How to Install a 34790 Mars Defrost Timer

If your heat pump is experiencing freezing issues and malfunctioning, the problem could be the defrost timer. One alternative to replacing it with a manufacturer's defrost timer, which may be expensive, is to use a substitute timer, such as the 34790 Mars defrost timer. The most important part of this procedure is to make sure the heat pump wires are hooked up to the board correctly.

Screwdriver
Socket/socket set
Wire cutters
Pencil/paper
Needlenose pliers

Instructions

- 1
  Turn off the power to the heat pump at the outside breaker. In the United States, most heat pumps will be installed on a 240V circuit, which means the breaker inside the control box will be one of the larger, double breakers. Turn the breaker to the position marked "Off" or "0".
- 2
  Use a screwdriver or socket wrench to remove the screws or bolts holding the service panel in position on the side of the pump. Set the panel aside.
- 3
  Make a note of the current wire connections on the board, if this installation will replace an existing Mars board. This will help you make the right connections. Use a pair of needlenose pliers to pull off the current wires. Loosen the screws holding the board in place inside the heat pump cabinet, and remove the board.
- 4
  Connect the bottom left terminals, marked "Cycle," to the power cables (usually colored black). Connect the red wire protruding from the center of he Mars board to the red wire from the heat pump. Consult the diagram you made earlier of the previous board to connect the rest of the wires. If this is a new installation, consult your heat pump manual to determine the exact order of wire replacement.
- 5
  Secure the Mars board inside the heat pump cabinet by inserting screws through the board body.
- 6
  Replace the electrical service panel.
- 7
  Restore the power at the outside breaker.

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