Cooling system does not work properly although the heating system works fine.

My thermostat is a Robertshaw 400 series installed by my home-builder in 1998. I was informed that the heating/AC system was a (dual) zone system with an upstairs thermostat.

I wish to replace this thermostat with a programmable Honeywell. I recently bought a Honeywell RTH6300B and installed it myself. The fan blows when the cooling mode is on but the compresor doesn't come on at all. I have two remaining wires, blue and brown.

Help! :confused:

In a conventional furnace A/C system the thermostat switches power to the Y terminal when calling for cooling. Then a yellow wire connects it to the Y terminal at the furnace to activate the blower, and a pair of wires runs out to the outside compressor relay coil from the Y terminal and a B or C terminal at the furnace. Unless you have a heat pump, the blue and brown may not be used. If the above connections don't work, post back with more details, type of system, terminal markings at the thermostat and furnace or air handler, etc. I, or others, may or may not be able to help you figure it out.