I've got an '86 740 with the manual 4 speed with OD. It had problems with not engaging its overdrive for a while (2 years) and then it finally gave up the ghost when reverse stopped working. I bought a transmission from a wrecker and replaced it.

Anyway, I'm anxious to make 5th gear work again. I replaced the switch at the top of the shifter knob, soldered some suspicious joints inside the OD relay, the solenoid rattles when you shake it, and am now looking at other spots. I can't find broken or worn wiring under the dash, and now I'm wondering about the switches on the transmission itself.

How exactly does the 4th gear selector switch work? I gather it keeps the OD from engaging when the transmission isn't in 4th, but how? I only see 1 contact, so how's it sending a signal? Is it possible to rebuild one similar to how some people salvage their window switches? How can I check if mine is bad?