Thermostats

As its name implies, this thermostat shuts off the heater element if a dangerously high temperature is reached. This can happen if the operating thermostat fails (welds closed) and demands heat constantly. When this happens, the limit thermostat becomes the operating thermostat. Unfortunately, the limit thermostat cycles at a temperature almost 100 degrees higher than the high temperature operating thermostat! This does your clothes absolutely no good. You should be able to smell that something is wrong.

Charred Thermostats

Sometimes the terminals on the high limit thermostat burn or char. This problem starts with oxidation of the spade terminals on the end of the red wires leading to and from the thermostat. The oxidation process is accelerated by the heat from the heater element. The oxidized contacts generate heat when the high heater current flows through them. The high heat increases the contact resistance and the heat goes higher. This causes the terminals to get hotter and hotter until the bakelite case of the thermostat breaks down and the spade terminal and the wire insulation turns black. When this occurs, the high limit thermostat and the spade terminal should be replaced.

Thermal Cutoffs

Thermal cutoffs

Whirlpool installed thermal cutoffs near the top of the chimneys. They provide additional protection in the event that the heat generated by the heater coil does not move out into the drum fast enough. A picture of a thermal cutoff is shown below.

The frustrating feature of a thermal cutoff device is that it opens when its operating temperature is reached but then it stays open and will not close when the temperature drops down. In other words, the cutoff works once and then has to be replaced. I have determined that this part frequently false alarms, resulting in a service call to replace it. Part number 279769 contains a new thermal cutoff AND a high limit thermostat. Install the thermal cutoff if you want to go back to the original equipment or install the high limit thermostat in place of the thermal cutoff if you don't want to mess with this thermostat anymore.

If you want to check the thermostats, follow this procedure:

Procedures for Checking Thermostats

  1. Disconnect the power cord and move the dryer far enough away from the wall that you can disconnect the dryer vent hose.
  2. Remove the 5/16" sheet metal screws securing the rear access panel. Save the top, middle screw for last. Set the panel aside.
  3. Visually inspect the operating thermostats on the blower housing. Do any of the paper labels look charred? If they do, replace them. Set your ohmmeter at its lowest scale and confirm that they are closed by measuring less than 1 ohm. If a thermostat is not charred and it is closed it is usually OK. However, a thermostat's contacts can weld together and pass this test. They will usually get very hot before they weld and char the paper label. If you noticed charred paint on the rear access panel, just behind the heater chimney, suspect a welded operating thermostat. Replace them if the ohmmeter shows an open circuit at room temperature. The thermostat's operating temperature (temp that it will open) is stamped on its base. An L155 thermostat will open at 155 degrees. If you want your dryer to operate at a higher temperature, replace its operating thermostat with one bearing a higher L number.
  4. Perform the same checks on the limit thermostat as for the operating thermostats in the preceding step. You should have smelled the problem with a failed limit thermostat. A limit thermostat will not cycle unless the selected operating thermostat has failed closed. If the dryer is operating on the limit thermostat, it has transformed into an oven and is baking your clothes in a convection chamber at a temperature of about 250 degrees! Limit thermostats cycling at these elevated temperatures are prone to failing closed. Of course things really get hot with all thermostats failed closed and the heater element constantly on. I have seen one dryer go this far. It had melted the plastic console down and was close to igniting the lint ( and probably the clothes). This is a very dangerous situation because dryers are usually operated unattended. You can order a replacement limit thermostat using the model number of your dryer.
  5. Take all suspect thermostats to your nearest appliance store and purchase new ones. If the paper labels are charred and illegible, remember whether they were operating thermostats or the limit thermostat. If you are replacing one of two operating thermostats, determine from the wiring diagram on the back of the dryer whether you are replacing a high (Cottons) or low (Permanent Press) temperature thermostat. Or If you had two operating thermostats and one was charred beyond recognition, copy the numbers off the one that checked out OK and purchase the other one from the parts list. Take your dryer model number along.
  6. Replace the thermostats (you can reverse the wires on a two-terminal thermostat with no ill effects).
  7. Replace the rear access panel and check for proper operation.