I have a True Freezer Model T-49. It works fine but every 3 or 4

Sounds like to have a defective defrost timer

Usually every 6 hours will keep the evaporator coil clear of frost. I set the time at 45 minutes per defrost. This unit has a defrost termination control that kicks it out of defrost when the coil reaches a certain temperature. The defrost timer is only a backup if the defrost control fails.

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Does it? Or is it just the display that reads 50? Don't forget the evaporator has to get to 50f to get rid of the ice forming on it. It will do this 3 or 4 times a day, but the product temperature should not creep much above setpoint. Check the food temperature with a digital probe when defrost occurs to see if it is only the display module ( which may have it's sensing phial too close to the evaporator or have lost it's insulation jacket)
Also check the grill in the base (condenser) is clear and clean so it recovers from defrost as it should..

Check the condenser coil is clear and clean, and thermostat set about 2 or 3. Also check the fan is running with door closed, and no food is too close to the top stopping the airflow over the evaporator. It may be as simple as a blocked drain backing up - check the drain tube next time it defrosts.

The defrost termination/ fan delay control is a temperature-activated, single pole-double throw switch. ).
The control is wired into the refrigeration circuit. The control is located high on the evaporator where the frost is likely to clear last. The function of this temperature-activated switch is to terminate defrost when the evaporator coil has been defrosted, and to delay the evaporator fans from coming on immediately after defrost.
The coil must reach a temperature of 25* F. for the fan delay to close allowing the fans to operate.

Either the defrost circuit has failed or the refrigerant is low - my money is on the defrost circuit.

Most defrost circuits have three main parts:

1) defrost electric heating coil
2) defrost terminator
3) defrost timer.

The heating coil and terminator are in the freezer compartment - behind a protective panel. The heater is usually piggy-backed on the freezer coil and the terminator is in contact with the freezer coil to detect its temperature. The defrost timer is a simple assembly of a clock motor with switch contacts that can be located anywhere the manufacturer desires. The timer turns on or enables the defrost circuit every 8 or so hours for up to 30 minutes give or take.

When the timer has enabled the defrost cycle, the cooling mode ceases; the compressor shuts off and power is sent through the terminator to the heater. The heater warms and melts any ice build up on the freezer coil. The water drips to a pan and flows down the tube to a pan under the fridge - where it is evaporated off. The heater warms the freezer coil until either a) the timer returns to cooling mode or b) the terminator senses a preset rising temperature on the freezer coil. Once either condition is present, power is interupted and heating stops. The compressor is energized through the adjustable thermostat in your fridge. Since it is warm, the compressor turns on and cooling begins.

If any of the components listed above (1,2 or 3) have failed, the defrost cycle never warms the freezer coil and the ice never melts to clear the freezer coil as intended. Air can not be circulated through the freezer coil since it is choked with ice, so even though the compressor runs, the fridge and freezer spaces never get colder. The adjustable thermostat never sees the temperature you've set so the compressor never shuts off.

Water dripping in the fridge is melting ice from the freezer space - as the freezer is not getting colder - only the protected space around the freezer coil is.

To fix this, you'll have to get the service manual or schematics for the fridge to determine where the parts are located and do some troubleshooting with a multimeter after disassembling the freezer compartment and wherever else to access the timer if needed. This is not a good first appliance repair job for a DIYer due to the danger of refrigerant and testing live electrical parts in closed in spaces.

I hope this helps.

Hi, sounds like your defrost clock is stuck in the defrost mode. On most true boxes they have a paragon defrost clock with adjustable pins you can set the amount of cycles and duration of the defrost cycle. Find the clock and give it a turn in the direction of the arrow or cw. It will come out of defrost and start to freeze again. If it continues, you may need a new clock. Let me know what you find. Keep me posted. True boxes are top of the line
Sincerely,
Shastalaker7

Freezers have a sensor that is referred to as a "Fan Delay/Defrost Terminator". It senses the temp during the defrost cycle. Usually, when the evap temp reaches about 50 degrees, there should not be any frost or ice on it. This device terminates the defrost cycle. It also keeps the fans off so warm air is not circulated. When the evap gets to about 20 degress or so, the sensor switches on the evap fans. If the fans are not coming on, either the sensor is bad or the evap is not reaching the proper temp to cause the sensor to turn on the fans.

Hope is helps to clear up some things and point you in a direction as to what might be going on.

Your unit should have a defrost timer.The timer may be stuck in a defrost.It should be located at the bottom of the unit next to the compressor.Open the door to the timer and observe the position of the timer.If you turn the knob and the unit starts up,it's the timer. If this is the case, you can purchase a new one at any local a/c supply.Let me know,Mike