When the on/off button is pushed on my whirlpool bath, nothing happens. We have checked the tube under the tub and it appears to be OK. We are sure the problem lies in the air switch, but are not sure what parts to order, what is your suggestion?

If the actuator tubing is ok the problem is either the actuator or the air switch.  To check the actuator disconnect the tubing where it connects to the switch on the motor. Place your thumb over the end of the tube and have someone press down on the actuator button. When the button is depressed release your thumb from the end of the tube. You should feel a puff of air coming from the tube. This is a pretty subjective test but will tell you if the actuator is compressing air in the tube. If the actuator is ok then the problem has to be the airswitch.