A thermostat is basically just a switch that opens or closes according to the temperature that it senses.
This switch can be used to turn the heating system (either gas or electric) on and off to maintain a certain air temperature range at a given place in the system. There may be several different thermostats side by side; for example 135 degrees for low temperature, 165 for high temperature, etc. You choose which thermostat is used by selecting the temperature on the dryer console.
There are exceptions; for example, Frigidaire used a specially-designed thermostat that has five leads instead of two. Naturally, it is a bit more expensive than regular thermostats, and difficult to test.
You can test a thermostat as described in section 1-4(b) by testing for continuity across its terminals. A cold operating stat or hi-limit stat should show continuity. A cold cool-down stat should show no continuity.
If a thermostat fails into a closed position, there is a danger that the heating system will continue operating until something catches fire. To prevent this, there is a high-limit thermostat that will cut out the entire heating system.
There is no way to repair thermostats. Replace any that are bad.If you suspect it’s defective, it probably is. Replace it.
A thermistor is a “variable resistor” whose resistance varies with temperature. Rather than just turning the heating circuit on and off as thermostats do, dryers with solid-state (computer logic board) controls can use a thermistor’s input to control the drum temperature more closely. This can result in lower energy usage.
Thermistors are tested by measuring resistance across them with a VOM. A cold thermistor should show no resistance. Replace if defective.
This fuse will blow when too high a temperature is sensed at the outlet; usually when one of the operating thermostats has failed.
AUTO CYCLES (More Dry-Less Dry)
In an “auto” cycle, the system has some way of sensing how much moisture is in the air inside the dryer drum. If the air is dry, the timer advances more quickly to end the cycle sooner. This is done in one of two ways.
When the air in the drum is moist, the water in it absorbs heat to evaporate. This keeps the air temperature lower, and it takes longer to heat up.
The thermostat on the drum exhaust will keep the heating system on longer. In these systems, that same thermostat controls the timer motor; while the heating system is on, the timer motor is not running, and vice-versa. So when the clothes get drier, the exhaust air temperature gets higher more quickly, the heating system doesn’t stay on for as long and the timer motor runs more, ending the cycle sooner.
The thermostats in these machines have three leads. One side goes to the timer motor, the other to the heating system. These thermostats are also difficult to test without any way to heat them up. But they’re pretty cheap. If the symptoms lead you to suspect that yours is defective, just replace it.
Besides heating more slowly, moist air also conducts electricity better than dry air. So another way the engineers design a humidity sensor is to put two electrical contacts inside the dryer drum. The electrical currents conducted by the air are so low that an electronic circuit is needed to sense when the air is moist, but essentially the same thing happens in this system as in the other. When the air is moist, the timer motor doesn’t run as often. When the air is dry, the timer motor runs longer and times out sooner.
The sensors in these machines tend to get coated with gummy stuff, especially if you use a lot of fabric softener in the wash or starch in ironing. If the timer is not advancing during the auto cycle, this is likely what has happened. Try scrubbing the sensors with a little kitchen cleaner, such as 409 to get it off. In extreme cases, use a little rubbing alcohol as a solvent. The circuit board could have gone bad, too; there is no good way to test it with out a lot of expensive equipment. If you think it has gone bad, it probably has. Replace it.
SPECIAL NOTE: In electric dryers with an automatic cycle, a special problem exists. The problem is that the heater operates on 220 volts, but the timer motor runs on 110 volts. There is a resistor in the system to cut down the voltage (see figure G-6(a) and if this resistor is bad, you will see the same symptoms as if the thermostat was bad: the timer motor will not run in the automatic cycle. If you have one of these dryers, make sure you test the resistor for continuity, in addition to the thermostat.
OTHER TEMPERATURE CONTROLS
Selecting which thermostat is used may be done inside the timer, or there may be a separate multi-switch that accomplishes this. Test internal timer switches as described in section 2-4(b).
Separate temperature selector switches are tested either by measuring continuity through each contact (figure G-9) or by jumping across the two correct terminals with your alligator jumpers.