I had this problem with my nine year old Samsung SRL 3928B frost-free fridge freezer and have now fixed it for £45 and a few hours work!
It is most likely to be due to failure of the heating element around the evaporator coil. This is switched on every 6 hours to melt the ice which forms on the coil. The fridge compressor is switched off at the same time. The heater is normally switched off again when the coil temperature reaches 1-2C (= all ice melted) and the fridge comes back on. If the heater is broken, the ice doesn't melt so the temperature of the coil stays below zero and the fridge stays off. If you switch it off and on at the mains, the timer is reset but the ice stays put so after a few cycles the fridge becomes ineffective.
A competent DIYer should be able to fix it in about 4 hours. It is not a job for the ham-fisted or faint-hearted!! First you need to make certain that it is the element that has failed and not the controller. After unplugging the fridge, remove the back of the freezer compartment. This involves undoing a few screws and releasing the clips around the edge (using a screwdriver). There are two covers, the second of which incorporates the circulation fan and control damper which you will need to unplug. The evaporator assembly is then visible although it will be iced up if you have been using the fridge. If so, it is best to allow the ice to melt before proceeding.
The heater is an aluminium tube wrapped around the OUTSIDE of the fins. The tube on the inside is the evaporator coil. The heater has wires emerging from each end of the tube which go to a connector. Disconnect this and use a multimeter to check the resistance of the heater. If it has failed it will be open circuit. If so, order a new evaporator assembly from you spares supplier. This comprises the evaporator pipe and fins, the heating element and the temperature sensor. I got mine from DES Service WG Ltd in Slough for £45.28 including delivery.
When it arrives, you will need to remove the heating elements from both evaporators so that you can fit the new heater to the old evaporator (in the fridge). This is necessary to avoid having to re-gas the fridge. It is probably best to work on the new one first because access is better and damage to the fins does not matter but avoid bending the new heater more than absolutely essential to remove it.
Remove the screws securing the old evaporator to the back of the fridge and gently ease it out so that you can get access all around. You will need to bend the copper pipes to do this but should not damage them if you are careful. You will also have to disconnect the temperature sensor. Avoid damaging the fins as you remove the old heater. Installing the new heater is very fiddly so take your time. Again avoid damaging the fins but don’t forget to ‘twist’ them slightly to hold the heater in place and make sure there is metal to metal contact. If necessary use some cable ties to secure the heater.
Replace the evaporator assembly and secure it with the screws. Plug the new heater and temperature sensor connectors in and then replace the inner cover with fan and damper, remembering also to plug these back in. Check that the fan rotates freely and then replace the outer cover.
Finally, plug the fridge into the mains and switch it on. The compressor should start immediately to cool the fridge and freezer sections. After six hours, the heater should come on briefly as described above. You cannot confirm this directly, but if the compressor comes on again after the first six hours, the problem is solved….
Good luck!!
Bicycle Repair Man
December 2009
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