Working of frost free refrigerators (defrost System)
There are many separate components in a refrigerator's defrost system that must work in consort for a frost free system to work properly. We will attempt to explain the workings using simplified electrical schematics.
The heart of the defrost system is the defrost timer. The timer is a motorized device that opens and closes electrical contacts. Each contact can be thought as a light switch, one controls the defrost cycle, the other the cooling system. When one of these is switched on, the other is switched off. A motor on the timer (NOT illustrated) turns a cam that opens and closes these contacts at set intervals (see below for other types).
Cooling Cycle
Refrigerator cycling on thermostat (cold control)During the cooling mode, the defrost timer closes a contact to the compressor circuit so it will run. The circuit to the defrost heater is open.
While in this mode, the thermostat (a.k.a. cold control) cycles the compressor and fan motors on and off to maintain an appropriate temperature.
Defrost Cycle
Active circuit in defrost cycle
The defrost timer eventually switches into defrost mode and supplies power to the defrost heater(s) to melt any frost that has accumulated on the evaporator (cooling) coil.
The cold control contacts remain closed but since the defrost timer is no longer feeding power to that circuit, the compressor does not run.
Once the defrost termination thermostat (a.k.a. defrost limit switch) senses a set temperature, it opens the circuit to the defrost heaters, shutting them off. The timer remains in the defrost cycle until the timer advances back to the cooling mode. Since the limit switch is open, the heaters are no longer on for the rest of the cycle.
Cooling Cycle
When the timer again advances back into the cooling mode, the compressor will start to run along with any air circulation fans. The defrost limit switch will remain in the open condition until it is reset by cold temperatures.
Shows defrost termination thermostat contacts closing and timer contacts to the heater circuit still openOnce a set colder temperature is reached, the defrost termination thermostat closes again. This is OK since the defrost timer is no longer supplying power to the defrost circuit, the heater does not get energized.
When the defrost timer again advances into the defrost mode, the limit thermostat will already be closed and will allow power to be supplied to the defrost heater to melt any frost that has developed on the evaporator coil again.
Fans
Fan(s) runningNormally the interior evaporator and exterior (if present) condenser fan motors should run whenever the compressor is running and vise versa. If the timer is stopping operation of the cooling system, neither the fans nor compressor should usually be running at that time.
Symptoms
The most common symptom of a defrost system failure is a complete and uniformly frosted (not iced) evaporator coil. Frost may also be visible on the panel covering the evaporator, usually in the rear of the freezer compartment.
Progression of evaporator frosting
Excessive frosting can be cause by the defrost heater or limit thermostat being open (ie. defective), a mechanical defrost timer sticking and never advancing into the defrost cycle or a problem in an electronic defrost control or one of its sensors failing to allow the defrost heaters to be energized.
Sometimes (but fairly rarely) both heater and cooling system can be energized by the timer at the same time. This can result in thawing then refreezing of food in the freezer compartment often leading to freezer burn on that food. In most cases the evaporator coil will remain mostly in an un-frosted state. The defrost heaters will cycle on and off as the defrost thermostat opens and closed due to the temperature it senses.
Refrigeration System Problems
Partially frosted evaporator coil
If the evaporator coil is only partially frosting or a ball of ice develops on just a small area of it (see illustration below), it is usually a sign of a refrigeration system problem in which case a trained refrigeration technician will be required to determine the cause and correct it. These conditions are not caused by a defrost system failure.
Defrost Component Locations
Top-freezer refrigerator with the evaporator cover panel removed exposing the evaporator (cooling) coil
On most frost free refrigerators, the evaporator (cooling) coil is inside the freezer compartment covered by a panel. The freezer fan motor is usually in the same general area.
The defrost heater is mounted onto or woven right into the evaporator coil in the freezer. The defrost termination limit switch is usually mounted on the side of the evaporator coil or on one of the connecting tubing.
Evaporator coil with defrost heater and defrost limit thermostat shown
The defrost timer can be in various places including behind the kickplate at the front of the cabinet, inside the fridge compartment possibly in a control panel along with the thermostat or on older models, at the back in the motor compartment by the compressor. Some strange Kenmore refrigerator models had the defrost timer built into the icemaker unit.
The Defrost Heater
Glass Tube Defrost Heater
The defrost heater is basically a wire filament enclosed in a quartz, glass, aluminum or other material, tube sheath which gets hot when powered. It will either have resistance (show continuity) and be good or will have infinite resistance (no continuity) and be defective. How much resistance it has is irrelevant as its resistance will not normally change except to being open (infinite resistance) when it fails.
