I have a "89 740 and the overdrive does not work. I have been reading other posts on this issue and instead of changing the relay and solenoid I would rather go with the bypass plate.

What I need to know is how to install it when I do get it?

 

Pull the solenoid and cut a groove between the ports on the solenoid with a Dremel.
Groove should about as deep as the smaller port.
Reinstall the solenoid and go.
I never used the underdrive anyway.
ED7