badbrick,
I'm no expert Duane is....but is this an m46 we speak of? Have you replaced/checked the 4th gear switch on the top of the trans near the speedo cable? These can be a problem as well.
There are two things that click.. the solenoid and the relay.
If you are under the car and hear the click then I'd guess that their are seals in the unit that need replaceing.
Duane sells a rebuild kit as does IPD.
I have a 85 740 gle with a m46.
I find that if you get up to about 60mph enguage od (see that green light on dash is lit etc.)and push the gas pedel up and down raising the revs to between 2800 & 3200 rpm mine will enguage eventually. It then normally will stay enguaged and re-engage the rest of the trip.
Search the archives for postings by Duane Holberg and read up.

Mark