The Defrost Termination Thermostat
The defrost termination thermostat (aka defrost limit switch) is basically a small SPST (single pole) electrical switch which is actuated by temperature. Depending on the temperature it is, it will either have no resistance (show continuity) and be good or will have infinite resistance (no continuity) and be defective. At room temperature it will usually be open (which is normal and not a sign of being defective) and only close when it gets cold. How cold it has to be to close will depend on its particular calibration but usually near or below freezing point.Some newer model refrigerators (Amana and Frigidaire in particular) and some older models (GE included) run power for the evaporator (freezer) fan motor through the defrost heater element and defrost limit switch. If either of those components should fail, remaining open, the fan will not run which will stop the circulation of cold air throughout the refrigerator.
On that design, the evaporator fan motor will not start running after a defrost cycle until the evaporator has had a chance to begin cooling again. While it is generally a good design idea so as not to blow the warm defrost air throughout the refrigerator, a failure in one part of the defrost system will usually render the whole refrigerator ineffective because of the lack of air flow.
Defrost Timers Types
Defrost TimerEarly production mechanical defrost timers would go into the defrost cycle after a set amount of time. Common timing periods were 6, 8, 12 and 24 hours. This meant that say every 6 hours, the refrigerator would go into defrost whether it needed it or not. The duration of time it would remain in the defrost cycle was fixed and could be anywhere from 18 to 30 minutes depending on the timer design but it would always be the same length of time.
As stated above, the defrost heaters may not be on for that full length of time, thanks to the defrost limit thermostat, but the cooling cycle would not start again until after the complete defrost duration was ended.
This original design was wasteful as the refrigerator would defrost regardless if it was necessary or not. A later design tried to help this a bit.
Cumulative Run Timers
The next design was called a 'cumulative run' timer. These timers were installed in a way were they would only count the time that the refrigerator (the compressor) was actually working. This makes sense since no frost could build up if the compressor was not running.
These later mechanical model timers would only advance the into defrost when the compressor had actually been running for a certain length of time, usually 6 to 8 hours of accumulated compressor operation.
Adaptive Defrost Control
ADC - Adaptive Defrost Control exampleThe latest, energy saving variation is electronically controlled and called an adaptive defrost control. Not only does the period between defrost cycles change but also the time duration of the defrost cycle itself. The device is programmed to keep track of the appliance usage and how long it takes for the evaporator coil to be thoroughly defrosted. It will then calculate the amount of time required and adjust itself accordingly.
GE's 'Mother Board'
Newer GE made refrigerators are almost totally electronically controlled. Their motherboard takes the place of both the temperature and defrost functions even controlling DC fan motors that can operate at several different speeds. On this system only the defrost heater is the same as described above. A defrost cycle is ended when a thermistor detects a temperature rise of the evaporator which is the signal to the main control to terminate the defrost and start the cooling. The termination thermostat on this appliance design only acts as a safety device to shut the heater(s) off in case of a malfunction before the plastic interior liner melts, otherwise it is never active. The control can not be manually put into defrost mode.
Timer Testing
Defrost Timer showing Advancing ScrewThe mechanical timer types described above will usually have a screw on their underside which can be turned clockwise to manually change its present cycle. If running, slowly turning it until it clicks once should put the timer into 'defrost mode'. When in the defrost mode, turning it until it clicks once should put the timer into 'run mode'. It can be left in the defrost mode to see if it will advance by itself to the run mode (like it should in less than 30 minutes) or the defrost heaters may be able to be tested to see it they are receiving power or not at that time.
NOTE: Once the compressor has been turned off, it should be allowed to sit for several minutes (5-10 minimum) before attempting to restart it. This is required to allow the internal refrigerant pressures to equalize so the compressor is able to restart again without putting excessive strain on it.
Test points and how to manually switch adaptive defrost controls into the different cycles vary from manufacturer to manufacturer. For these, check the appliance's mini-manual for instructions which is most often located behind the kickplate at the bottom front of the refrigerator but may also be inside the actual control panel with the defrost control.
DIAGNOSIS to Fix the Generic problem for all Refrigerators
if symptoms of damage such as: fridge not cold, no noise from the compressor, how to analysis and improvement:
1. Check the incoming voltage to the compressor, make sure there is voltage coming into the compressor, a simple way to hold the engine compressor. if there are hot that means the voltage has been entered. the part that does not work is the compressor. you should replace it and fill freon. you must have special equipment to be replaced, such as welding equipment, analyzer to check freon pressure and pressure compressor. and many more tools that should be necessary.
2. Thermostat how to replace it: simply replace the existing thermostat in the freezer space, there is a screw that must be opened, open by pulling the cover and then remove from the holder thermostat. replaced in accordance with the size and spec. there are only two wires are connected there, no matter if inverted in installing.
