To go into overdrive (4th gear) the OD solonoid gets 12 volts through the activated relay, at this time the light is out as it now has 12V on each side. When the relay and solonoid are dropped the car goes out of OD (into 3rd), the light is supplied with a ground path and comes on. Sounds strange but that's how she works.

Of course the throttle cable will overide OD when you stomp. Check the adjustment, its a common sense one, no book needed and make sure the cable is free.

Even those relays can be bad when fairly new, resolder the heavy current carrying solder points on the relay PCB and do your fuel pump/system relay at the same time. They will be better than new. If still a problem you should then check for 12V accross the solonoid itself. You also may want to perform a flush on the tranny, simple to do as described in the FAQ section.

These are the first easy, no or low cost steps before going into valve bodies or pulling solonoids and other nasty stuff.
--
David Hunter