I cleaned the fins on my Trane split system cooling condenser (model ttb042c100ao), when I went to put the fan back, I noticed a loose black wire - I hooked it up to the contactor switch (to a shiny plug) and finshed putting screws back... started unit... it ran but the fan motor was very slow... there was a slight hum from the contactor area. Although it cooled (there was no frost)... the fan motor was at 20% its normal speed... help

The wire you put on the contactor (black wire) may be a extra wire not being used(for a higher horse power) and now that you hooked it up your motor may be trying to run on two speeds at once and in turn will burn the motor out in a hurry if kept that way.

Thanks for your answer... unfortunately, when I disconnected the black wire... and restarted the unit... I did get a compressor hum but no fan motor running... so I took needle nose plyers.. and touched the contactor on the both sdes of the contactor block... the opposite side (where there was a yellow wire)... motor started up again... but at 20% of its normal speed... then went back to riginal spot... same results... suggestions?

Have a tech read the wiring diagram and connect wires accordingly.
If you replaced the OEM fan motor with a universal motor, be sure that you have wired the motor and capacitor correctly and that the motor has the correct replacement ratings.
Motors are usually multi-speed with wires that are normally not used.
You probably did exactly what T-Top referred to and added a wire that was not meant to be connected.

Your compressor,fan motor or capacitor is going to ground if the motor will run with the black on the same side of the common wire(that may be yellow from what you say). You never did say what the other wires left over from the motor was doing are they caped off, being used or bare and going to ground.