Introduction
The Marine Air Passport II Air Conditioner is used in many boats and yachts. The control board has a mechanical relay soldered on the main board that controls the operation of the compressor and seawater pump. The relay eventually fails and will either not operate or welds together preventing the compressor and seawater cooling pump from shutting off. Welded contacts can be confusing as the unit appears to work normally when power is supplied, yet will eventually trip the fuse or breaker protecting the unit. The unit (assuming it is otherwise in good mechanical order and properly charged with refrigerant) will cool normally. With welded contacts when the unit reaches the desired cooling temperature the fans will shut off but the compressor and seawater pumps will continue to operate. This will eventually result in the compressor ingesting liquid coolant, stalling the compressor. The resulting locked rotor current will trip the input breaker. This results in a mysterious failure - by the time it is noticed that the breaker tripped the liquid refrigerant will have returned to a normal state and the unit will again operate apparently normally.
Parts
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The Marine Air Passport II Air Conditioning unit uses a mechanical relay for the compressor. The relay will eventually fail resulting in the contacts welding together and the compressor and water pump running continuously, or failing to run. Replacing the relay is a simple fix.
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Desolder the locations marked in red in the photo to remove the relay. Note that the upper middle hole may not have a relay pin. Be careful not to use excessive heat or damage the PCB traces.
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The original relay is discontinued. An acceptable substitute is a Potter and Brumfield T90S1D12-5 available at Digikey Electronics. Solder into place.
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To reassemble your device, follow these instructions in reverse order.
