420AC - AC Seawater pump relay

K

ktcanuck

New Member
Sep 13, 2010
275
1000 Islands Ontario
Boat Info
'96 420 Aft Cabin
2012 310 Merc RIB/2011 9.9Merc2S
Engines
420 3126 Cats.
Well, the boat's in the water here and I am progressively commissioning each system after this boat's first real winter (it was a Florida boat until last year). Trying to get heat on and no water from the seawater pump. Removed the pump and it functions fine at home. I am trying to locate the pump relay to check it out even though I am getting a switched 220V at the pump. The drawings show it located next to the main cabin a/c controller but it isn't there. I can trace the relay cables from each of the a/c units heading to the area around the breaker panel but nothing is visible behind it. I will drive down to boat and re-install the pump today and check behind the microwave.

Have you had to locate your relay box on this boat? Where was it?
 
First, I don't know the answer on your specific boat, but the usual installation is to enclose the a/c triggers in the main panel box at the salon a/c unit. Try to follow the wiring harness from the front unit into the rear one and t he wiring out to the pump and you should see it. Also each unit has a trigger that turns on the pump independant of the other unit. If you are only working with the rear heat, try the forward one to see if it comes on.
 
fwebster said:
First, I don't know the answer on your specific boat, but the usual installation is to enclose the a/c triggers in the main panel box at the salon a/c unit. Try to follow the wiring harness from the front unit into the rear one and t he wiring out to the pump and you should see it. Also each unit has a trigger that turns on the pump independant of the other unit. If you are only working with the rear heat, try the forward one to see if it comes on.
Click to expand...

Hi Frank, the triggers from each unit appear to work ok in that I am getting ~220v switching from the a/c units at the pump but with the pump disconnected so something is happening at the relay box. I can trace the trigger cables to a point before they disappear into harnesses somewhere adjacent to the aft part of the galley and forward of the electrical panel. The drawings that came with the boat do show a separate relay box from the main cabin a/c control box but I will certainly look at your suggestion. I guess if they are combined there should be 2 220v cables entering what looks like the main cabin a/c control box. I am assuming there is a control box for each a/c unit although I cannot determine this from Dometic on-line manuals.

The pump does get warm even though there is no rotation. I am going back to boat tomorrow to reconnect the pump and check the voltage with the pump connected using a temporary terminal block. I had thought I might have had a bad crimp connection at the motor cable to relay cable splice but I can't figure why the motor would become warm if any one of the three wires had a poor connection.

As always, many thanks Frank.
 
OK.......the warm pump with no rotation is a clue.........look for no/low voltage on one leg of the 240V service to the pump. You even get something close to 240VAC with no load so check it under load and I bet you find a bad breaker or relay not passing voltage on its bad side.
 
fwebster said:
OK.......the warm pump with no rotation is a clue.........look for no/low voltage on one leg of the 240V service to the pump. You even get something close to 240VAC with no load so check it under load and I bet you find a bad breaker or relay not passing voltage on its bad side.
Click to expand...

That's along the lines I was going too.

twyant said:
under the floor below the ac control in the storage aera
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Hi Twyant. Can you elaborate a bit? My main cabin a/c unit and its control box is in the under salon floor compartment just forward of the engine bulkhead. Is that what you call the storage area? I believe some 420ACs have their water heater here but mine has it's spare props here with the water heater port of the washer dryer.
 
I hate solutions like this. I re-installed the seawater pump and wired it temporarily through a terminal block to measure voltage across L1 - L2. After the customary count-down the water pump came on and ran for perhaps 20 seconds and stopped. Turned power off and re-powered, ran definitely longer. Cycled power again and it hasn't stopped since. Purging air from the system? All 3 units are now pumping much needed heat.

Twyant: I am still interested in knowing where you think that Relay box is.
 
on my 420 it is where you said forward of the engine bulkhead on the aft wall my water heater is under mittle berth bed I moved the relay box and put a hatch to get to the back side of generator foe easy axcess to pump so that but that where relay box on mine .
 
twyant said:
on my 420 it is where you said forward of the engine bulkhead on the aft wall my water heater is under mittle berth bed I moved the relay box and put a hatch to get to the back side of generator foe easy axcess to pump so that but that where relay box on mine .
Click to expand...

Ok. I will keep looking!
 
With 3 units on that boat, the pump is probably going crazy with starts and stops. We also have 3 units, and have wired the pump to run all the time. No more starts and stops. There should be instructions on how to do this in your wiring diagrams, or a local AC should be able to do it for you. I think you will find this to be the best way.
 
ktcanuck said:
Ok. I will keep looking!
Click to expand...

You were right. I thought I was looking at the control box but the part number shows it is the relay box. Good idea on the hatch.

The pump is working fine now. However now the fan motor on the salon a/c has seized. Thankfully there's enough heat from the fore and aft units to keep us going until i can find a replacement.
 
Yes change it and the compressor but mine is easy to get to under the center state room
 
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