Defrost heater does not cycle on

The first thing you want to do is open the freezer and see if the fan inside there is running. The reason is the freezer cools the refrigerator side by circulating air from the freezer. If the fan is running then your problem is one or more parts of the defrost system is malfunctioned and in return your problem arises. There are three parts to the system, the defrost timer or in some cases control board, defrost thermostat also known as a bi-metal, and the defrost heater. The defrost system is set up to operate as so. The defrost timer or control board will turn the machine off, in some cases every 8 hours, some 12. During this time period the defrost heaters, located behind the freezer wall and rest underneath the evaporator, yet wires to them run up the side of it, are energized and emit heat that raises upward to melt the ice that has formed on the evaporator coils. The bi-metal or defrost thermostat is at the top of the evaporator. When the heat from the heater starts to reach the bi-metal, it breaks the circuit turning the heater off. The whole purpose of this is because the freezer cool the refridgerator side by drawing air across the coils and blowing to the other side via the evap fan motor. In my opinion, if you change one part of the defrost system change them all because usually when one part fails another will follow, so save yourself the time and the food loss, the cost difference is minute.

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The defrost timer or the defrost heater element has probably failed. Go to the link below and enter your exact model number; you should find a line-drawing illustration and parts list for your fridge that you can use to locate these items and normally also determine how to remove or install them:
Sears PartsDirect

check to make sure the fan in the freezer section is working as this pushes air into the refrigerator section... also check to see if there is frost on back panel of freezer if so you may have either the fan not working or a defrost problem... will require that you remove freezer shelves and back panel of freezer on the inside of unit... you will see the fan, a heater at the bottom of coils ,and defrost thermostat attached to the coil usually toward the top side ... check the defrost thermostat with a meter for contuinity while it has ice on it as it closes at very cold temps... remove wires from heater and thaw out coul with a hair dryer now that you have tested defrost thermostat you can thaw out and check heater for contuinity... if both good then you have a bad defrost timer... if the unit is not froze up in back panel of freezer and fan is functioning properly. then possible unit has developed a leak... not sure what vents you are referring to that have ice on but if it is what i think your describing frost/ice on back panel of freezer or between freezer and refrigerator area then either a fan has failed or you have a defrost issue...good luck to you

Defrost problem: 3 components. Defrost Heater. Timer (circuit board or motorized) & Thermostat.
Remove cover on the inside back wall (usually 6 screws). Note frost pattern. (bottom of evaporator coil)
Put refrigerator into defrost cycle and note the defrost heater come on, or not. (careful heater can glow red hot)... Unplug Frige and test defrost components. Replace bad component... Refrigerator should defrost every 8 hours. when the cycle slows or quits the freezer will freeze over the evaporator coil and no air flow = no cold air...

You have a problem with the defrost circuit of your refrigerator. The parts of this circuit include the defrost timer/module, heater and thermostat. The function of the defrost circuit is to periodically stop the compressor and fan and turn on a heater mounted on the evaporator coil to melt any frost buildup. The fact that ice is building up and even stopping the fan indicates that the defrost circuit has failed for a while and the temperature in the refrigerator compartment is probally higher than it should.

Typical failures with the defrost are the timer/module and heater. Can't tell if you have a timer or module since you did not supply a model number. You can verify if the heater is bad if you have a meter. You will need to unplug the refrigerator and remove the panel in the freezer and use a hair dryer to melt the ice from the coil. Once the heater is exposed, a meter will be able to determine if the heater has failed.

If you are not able to troubleshoot the problem, recommend that you call a service company. Hope this helps you.

Is the compressor motor running.If no check starter/overload relay.
Check to see if the condenser fan at the back underneath near the compressor is running. If no check fan.

Are the condenser coils near there warm or room temp. should be warm


How to check stuff>
http://www.acmehowto.com/howto/appliance/refrigerator/refrigerator.php

Below the evaporator fan is the evaporator coils. Remove the back cover in the freezer to observe the frost pattern. Light frost everywhere(NORMAL) or a partial pattern of ice(LOW ON FREON) or nothing(LOW FREON OR COMPRESSOR PROBLEM).

If the evaporator coils behind the back panel of the freezer are icing up because of auto defrost failure that will stop the circulation of cold air and eventually affect the freezer too.

check defrost timer, defrost heater, defrost thermostat. In most newer models the timer has been replaced by an electronic control board. If the heater and thermostat are ok it’ll be the control.

You can turn the defrost timer till it clicks and everything shuts down. The heater should now come on. If it does, replace the timer. If it doesn't, check the heater and defrost thermostat. Turn the timer again till everything starts back up to end the defrost cycle.

Sounds like the automatic defrost system is not working. The usual problem is the defrost heater, the defrost thermostat, or the defrost timer. If any of these parts are bad, it will not defrost.

You have a defrost problem. Common problem on most frost free refrigerators because the ice buildup does not allow efficient heat transfer to evaporator coil. You will need to remove the cover in the back of the freezer compartment (you know , where you scraped of the ice).When you remove panel you will see a buildup of ice on your evaporator coil. This will have to be thawed out by hairdryer or some other means before you move onto next step.Next you will see a heater element (black thingy ) or glass tubes with internal elements .These are your defrost heaters. They can usaully be visually inspected but best to use an ohmeter to check continuity.If you have good heaters you will want to put frig into defrost by manually turning the defrost timer clockwise untill you hear frig cut off. (Look for timer inside right frig compartment next to thermostat.) power will now be diverted to heaters and will get hot for duration of defrost cycle. If defrost thermostat is bad this could be the case of problem( round looking disk thingy clamped to coil and wired before heater) You can check by bypassing the defrost therm directly to heater and see if heater works while in defrost cycle.You will want to replace it as this is the device that turns off the heater so as not to overheat and melt plastic in freezer.Just reinstall everthing and you should be good to go.

probably a bad def sensor. attached to the top of the coooling coil is a round sensor that senses heat form the def heater and terminates defrost when the coil cleared of ice. the part is model specific and is fairly inexpensive.

PLEASE DISREGUARD GUEST FIXYA SOLUTION,FREEZER PROBLEM PROCEDURE IF NOT DEGITAL,FIRST CHECK IF ICE ON BACK FREEZER WALL. STEPS-FIND DEFROST TIMER,BOTTOM FRONT ,BOTTOM BACK, INSIDE FRIG.ON CONSOLE,LISTEN TO SEE IF FAN IS RUNNING IF SO TURN THE DEFROST TIMER CLOCKWISE TILL YOU HEAR A CLICK SEE IF FAN TURNED OFF IF SO YOUR FREEZER IS IN DEFROST,FREEZER WILL COME OUT OF DEFROST IN 20-30 MINUTES IF IT DOES AND THE FAN COMES ON YOU HAVE ELIMINATED DEFROST TIMER&FAN,NEXT REMOVE BACK PANEL TO FREEZER IF THERE IS ICE FORMING REMOVE BOTH LEADS TO HEAT AND CHECK WITH VOLT METER ON OMES,IF YOU GET ANY READING THE HEATER IS GOOD,IF NO READING REPLACE HEATER AND CLIPON TEMINATOR,IF HEATER IS GOOD DEFROST COILS WITH HAIR DRYER TOP TO BOTTOM AND THIS WILL ALSO CLEAR THE DRAIN,IF EVERYTHING CHECK OUT GOOD DEFROST TIMER ,FAN ,HEATER, DRAIN, YOU NEED ONLY TO REPLACE THE LITTLE ROUND CLIP ON TERMANATOR,LET ME KNOW-MIKE