Question on adding gas to BD35 compressor

[s/v Jedi]

Originally Posted by wingssail

My point above is that since your unit is still partially working I'd suspect that a shot or three of gas will put you right. I would not take apart the quick disconnect fittings unless you are equipped to vacuum the system and completely refill it. Plus, of a certain age the fittings are not reusable without gas loss at least.

A real easy leak check is to wipe the fittings with your finger. The gas has oil in it and if it is leaking out there will be wet oil on the fitting, usually the high pressure side. Even then, you could stop the leak with a little tightening of the fitting (use care and two wrenches so as not to break the solder joints). AND they sell stop leak which is supposed to be good.

So my recommendation is, do not open those fittings unless you have to.

Well, you have to. The O-rings need to be replaced every 5 years because they start leaking after 6-7 years. My 15 year old fittings still work as designed, easy disconnect and connect without the need of evacuating the system. Sure a little puff of gas escapes but that is how they are designed.

The first time my fittings started leaking was after 6 years. The second time after 7 years (bet my replacement O-rings were newer than those that came on the fittings at the time).

I strongly suggest NOT to use any stop leak additives. These are hermetically sealed units (no shaft seals etc.) and the only leaks are at the quick connect fittings or a fail3d component like my keel coolers

[a64pilot]

Yes, never use any kind of stop leak, especially on these little systems.
The orifice on our little systems is likely small enough to be leak stopped

[Warby12]

Hi there, I was able to reveal the connectors today. (I'll take a pic tomorrow). The return low pressure side had oil on it!

Yes, I don't want to open it up until absolutely necessary. I will try the soapy water test, and after seeing what there is to see, tighten the low pressure connector. Waiting for the post for parts now.

Does anyone know which O rings are needed? I will keep searching the forum and elsewhere but if anyone knows off the top of their head? It's an Isotherm BD35, I think 2006 vintage

[s/v Jedi]

Originally Posted by Warby12

Hi there, I was able to reveal the connectors today. (I'll take a pic tomorrow). The return low pressure side had oil on it!

Yes, I don't want to open it up until absolutely necessary. I will try the soapy water test, and after seeing what there is to see, tighten the low pressure connector. Waiting for the post for parts now.

Does anyone know which O rings are needed? I will keep searching the forum and elsewhere but if anyone knows off the top of their head? It's an Isotherm BD35, I think 2006 vintage

I have the Frigoboat connectors so can’t help you there. You found the leak, no need for soap tests. The bad news is that it is at the low pressure side, so it may have leaked the other way as well if it pulled a vacuum. Is this a fridge or freezer line?

Do you have a gauge set? You need to equalize pressure between high and low side (so that there is pressure inside this return line, then like described before carefully tighten the connection with two wrenches. Then clean it, top up the R134a and run for a while, keeping an eye on oil residue.

On topping up the gas: I think you have an evaporator plate, not a holding plate, right? First bleed the air out of the hose between bottle and compressor, then tighten it to the low side service valve. Add gas while running and keep an eye on the part of the evaporator that gets frost. It would be great if you have access to a couple of inches of the return line. Add a bit of gas and see how far the frost expands. Stop when the frost appears on the first inch of return line or, when you don’t have access, when the whole evaporator plate is frosted. Ignore pressure recommendations.

BTW: connecting and disconnecting to the service valve has the same risk of failure than taking the quick connectors apart. These are valved connections that rarely fail

[Warby12]

Hi s/v Jedi, Thanks for that! Yes it's on the return side so that creates a risk of moisture being sucked in doesn't it? The flow into the evaporator seems ok at the moment. I think I have heard that if there is moisture, the first sign will be the flow starting and stopping as the moisture freezes, blocking the pipe. Of course it's not getting very cold at the moment so maybe once I gas it up, a moisture problem will assert itself?

It is a fridge, so think that means the suction is less on the return side, so maybe I'm lucky with sucking in moisture? You're right of course, no need for leak test when it obviously leaking haha.

I will turn it off for a while this morning to allow equalisation, then tighten the connector and test it. I have ordered a gauge set but have not received it yet. Thanks for the procedure on topping up, as soon as I get the gas I will, can't wait! During the day it pretty well runs continuously, which jangles my nerves. When it is warm to hot outside, it can't cope at all and the box temp starts rising. I can see the return pipe as it exits the insulation so will watch it for frost.

Plan is in place, just need the gas (and the gauge set)!

[s/v Jedi]

Originally Posted by Warby12

Hi s/v Jedi, Thanks for that! Yes it's on the return side so that creates a risk of moisture being sucked in doesn't it? The flow into the evaporator seems ok at the moment. I think I have heard that if there is moisture, the first sign will be the flow starting and stopping as the moisture freezes, blocking the pipe. Of course it's not getting very cold at the moment so maybe once I gas it up, a moisture problem will assert itself?

