Month: August 2018

Oil pressure controls… Oil failure controls…  Oil safety controls…   They’re a pain in the neck when they trip and diagnosing those problems can really tax even the best of techs.


As semi-hermetic compressors get larger, they can no longer rely on simple splash or “sling” type lubrication strategies where oil is just flung around inside the compressor and that is sufficient to provide lubrication.   Once we cross that threshold, we need an oil pump to force oil through ‘galleys’ machined into the crankshaft to lubricate all of the bearings.   To ensure that the compressor has adequate lubrication at all times, manufacturers require a safety control to prove there is sufficient oil pressure and shut the compressor off if there is not.


An oil pressure control is, effectively, a differential pressure control with a built-in time delay.




Let’s look a little closer.


The first thing we have to do is look at how we measure oil pressure.   The oil that we’re pumping through a compressor starts in the crankcase, so it’s already under a certain amount of pressure, depending on the suction pressure of the system.  That suction pressure affects the output pressure of the pump.  To properly measure oil pressure, we can’t just look at the pump outlet pressure, we have to look at the pump outlet pressure MINUS the crankcase pressure, this is called NET oil pressure.


Measuring crankcase pressure rather than just measuring suction pressure will become important later when we get into troubleshooting.  This is why the oil pressure control has 2 pressure ports and is measuring the differential between those 2 pressures.  The pressure control has a time delay because, on startup, the system needs a period of time to stabilize.  This time delay is usually fixed at 90 or 120 second, depending on the manufacturer and the brand and type of control.


Most new controls are electronic, but there are still a lot of mechanical controls out there, so let’s talk about how they work and, once we have a solid understanding of mechanical controls, the electronic ones are pretty easy to understand.


The first thing to understand about oil failure controls is that they require a minimum of 3 wires. They’re more than just a pressure control, remember…


So, we’ve got line voltage going to the control AND we’ve got 2 wires for the control circuit. Typically, you’re going to see terminals “2” and “M” jumpered together so everything I say assumes they are connected electrically.   Some rare applications will require them to not be jumpered but that’s beyond our scope here.


Depending on our control voltage, we’ve got line voltage at V1 or V2, one leg of the control circuit at M and one at L.  It’s very important to understand that L also acts as both one leg of the control circuit AND the second leg of the circuit for the heater (H) in the control itself.  This means that, unlike most other controls, we have to be careful where we put this control because the leg from L cannot have any other switches in it.  This requirement will be made clear in the paperwork included in the control and now you understand WHY.


Now, let’s look at the switch in the control labeled PC.  This switch, pictured as normally closed, will OPEN when there is sufficient pressure differential.  So, when we start the compressor, power is applied to V1 (or to V) and once oil pressure builds up to the 9-12 PSIG range, it will open switch PC and de-energize the heater.


Now, if this switch(PC)  is closed and the control is energized (power to V1 or V2) then the heater H is energized.   Once that heater reaches a certain temperature, switch TD, which is a thermal type switch similar to the one found in an electric heat sequencer, will open, breaking the control circuit.


This may seem complicated, but really stop and take the time to understand this.   You’re not really going to be putting a meter across switch PC, it kind of just works in the background, you just need to understand that it’s there and what it does to make the control function.


Electronic controls integrate these functions into an electronic control board and a small differential pressure switch that screws into the oil pump on the compressor.  They still have the same basic wiring requirements, having to have the third wire for power and an uninterrupted wire from L to the load.  The time delay feature is electronic rather than thermal, but there is still a small differential switch to monitor the net oil pressure.


In operation, an electronic oil failure control works pretty similar to the mechanical.   Power on the line voltage terminal and L supply the PCB (Printed Circuit Board) with power while M and L act as the control circuit.   The PCB monitors the differential pressure switch.  This is typically a brass assembly threaded directly into the oil pump.   If oil pressure drops too low, this time, the differential switch will typically open, signaling the PCB to begin the timing that was handled by the heater in the mechanical control.


Electronics have some advantages over mechanical controls.   More accurate timing being first among those.   A thermal switch is somewhat dependent on its environment.   The same switch in a warm ambient is going to time out faster than it would in a colder ambient.   With electronics, timing is repeatable across a wide range of ambient temperatures.

