ELECTRIC Dryer no heat or little heat, or shuts down to fast:
Check your venting and lint basket. Check blower for lint build up and blower wheel obstruction., test by trying to turn the wheel manually by hand (should be easy) May have to remove cabinet or front/back plate to get to it)
Next check the heating element itself with a meter for continuity OHMS CLOSED CIRCUIT. If not its defective or has a short if its grounding out? Which in turns causes blown fuses or thermostats or
The heating elements are located inside the heater ducts. If you think a heating element is faulty, test it with avolt-ohm-multimeter (VOM)set to the RX1 scale. Disconnect the leads from the power terminals and clip one probe of the VOM to each terminal. The meter should read about 12 ohms. If the reading is higher ohms, the heater is faulty and should be replaced. Replace a faulty heater with a new one of the same type and electrical rating. A heater connected to a 115-volt line usually has an 8.4-ohm resistance; a heater connected to a 220-volt line usually has 11 ohms resistance.
Check dryer Terminal block prongs both outside prongs should give combined 220, and 110 each if u check 1 outside & 1 center (ground) prong. Also check house electrical outlet for full voltage. 220 because if u only get half or 110 volts you will be able to run the machine which uses only 110 to run motor but not the heater which uses a full 220,
OR you may have a broken centrifugal switch in the motor because this switch activates the motor and the heater as well. supposed to be if the motor does not run , the heater should not heat in order not to create fire but you said that even the motor is not running, the heater is still heating, then there could be a problem with the motor centrifugal switch that is connected to this interlock switch that should trigger the heater.
Check the thermal cut off, the cycling and the hi limit thermostats.
For continuity or OHMS. If no ohms or resistance they need replacement.
In some dryer's the control panel relies on a thermistor rather than a CYCLING thermostat to regulate the drum's air temperature by monitoring the component's resistance changes; resistance goes down as temperature increases and up when temperature decreases. Once the drum's air temperature reaches a certain level required to dry clothes, the control panel shuts off the heater. The panel will turn the heater on again and begin another heating cycle when the thermistor indicates that more heat is needed to keep the air temperature constant inside the drum
Lastly check your moister sensor. ( located inside the dryer door usually) Especially if machine seems to shut down early and clothes are still wet.
Test with a meter at room temperature and it should show continuity.
A failed moisture sensor will affect the dryer run time in the automatic moisture sensing cycle but it will not affect the heating of the dryer or the timed cycle. Which are reflected by the thermostats.
Read more :http://www.ehow.com/info_12203962_check-dryer-thermistor.html
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Dryer will not shut off:
The timer: Located in the control console of the dryer the timer consists of a small motor connected to a series of gears that rotate cams, which turn electric contacts off and on inside the timer. The timer contacts control the dryer motor as well as the heat circuit and the timer motor itself. A defective timer motor or a defect in the timer's electrical contacts can cause the timer motor to fail and the timer will need to be replaced. The contacts can be checked for continuity with a multi-meter. This test should only be made when the dryer is unplugged. Another test u can do is observe by eyesight and see does the timer advance or is it stuck on one cycle?
1. On gas and electric dryers that have an "Auto Dry" cycle, the cycling thermostat is often used to advance the timer as well regulate the drum temperature. Essentially, thermostat will alternately turn on the heat source or the timer motor when the temperature has been satisfied. To check this thermostat, you will have to check for power to the timer motor with a multi-meter, during a cycle. This is a live voltage test and caution should be used.
2. Some dryers will use a cool down thermostat to tumble the clothes without heat, at the end of the dry cycle. Power is routed through this thermostat to the drive motor to keep it running until the drum temperature has dropped to a specific temperature. If this thermostat fails it can cause the drum to turn indefinitely or until the door is opened. You can check the thermostat for continuity with a multi-meter. This test should only be made with the power removed from the dryer.