When electric dryers appeared on the market more than forty years ago, the housewife guessed how many minutes it would take to dry the clothes. This was an inexact science, at best, and twenty years or more later, schemes were invented to sense when the clothes were dry and to shut the dryer off. Some of the designs to sense moisture in clothes were very complex. Various sensors were imbedded in the back wall of the dryer drum. Most of them were based on measuring the increasing resistance of something when the moisture content of the drum decreased. As often happens, the complexity of the moisture measurement systems led to reduced reliability and even ruined clothes!
Many Whirlpool built automatic dryers employ a very simple, reliable design for sensing when the clothes are likely to be dry. It is based on the fact that dry clothes will not absorb as much heat as wet clothes inside the drum. Therefore, the dryer exhaust temperature will go up quicker when the operating thermostat switches the heater coil on when the clothes are nearly dry. A 4000 to 5000 ohm "power resistor" is switched in series with the synchronous timer motor for the automatic cycles. The timer motor and resistor are connected in parallel with the operating thermostat. The timer motor, resistor and thermostat are then connected in series with the heater coil.
The timer motor advances only when the operating thermostat is open. A small current flows through the heater coil, the resistor and the synchronous motor when the thermostat is open. The amount of time that the operating thermostat is open increases as the clothes get dryer. The synchronous motor merely moves the timer dial to the "off" position. If a large load of clothes that will take longer to dry are loaded into the dryer, the timer is set such that it will take the motor longer to move the pointer to "off". There is no "sensor", just the operating thermostat which never actually touches the clothes. Automatic dryer cutoff is achieved with the addition of that resister and a slightly more complex timer.
The power resister is usually found inside the control panel often in close proximity to the timer. It could be of various physical shapes; a small disc (see the image above), rectangular with attached metal mounting bracket or just an inline design. All are replaced with the same kit, part number 279872, which is an inline style with insulated terminals that covers all applications and comes along with installation instructions.