Moving into the future

As electronic controls advance, manufacturers are integrating more features into them.  What was once a single purpose control monitoring the compressor’s oil pressure is now turning into what amounts to a central control unit, taking oil pressure input along with motor current data, high and low-pressure switch inputs, motor temperature inputs and acting as an integrated safety for the machine.   Not only does it provide troubleshooting data to the tech in the form of error codes, some are acting as data recorders allowing more detailed troubleshooting if the tech connects to the controller with a laptop or other device and downloads the data.

— Jeremy Smith CM



I see a lot of techs and installers use tape over foam tubing insulation joints (Rubbatex, Armaflex etc…) rather than using the glue that is designed for it.

Some of them use tape because they don’t know the glue exists, some use tape because it’s what they happen to have on the truck and others don’t use the glue because they attempted to use it and they couldn’t get it to work right.

There is a trick to using contact adhesives like tubing insulation glue and spray glue. You need to allow it to dry a bit and “tack up” before pressing it together.

Here is the step by step –

  1. Make cuts clean and even 
  2. Apply glue on BOTH sides of the cut 
  3. Hold the sides apart and allow them to dry and get tacky 
  4. Press together evenly and hold
  5. Celebrate your new skill by attempting to pull it apart and being unable!  (This part is based on a true story, no armaflex was harmed in the making of this photo)


Testo 570 Premium Manifold

This is the article you read BEFORE you call and ask a senior tech what your subcool should be, or the one you send to a junior tech when the call and ask you.

So what is subcooling? (or subcool as many call it)

Subcooling is a measurement of temperature DECREASE of a liquid below its saturation (mixed liquid/vapor) temperature at a given pressure.

For example, water boils at 212° Fahrenheit at sea level (atmospheric pressure of 14.7 PSIA). If water is at 212°f and at atmospheric pressure at sea level you can be sure it is at saturation, which means it is either in the process of boiling or condensing. If you measure that same water and it is at 202° you can be sure that it is fully liquid and that it is no longer in the process of either boiling (changing from liquid to vapor) or condensing (Changing from vapor to liquid). Because the water is at 202°  instead of 212° we know it is liquid and we can also say it is subcooled by 10°. This 10° of subcooling PROVES that not only is it fully liquid but that it has given up more sensible heat energy enough to drop 10° below the boiling temperature at that pressure.

With refrigerant, we measure the subcooling between the condenser and the metering device and it gives us a lot of information. It not only tells us whether or not the line is full of liquid it gives us indications of refrigerant charge as well as condenser efficiency when viewed in conjunction with the condensing temperature (high side saturation temperature). Now be careful, like with all measurements, it is only as accurate as your tools, it must be taken using liquid line pressure and temperature (Line between the condenser and metering device) NOT discharge line pressure and temperature (line between the compressor and the condenser) AND you must have a good connection to the port. I can’t tell you how many times green techs have called me with “crazy” readings only to find out their hose was not depressing the Schrader core fully.

So what should it be?

Generally speaking 10° – 12° of subcooling at the outlet of the condenser coil is most common but you must look for the proper design subcooling for the particular system you are working on. Some systems will require subcooling readings of up to 16° for maximum efficiency and capacity.

Many techs will say that subcooling  is how you “set a charge” on a TXV / TEV / EEV metering device system

Subcooling is one of many factors you consider when setting a charge but you first need to make sure that your equipment is properly matched with the correct metering device. The air flow is set in properly, the blower, air filter, condensing coil and evaporator coils are clean and WHENEVER adding or removing charge use a scale so you can monitor your progress.

While it is true that subcooling is the primary charging measurement on a TXV /TEV / EEV system, subcooling is important to check on every system, every time you connect (whenever possible).

Negative Subcooling isn’t possible if the liquid line temperature and pressure are taken at the same point. What is possible is to have a miscalibration of your tools that make a zero subcooling look like a negative subcooling.

Zero Subcooling means that the refrigerant in the liquid line is a mix of liquid and vapor, this is not an acceptable condition except in cases where the system is designed to inject discharge gas into the liquid line on purpose to increase liquid pressure (headmaster).

Low Subcooling is an indication that not enough refrigerant is contained or “packed” in the condenser. This can be due to undercharge, poor compression, or a metering device oversized or failing open (overfeeding).

High Subcooling is an indication that more than the designed amount of refrigerant is “Backing up” or “packed” into the condenser.  This can be caused by overcharge, restriction (such as a contaminated line drier or kinked liquid line) or an undersized or failing closed metering device.

Keep in mind, the subcooling can often read in range on a system that still has issues. Many times this is because the previous tech simply “set the charge” by subcooling without fully testing all aspects of the equipment such as airflow.

— Bryan

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