Electrolux Fridge getting warm?

My fridge freezer is faulty and I want to know if I should repair it or replace it. It’s a Swedish made Electrolux model TR 71/55, Product No. 9240811, serial no. 410-1736. The fridge has an auto defrost capability. There is a separate compressor and mechanical thermostat for each compartment. The freezer compartment has built up lots of ice at the top over many years so I assume it does not auto-defrost but it keeps food frozen.
However the cooling panel in the fridge gets warm and heats up the food. I assume the auto-defrost phase is on for too long. There is no timer and the cooling panel is a simple single loop.
How does this type of fridge auto-defrost?
What could cause this fault?

Regards,

Brendan
February 2009