It is a fridge, so think that means the suction is less on the return side, so maybe I'm lucky with sucking in moisture? You're right of course, no need for leak test when it obviously leaking haha.

I will turn it off for a while this morning to allow equalisation, then tighten the connector and test it. I have ordered a gauge set but have not received it yet. Thanks for the procedure on topping up, as soon as I get the gas I will, can't wait! During the day it pretty well runs continuously, which jangles my nerves. When it is warm to hot outside, it can't cope at all and the box temp starts rising. I can see the return pipe as it exits the insulation so will watch it for frost.

Plan is in place, just need the gas (and the gauge set)!

You don’t really need a gauge set, although it is much easier. The valve that goes onto the R134a canister and pierces it... you can connect that directly to the low side service valve using a small hose they sell for that. Don’t forget to bleed the air out!

You are correct to assume you’re probably okay with it being a fridge as that doesn’t create a deep vacuum on the low side at the end of a cycle.

We always carry a gauge set, ample R134a and even a vacuum pump. I once sold these, then had to buy new again so I’ll just hang on to it now

[Warby12]

So, thank you again all! I have successfully added gas and it instantly improved the performance of the fridge. The cold plate frosted up to 90% (roughly) and started cycling properly. Very good stuff!


Unfortunately the leak is still there. Inspired, I have bitten the bullet and bought a vacuum pump, so would like to do the job properly now but I'm stumped and may be defeated by the leak. The low pressure coupling has oil residue all around it, from below the coupling, up to where the capillary is soldered into it on its way to the evaporator. I have tried a few times to leak test with soapy water but no luck! I know how to do this because I've done it with propane connections many times, but no bubbles! I did tweak it a bit tighter so maybe that bought me some time.


I can't easily locate replacement o-rings for this older Isotherm, so I was thinking of cutting out the couplings and soldering them together with a bridging section of tube and some couplings but the tube sizes defeat me..... the low pressure line back from the evaporator to the coupling seems to be 6.2mm O.D. (1/4"?) then from the coupling on to the compressor approx 5.5mm (3/16th?). And also the high pressure line from the compressor about 4.1mm!

Maybe it's the way they are bent that is confusing things but can anyone help me with what size tube, I can replace the couplings? It is an Isotherm, Compact Classic (maybe 2006). The manual lists inside diameter for the tubes but not the outside diameter, which is what I need.

Sorry, very long winded and probably not very interesting but I live in hope

[s/v Jedi]

Do you mean that they don’t sell the size O-rings you need or that you don’t know which size you need? It’s easy to mail O-rings.

I don’t think cutting the coupling out is a good idea because of the different tubing diameters. But maybe the one simply fits inside the other? Do you have reaming tools?

[Richard Kollmann]

Originally Posted by Warby12

So, thank you again all! I have successfully added gas and it instantly improved the performance of the fridge. The cold plate frosted up to 90% (roughly) and started cycling properly. Very good stuff!


Unfortunately the leak is still there. Inspired, I have bitten the bullet and bought a vacuum pump, so would like to do the job properly now but I'm stumped and may be defeated by the leak. The low pressure coupling has oil residue all around it, from below the coupling, up to where the capillary is soldered into it on its way to the evaporator. I have tried a few times to leak test with soapy water but no luck! I know how to do this because I've done it with propane connections many times, but no bubbles! I did tweak it a bit tighter so maybe that bought me some time.


I can't easily locate replacement o-rings for this older Isotherm, so I was thinking of cutting out the couplings and soldering them together with a bridging section of tube and some couplings but the tube sizes defeat me..... the low pressure line back from the evaporator to the coupling seems to be 6.2mm O.D. (1/4"?) then from the coupling on to the compressor approx 5.5mm (3/16th?). And also the high pressure line from the compressor about 4.1mm!

Maybe it's the way they are bent that is confusing things but can anyone help me with what size tube, I can replace the couplings? It is an Isotherm, Compact Classic (maybe 2006). The manual lists inside diameter for the tubes but not the outside diameter, which is what I need.

Sorry, very long winded and probably not very interesting but I live in hope

Your picture shows Aero-Quip manufactured line connectors that do not seal the final connection with O ring seals. The O rings inside are designed to only retain refrigerant when lines are not connected. Final leak proof connector seal on these couplings is by metal to metal connection. Lack of proper torque or corrosion on these steel surfaces will cause loss of refrigerant.

[s/v Jedi]

Originally Posted by Richard Kollmann

Your picture shows Aero-Quip manufactured line connectors that do not seal the final connection with O ring seals. The O rings inside are designed to only retain refrigerant when lines are not connected. Final leak proof connector seal on these couplings is by metal to metal connection. Lack of proper torque or corrosion on these steel surfaces will cause loss of refrigerant.

[Warby12]

Thank you very much, SV Jedi and Mr Kollman.

It's funny, as I was writing my question, it occurred to me that cutting them should probably be avoided but I was very curious about the tube sizes in case I needed to.

