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Trouble shooting marine refrigeration and marine air

By: Scott Fratcher - Marine Engineer/Captain

Contrary to popular belief, troubleshooting a boat’s refrigeration can be tackled without gauges. Many system malfunctions can be solved by DIYers using nothing more than a multimeter and a little forethought

An important first step. Forethought testing involves taking a series of “normal” measurements from the boat’s refrigeration system when everything is running correctly. It is this “base-line” of tests we refer back to when attempting to solve a refrigeration issue. All readings should be kept in the ship’s log.


Tools needed for tests:

* Stopwatch
* Temperature probe or laser temp gauge
* Multimeter

Before beginning the tests, take the temperature of the seawater, the ambient air and inside the fridge box. Also note battery voltage and amount of food in fridge box.
The base-line readings we’re looking for are:

1. The time required to cool the refrigerator box from start up
2. How long the compressor runs per cycle after the box has reached target temperature
3. How long the compressor remains off between cycles
4. Length of time before all bubbles stop flowing through sight glass
5. The amount of cooling water flow being pumped
6. If possible, temperatures of all in/out points on the system including the compressor, condenser, expansion valve and cold plate
7. Note any areas where water condensates or ice forms on the refrigeration lines
8. Take voltages from the input of the fridge’s electrical junction box and the compressor itself

Taking these measurements when the refrigeration system is working properly is important – they provide a simple method of comparison when something goes wrong.
It’s not cooling

Don’t delay – the sooner we start to record new observations of the system, the quicker we’ll find the problem (and maybe prevent burned up components). Let’s take a look at what each change of parameter might mean.
Length of time to cool

Often the fridge is making all the normal noises but it’s taking its time to cool down. Note, if you’ve just filled the box for a boat trip, it will take a bit longer to cool down. But let’s assume the cold box load is the same as usual. And let’s say that where the fridge was originally running 15 minutes per hour to maintain temperature, it now takes 30 minutes.

If you have a water-cooled system, the first thing to check is water flow. Any restriction causes inefficiency, and it’s easy to check and correct. It may need a back flush or rinse with muriatic acid (available at Sailor’s Corner), a cleaning of the sea strainer or maybe it’s a pump issue.

For an air-cooled system, check that the circulation fan is functioning and whether the cooling fins have been bent, crushed or are full of lint. If so, use a “fin comb” to straighten or clean the fins (see picture above). Also check that there is a clear air-flow around the fins. Any restriction will compromise performance.

Next, look at the sight glass. It’s a window into the inner workings of the refrigerant. If you see a long string of bubbles, the system may have a leak. This means finding the leak and a recharge (and usually means calling in a technician).

Lastly, compare all the new temperature readings with the base-line tests. Any significant difference will point to the troubled component. For example, if the base-line test showed a 3oC rise in sea water temperature through the condenser, and the new test only 1oC, this could mean a clogged condenser, or an insulating residue such as oil building up inside the condenser.
No noises

You don’t hear any clicks, nor the typical hum of the compressor in operation. The first step is to check voltage at the fridge electrical junction box. Compare this reading to the base-line reading.

If there is no voltage at the junction box, check the fuse or breaker. If the breaker has blown this could mean the compressor is drawing too much current.


Overload trips may also be caused by:

* The start relay not opening
* The breaker itself failing
* Poor feed wire condition
* Shorted motor winding
* A seized compressor

To check for a shorted winding, disconnect the compressor motor wires completely from the electrical system. Be careful here if you have a 240-volt AC system – first disconnect the power source to eliminate the possibility of electrical shock.

Check for continuity from the motor leads to the case. There should be no continuity except for the green ground lead. Any reading on the multi meter’s ohm scale means the motor windings are failing and will need to be replaced.

Next, check for voltage at the compressor. If we have input voltage but no compressor movement, look for indications the motor is trying to “start” (for example, a jerking motor or loud hum).


No indication of a motor trying to start could be caused by:

* Open circuit electric motor IE burned winding
* An internal connection wire may be fatigued and cracked
* Worn out brushes
* A very dirty commutator


A loud hum may mean:

* A seized compressor
* A compressor that has hydro-locked
* A failed capacitor
* A failed start relay
* A stuck centrifugal disconnect switch
* A seized motor or compressor bearing

If a gentle "thunk" with the palm of your hand makes the motor start, this indicates worn brushes or a dirty commutator.