3. Timer: Timer can be damaged, causing the compressor does not work, the indication: there is no noise from the compressor. timer location near the compressor (in box) or in room fridge, usually near the refrigerator light. first try to turn the timer until the refrigerator running. if it still does not work just replace the timer.
4. check the fuse and the thermo bimetal (thermo switch) and the heater as a function of defrost in the refrigerator. these parts have anything to do with the function "timer", if one is not functioning will cause the refrigerator does not work normally even the refrigerator is not cold.
One cause of icing up and over freezing in a fridge or freezer can be a poorly fitting or damaged door seal. If the door seal becomes distorted the seal will start to allow warm moist air into the freezer or fridge. This will cause snowy ice to deposit around the point of entry which will eventually cause the compartment to completely ice up.
If the door seal is worn, then clearly it needs replacing but sometimes it’s just a matter of re-shaping the door seal with the aid of either a hair dryer or a warm cloth. Carefully heat the seal and try to form it back into the right shape closing the door and letting the seal cool before opening the door again.
Another possible cause of frost in fridge or freezer
If the fridge or freezer is not level, the cabinet may become twisted, this causes the door to not fit properly and the seal to not seal properly. This can allow gaps which again allows warm moist air to be drawn into the appliance. If affected make sure the appliance is properly level by adjusting the feet or if necessary by placing some packaging under the feet. You can always try the paper test, which is to place a piece of paper between the seal and the frame and see if it will hold, if it doesn’t the seal will be letting warm air into the unit.
Note: Many modern fridges and freezers do not have replaceable door seals. Only complete new doors. This is disgraceful but just another symptom of manufacturers making appliances less repairable in the race to make finished goods cheaper.
On the subject of door seals it is not advisable to open any fridge or freezer door by curling your finger’s around the frame as this can result in the seal splitting. Always use the handle, and don’t forget if you do reshape a door seal always fully defrost the unit afterwards. Another point to remember is that if the door seal has split on the face that touches the frame it needs to be changed. If it is split on the outer side crease it is only cosmetically unsightly.
REFRIGERATOR DEFROST CYCLE DIAGNOSIS
- 1) Unplug the refrigerator.
- 2) With the freezer frozen down, (it must have been running at least a couple of hours to give the contacts in the defrost thermostat a chance to get cold and close) remove the evaporator cover (the cover in the freezer that covers the cooling coil)
- 3) Plug the refrigerator in.
- 4) Locate the defrost timer and, with a screw driver, turn the timer to the defrost mode. ( There is a slot on the timer that the screwdriver will fit in. Turn it clockwise. The refrigerator will stop running when it goes into the defrost mode.)
- 5) Wait about 5 minutes, then see if the defrost heater is heating. It will get VERY HOT, so be careful! If it is heating, the defrost heater and defrost thermostat are good. The timer should be replaced.
- 6) If the heater is not heating, then the timer is probably good. (While this is not true ALL of the time, in my 15 years in this business I have seen this NOT true only twice. You could still have a bad timer, but is is unlikely)
- 7) UNPLUG THE REFRIGERATOR!
- 8) Use a hair dryer to defrost the evaporator (cooling) coil.
- 9) Visually inspect the defrost heater. Check for a broken or burned area on the heater. Replace the heater if you find this.
- 10) If you do not have a multi-meter, skip to 13. If you have a multi-meter, check the resistance of the heater by disconnecting the wires from both end of the heater and, with the meter set to "OHMS", put one meter lead on each end of the heater.
- 11) If you get no resistance reading, replace the heater.
- 12) If you get a resistance reading, replace the defrost thermostat. (See 6 above.)
- 13) If you do not have a mulit-meter, make sure the refrigerator is unplugged, then locate the defrost thermostat. It is a round, quarter sized part with two wires coming out if it. It usually mounts directly to the evaporator coil.
- 14) Make a note of how each wire is attached to the thermostat.
- 15) Remove all wires from the thermostat.
- 16) Using a wire nut (you probably removed two of them to remove the wires) connect ALL of the wires that WERE attached to the thermostat together. (NOT THE WIRES THAT ARE MADE ONTO THE THERMOSTAT, BUT THE WIRES THAT WERE ATTACHED TO THE THERMOSTAT)
- 17) Plug the refrigerator back in. Make sure the timer is still in the defrost mode.
- 18) Wait a couple of minutes, then check to see if the heater is heating. It will get VERY HOT, so be careful.
- 19) If the heater is heating, replace the thermostat.
- 20) If the heater is not heating, replace the heater. (See 6 above)
