I just replaced the water pump on my 1978 Mercury 90 (inline 6). When I put the lower unit back on, the gear control box is in neutral and the prop spins freely in both directions as it should. But, when I put the gear control box in forward (all the way) and the prop still spins freely in both directions. Same with the reverse. I pulled the lower unit off while the gear control box was in neutral, but I can't figure out how to get it to engage now that it's back together. Any ideas on where I went wrong and how I can get it to work?