If you have a belt-driven compressor, try giving the motor a little start spin. If it begins to rotate, replace the start capacitor.
No compressor voltage

If there is no voltage at the compressor but there is voltage at the junction box, we have a control system issue. This is typically a good thing, as control systems are often less expensive to repair than a compressor or other major system component.

The refrigeration control system is simply an on/off thermostat switch that monitors temperature. To test it, find the leads from the thermostat and briefly cross them. If the compressor jumps to life, you need a new thermostat.

If the compressor doesn’t start after bridging the leads, try pushing the relay or contactor closed. If the motor begins spinning, test for voltage on the control side of the relay. If you have control voltage but the relay is not closing, you need a new relay. If you don’t have control voltage, it’s likely the circuit board has failed.
Sight glass bubbles

If the refrigeration system sounds normal (clicks, hums, and compressor noises), but the cold box is not cooling, check the sight glass. Most boat refrigeration systems have a sight glass for monitoring basic performance. From initial start up, the sight glass should clear within a few minutes.

Note: no bubbles may mean all the refrigerant has leaked. If you suspect this, stop the compressor immediately as the refrigerant is also a lubricant in some systems. Running a “dry” system may cause damage.

If the sight glass shows continuous bubbles, you have lost refrigerant and the system probably needs a recharge.
Expansion valve

Every cooling system needs to release the liquid refrigerant in a controlled manner. This is the job of the TX valve. It’s normally trouble free, but there are a few tests to check it.

With the system operating, find the TX valve and listen for hissing. Hissing may mean low refrigerant.

If the system begins to cool but loses its ability to pull the temperature down after a few minutes of running, the TX valve may be icing up. Turn off the system and let the TX valve defrost for a few minutes. Turn on the system again, and if it begins cooling only for a few minutes, it’s likely the TX valve is icing up on the inside and blocking the flow of refrigerant. This requires a pump down and a new moisture filter.

Insulation is the bottom line for maintaining efficiency in any refrigeration system. The better it can hold the cold, the less the fridge is going to work and the less energy the system consumes.


Common tricks to increase chilly box efficiency:

1. Make an insulating blanket that lays over the top counter or sides of the fridge box. Some boaties even make a fridge blanket that hangs over the side of the outer hull to add extra insulation and shade to the side of the box against the hull.
2. Use a laser temp gauge to look for cold spots around the fridge. Often most of the insulation is working well, but a small area needs to be renewed. Any fridge area showing condensation is a sure bet large amounts of energy are being lost.
3. Fill unused areas of the fridge or freezer with plastic bottles of water. The water acts as a “battery” to help maintain temperature.
4. Pay particular attention to the door/hatch seals. Worn seals waste power and cause air circulation inside the chilly box. Air circulation causes condensation on the cooling plates. This condensation causes ice to form, meaning fewer days between defrosting cycles. Condensation can also cause water to soak into the insulation, causing excess corrosion on the cooling lines meaning early failure.

While you may not be able to always fix the cooling system problem, at least being able to identify the fault will allow the boat owner to make a clear decision, thus saving time
and money.

Boat refrigeration systems

There are three basic styles:

  • Belt- or electrically-driven compressor
  • Small, portable, hermetically-sealed plug and play system
  • Propane-heated plug and play system

    Choosing a system that’s right for your boat is a matter of personal choice. A quick look at the benefits and restrictions of each system may help you decide.



    Belt driven systems are powerful, allowing quick temperature pull downs. This is the system of choice for the DIY and those who want to freeze the occasional large fish. The disadvantage is that they are high maintenance and can mean running the engine every day.


    Portable system

    Small portable systems are simple to install. Just connect the power cables and turn it on. The power drains for portable units has dropped considerably in the last few years due to increased design and insulation efficiency. The big disadvantage is the inability to quickly pull down the temperature of large loads. This means catching and preserving large fish can be a challenge.


    Propane system

    Propane systems have a devoted following due to easy installation, completely silent operation, extremely low maintenance, 20+ year life span and no need to upgrade the yacht’s electrical system. The disadvantage is the need to carry extra propane and the ever-present explosive nature of propane.





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