Aero-Quip! Thank you Richard, that sheds a bright light on it. So no o-rings. I did tighten the coupling (and checked the other one) and it was loose. If it is metal to metal, that would definitely leak. Fingers crossed, that was the source.

I still believe there is moisture in the system though because when it gets hot, the compressor stops and restarts and the led I put on it flashes three times. At the same time the evaporator plate defrosts. From my reading on the forum, I hear this may be caused by moisture in the filter dryer boiling off and then freezing and blocking the capillary tube?

Does this make sense?

If so I will do the vacuum and recharge. Trouble is I am on anchor so don't have unlimited AC power for the vacuum pump and can't run a heat gun to warm up all of the components to help with clearing the moisture. I will pick a hot day and maybe a bit of careful propane torch application, fingers crossed!

Thanks again guys, I really appreciate your help. Refrigeration is so important.

[Richard Kollmann]

Warby12, This is not your mother’s refrigerator compressor it has no high pressure or high temperature shut down device. High pressure or temperature might blow or burn through the pump’s tiny cylinder head gasket permanently destroying compressor. If refrigerant charge is correct and system performs well it is unlikely moisture will cause an overload amperage as the trouble LED three flashes is indicating. Overload on this BD35 compressor is a result of high amperage on compressor start or unstable three phase electronic pulse while running normally caused by poor boat wiring between battery and compressor module. I would first check starting and running amperage, if ok I would run a jumper wire from battery ground to module ground.

A very small amount of moisture in refrigerant will over time cause corrosion or acid oil problems. One drop of moisture more than filter dryer can safely capture will freeze and thaw in expansion device but will not stop compressor ounce running.

[Warby12]

Originally Posted by Richard Kollmann

Warby12, This is not your mother’s refrigerator compressor it has no high pressure or high temperature shut down device. High pressure or temperature might blow or burn through the pump’s tiny cylinder head gasket permanently destroying compressor. If refrigerant charge is correct and system performs well it is unlikely moisture will cause an overload amperage as the trouble LED three flashes is indicating. Overload on this BD35 compressor is a result of high amperage on compressor start or unstable three phase electronic pulse while running normally caused by poor boat wiring between battery and compressor module. I would first check starting and running amperage, if ok I would run a jumper wire from battery ground to module ground.

A very small amount of moisture in refrigerant will over time cause corrosion or acid oil problems. One drop of moisture more than filter dryer can safely capture will freeze and thaw in expansion device but will not stop compressor ounce running.

Thanks Richard, very interesting and helpful ..... The fridge runs steadily on about three amps, it increased about .5 amp after adding gas. At startup about 4.5 amps.

I have a temperature sensor on the line just after the condenser to the evaporator. If this gets much above 32C (90F) the compressor starts stopping and restarting repeatedly. It should be able to handle a much higher temp from after the condenser shouldn't it?

I will let it get hotter today and observe the amps (and reconnect the LED). I will bypass the thermostat again too. It hot here today so should be a good test.

The wiring is all new installed by me and direct to the battery, but I will check again.

Thanks for your help.

[Richard Kollmann]

If you bypassed the thermostat and its wiring at the control module BD35 would be running at 2000 rpm drawing 3 to 3.5 amps. Even if current draw reached 5.5 amps compressor would not trip the three LED code. Are you sure that with thermostat bypassed and left running many hours compressor will still stop and display a three LED code?

Blockage of refrigerant flow would not stop running compressor.

A compressor that cycles off on a low voltage spike can when restarted cause a low voltage single flash code to be covered up by a overload three flash code.

It has been the cases where air contaminated refrigerant overloads amperage but this shows up as an amperage overload code.

The fan circuit of module terminals small + and F if current draws more than 1/2 amp will prevent compressor from starting.

[Warby12]

Originally Posted by Richard Kollmann

If you bypassed the thermostat and its wiring at the control module BD35 would be running at 2000 rpm drawing 3 to 3.5 amps. Even if current draw reached 5.5 amps compressor would not trip the three LED code. Are you sure that with thermostat bypassed and left running many hours compressor will still stop and display a three LED code?

Blockage of refrigerant flow would not stop running compressor.

A compressor that cycles off on a low voltage spike can when restarted cause a low voltage single flash code to be covered up by a overload three flash code.

It has been the cases where air contaminated refrigerant overloads amperage but this shows up as an amperage overload code.

The fan circuit of module terminals small + and F if current draws more than 1/2 amp will prevent compressor from starting.

Hi Richard, thanks again. I have it running now with the thermostat bypassed at three amps and no problems so far. I will keep watching, it's not very hot now, also the fans are on a relay so can't draw too much.

But! I may have to put that to one side because I have noticed the line is still leaking somewhere, as can be seen in this photo I have just taken, there is a drip of (I guess) oil on the capillary tube.

Where can that be coming from? Above that tube is the entry point for the capillary into the low pressure side. If so can that be fixed or do I need a new evaporator, do you think?