A518 won't shift into overdrive

Hey guys, a little help needed.

First off I installed my 518 behind my 360 using a automatic OD engage kit from Performance transmission center. It worked for about three days and now my OD won't kick in. The tranny is a rebuilt one with less than 250 miles on it. The kit I purchased cam with a vacum switch, two oil pressure switches (one for OD, one for lockup). Does anyone know of a way to test to see if the switches went bad or if it's the tranny? The tranny shop I bought the trans from is 80 miles away and I don't want to haul it until I have checked everything.

Your help is appreciated!
Grant