Two Compressors - One Pump

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Chainplate's picture
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Joined: 2016-12-31

I hope someone here can help me. My wife and I live on a small sailboat. When our refrigerator and freezer began to have problems, we sought the help from a recommended refrigeration technician. He said he needed to remove everything to bench test at his shop. After two months of living out of a cooler and no answers to our phone calls/texts/emails, we threatened with legal action. Shortly thereafter, he returned everything to us - in a box! I was able to hire another technician to put most of it back together, but he can't figure out how to make the water pump work.

We have two compressors, one for the refrigerator and one for the freezer. We use sea water (raw) to cool the refrigeration's condenser. We have one pump that circulates the raw water. What we're struggling with is how to wire it up so that regardless of which compressor is cycling, the raw water pump runs. Of course, it was working fine until the 'recommended' technician tore everything apart. We have a series of relays but we aren't smart enough to figure out how to wire them. Each compressor outputs 12VDC when the compressor is cycling for the purpose of powering a pump.

Any help would be greatly appreciated.  Happy New Year!

USS Hornet's picture
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Joined: 2015-01-07

Connect the coil of a 12VDC relay having one N.O. contact across the refrigerator compressor motor. Connect an identical relay across the freezer compressor motor. Connect the two N.O. contacts in parallel and in series with the pump motor. The relay contacts must be be heavy duty enough to prevent them from burning due to the arcing when they open. You might want to call in another electrician to do this.

100 td's picture
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Joined: 2017-04-20

What is the load current of the water pump?

user-1207832's picture
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Joined: 2017-04-21

USS Hornet is right if each of the two 12VDC conductors (1 refrigerator & 1 freezer) are only control wires for energizing a 12VDC coil. But if your former electrician, from FBN Marine Electric brought ALL of your parts back and you don't have a couple of 1-pole relays with 12VDC coils in the pile, then I think it'll be a lot easier. From here on, if your results don't match my story line, then follow the Hornet.

My guess is that pump is similar to a little water pump you'll find in a camper. I bought a couple of those awhile back. It's no bigger than your fist and has two 16 gauge insulated stranded conductors. If so, either or both of the 12VDC conductors from the refrigerator and/or freezer (when energized) will run the pump (THE OTHER CONDUCTOR FROM THE PUMP CONNECTS{makes} TO THE ROUNDED/COMMON/FRAME/METAL/etc.). If each of these 12VDC conductors from appliances originate from the same 12VDC system, then there will be no difference of potential between them when energized. So, when you have everything in place and wired up, except the three conductors (fridge wire, freezer wire & pump wire) have your tester ready and set to... let's say the 50VDC scale. But first, connect the pump wire to the fridge wire and turn the fridge on. If the fridge and pump are running, good. Shut the fridge off and separate the pump wire from the fridge wire. Now, connect the pump wire to the freezer wire and turn the freezer on. If the freezer and pump are running, good. Separate the freezer wire from the pump wire.

Now you're going to check for any difference of potential. In this case, voltage between the refrigerator wire and freezer wire, when both appliances are energized. I know you're supposed to be pumping water when these appliances are running, but this is a short and necessary test. Have your tester ready. Okay, turn both appliances on and check voltage between the two wires (appliances). If there isn't voltage, awesome. Shut both appliances off.

If you had NO difference of potential, in this case voltage, then tie the conductors (one from pump, one from refrigerator, and one from freezer) together with a twist on type connector or whatever is appropriate. Then, tighten, secure and label as needed. You're ready to sail Sinbad!!

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