If you’re working in circumstances where this isn’t feasible for some reason, you’re going to have to change out the controls and sensors with replacement parts off the shelf of your nearest dealer.
I’ve done this several times as a last resort, and my advice would be to draw out a complete and accurate schematic, use parts that are designed to work with the voltages, pressures, and temperatures of the unit, and always make sure the customer understands exactly what is going on.
When you’re troubleshooting a refrigeration control system, keep in mind that no matter how complicated the schematic might look on paper, the control system is actually only trying to provide a few simple outcomes:
• control the compressor(s) and condenser fan(s);
• control the evaporator fan(s) and defrost heater(s) (if installed);
• control the liquid line solenoid (if installed); and
• maintain the desired temperature in the box.
Whatever component in the system isn’t working, find it in the schematic, trace back through the circuit to the switch contacts that control it, locate them in the equipment, and test them one by one. Do the same with the actual component and the wiring attached to it.
Follow the wires to the control contacts, and continue all the way to the power source. As you trace the circuits out, you will build your understanding of how the system works.
Lights, mullion heaters, door heaters, and door frame heaters might or might not be wired parallel with the rest of the controls, but they won’t be controlled by anything in the refrigeration control system.
In medium temperature walk-ins, and some reach-ins, the liquid line solenoid will be near or in the evaporator, and the liquid line solenoid coil and thermostat will be wired in parallel with the evaporator fans. This circuit will usually be totally independent from the condensing unit control circuit.
In this type of refrigeration control system, the thermostat and liquid line solenoid control the compressor by controlling the low side pressure. The compressor is usually controlled by a dual pressure control. The low side contacts of the dual pressure control will automatically reset in response to pressure changes. The high pressure side of the dual pressure control is set to prevent the compressor from running at dangerously high pressures, and is usually manually reset.
When the box is above the set point, the thermostat contacts close, the liquid line solenoid coil energizes and the liquid line solenoid valve opens, refrigerant flows into the evaporator, low side pressure rises to the low pressure control “cut in” set point, the dual pressure control contacts close and energize the compressor contactor, and the compressor runs.
When the box temperature drops to the thermostat set point, the thermostat contacts open, the liquid line solenoid coil de-energizes closing the liquid line solenoid valve, the compressor pumps refrigerant out of the low side and into the high side until the low side pressure reaches the “cut out” point of the dual pressure control, the dual pressure control contacts open, the compressor contactor de-energizes, and the compressor stops.
The evaporator fans will run continuously.
If there is an oil safety switch in the refrigeration control system, its contacts will be in series with the compressor contactor coil. If it senses a low differential between oil pressure at the outlet of the oil pump and suction pressure in the crankcase of the compressor, it will de-energize the compressor contactor after a pre-set time delay. It will be a manually reset switch.
If you are working on the refrigeration control system of a medium temperature unit that is expected to maintain a box temperature of around 35°F, the evaporator coils will possibly ice up.
I’ve encountered this occasionally, and a simple solution is to install a defrost timer, and turn the compressor off for 20 to 30 minutes several times every day, to let the evaporator fans run and melt any ice off the coil.
The refrigeration control system for a freezer has the same types of controls and safeties, but is slightly different.
In both walk-in and reach-in freezers, there is a defrost control that will turn off the cooling circuit (condensing unit, evaporator fans, and liquid line solenoid), and turn on the defrost circuit (electric or hot gas), at pre-set intervals.
For smaller reach-ins, the defrost control contacts can handle the load of the compressor, evaporator fans, solenoid, heaters, etc. On larger freezers, the defrost control will energize relays and contactors to turn on evaporator fans, heaters, etc.
The Paragon type of defrost control is the most common, and other manufacturers make similar timers that will replace it wire for wire, terminal for terminal. Be careful if you have to use a timer with a contact arrangement that is different from the original timer.
Don’t assume that the X terminal is the defrost termination terminal. On some timers, the X terminal is simply for the neutral or second line, so check the schematic to be sure.
On almost all freezers, there will also be a drain line heater that prevents water freezing up and plugging the drain line. This heater should be wired to be on all the time to keep the drain line warm. Follow the instructions when installing a new heater, then insulate the drain line inside the freezer if required.
If you’re troubleshooting a refrigeration control system and don’t have a factory schematic, be patient, trace the controls out wire by wire, and draw a careful schematic. By the time you’re about halfway done, the way the system works will start to make sense, and the rest of the circuit will seem easier to trace out.
You might also be interested in taking a look at our free onlineRefrigeration & Electrical Troubleshooting Chart, which is a 10-page PDF file reprinted from a military training manual.
Publication date:06/08/2009
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