I replaced the start up contactors in my trane xe900 a/c unit,but the only thing that runs is the compressor & not the fan. I follwed the wiring diagram to the letter, I some suggestions as what to do next

Probably replace the fan motor.
Double check,make sure all wires to and from motor are connected,

Look on the top (typically) of the contactor where you hooked up all the wiring. This is referred to as the load side of the contactor. If all you did was swap out the contactor then make sure you put the fan wire on the correct side of the contactor. You may have one 'hot' bar and one that is operated by a plunger. The wiring diagram should tell you which one.

If the fan motor worked when you swapped the contactor check the wiring to the run capacitor and from there to the motor.

Can you tell us how many wires are off the fan motor and what they are hookedup to?