Airconditioning Raw Water Pump Relay Box

[Capt. Rich]

I am not sure of the device but it is the relay for the water pump">raw water pump and all ac units. The shore and or gen power comes into the box and energizes the ac units. When one or all ac units are turned on the raw water pump is also turned on. Last week while at anchorage for New Year's Eve I thought I should turn the ac units on since they have not been used for about 3 months. When I turned on one of the ac units the raw water did not come on. I thought the pump was seized up due to lack of use. It is an Oberdorfer pump with a 1/2 hp 110 volt Dayton electric motor. I was correct the pump was seized and had to be manually turned over a few times to free up the pump. After the pump turned over easily I went back and turned on one of the ac units. The pump still did not come on. I checked the voltage at the relay box and the incoming voltage from the gen was about 121 volts, the ac unit leads read the same , but the raw water pump leads had no juice. The relay box is some sort of printed circuit from Taylor Made / Airmarine. I have no way of checking anything but the leads to each unit. All units become energized at the box when turned on at their respective thermostats. The raw water pump does not become energized. Did I fry some circuit for the raw water pump? What is the best replacement for this relay?

[svHyLyte]

Why not simply rig a solenoid to the compressor power line? When the compressor is power up by the thermostat, the solenoid closes connecting the power circuit to the pump. It should be a fairly easy fix although there are electrics types here that are more qualified than I to opine.

FWIW...

[osirissail]

In the modern (newer) version of marine air conditioning units a relay to energize the raw water pump is included in the air conditioning unit. So an external relay box is not necessary.

However, for the older marine air conditioners there is a separate metal box with relays in it that are activated by the turning on of the air conditioner allowing ship's AC power to flow to the raw water pump. In the older units the box had the old physical solenoid type relays. I suspect the intermediate age units switch to solid state and then in the new air conditioners it was all incorporated into the air conditioner control box actually on the air conditioner.

If you apply ship's AC power to the box and then with the air conditioner turned off and the raw water pump wires disconnected - you should observe no voltage at the raw water pump terminals.

Turning on the air conditioner should result in seeing full voltage appear at the relay terminals going to the raw water pump. If not, then the relay is defective or the control wire from the air conditioner to the relay box is not supplying power to close the relay.

If there is full voltage to the raw water pump terminals and the pump does not operate then the pump has failed. Energizing a "stuck" pump may have "burned-out" something in the pump, but with modern pumps this is rare.

However, in my experience, air conditioning raw water pumps fail all too often and at hundred of dollars each, it pisses me off to have to replace them. A lot of them are "magnetically coupled" pumps where the motor shaft rotates a magnet that then tries to rotate the impeller in the pump. The impeller are sometimes only a plastic flat bar that "slaps" the water to the output hose. I believe this was done to limit the "clogging" of the pump with sea debris.

I have replaced and tried many different brands of AC raw water air-conditioner pumps and have found rusted shafts; scored plastic pump housings which prevent the pump magnet from rotating and broken paddles. I have even found bent motor shafts where most probably the pump had experienced something heavy landing on it.

The diagnosing is rather simple - take the raw pump out and hook it up to an AC power source and see if it runs. Then attach some hoses and put the inlet hose in a bucket of water and energize the pump. If water flows out the "outlet" hose which you have raised to the equivalent "head" height of the air conditioner coils then the pump is fine. The problem is in the wiring or relay box.

[AnchorageGuy]

One issue with connecting the pump to energize with the compressor is that some units require the pump to start before the compressor so that water is present and flowing when the compressor comes on. This might be a temporary fix but the problem needs to be solved and the unit operating as designed. Chuck