[denleep64]

When I switch my a/c on nothing happens there is a relay switch on the unit outside when you push it in the fan kicks on but when you release it shuts back off.

[hvac1000]

From your description you are missing the low voltage control circuit going to the outside unit. There should be a wiring diagram of the outside unit on the access door. You can use this diagram and the diagram on the furnace/air handler located on the inside of the blower door to find all the connections and wires for this low voltage problem. You will need a meter and know how to use it in order to track the problem down.

Some usual problem areas are.

A blown fuse on the interior unit circuit board.
A thermostat that is not set to cooling.
A broken wire going to the outside unit. (dogs and animals love to chew on the low voltage wires.)
A defective thermostat not sending the cooling signal to the outside unit.
A defective low voltage transformer not supplying the low voltage for the circuit.
A defective low or high pressure control in the outside unit itself.
A defective time delay circuit if one is used for your outside unit.
There can be other items that can also cause this problem so a good check of all low voltage circuits is in order.