I had the same issue on our 91 240. It was an insulation failure inside the shifter housing of the wire that runs down to the solenoid. The short killed the relay. Ran a new wire and replaced the relay and 4th gear worked.

 

Just a thought that if the button was missing when you bought it, perhaps a mechanic disconnected it because it was faulty and the mechanic thought you really don't need it in most situations.

 

Simple solution: Get the soleniod bypass kit from IPD and in 15 minutes you have the transmission shifting through all four gears.

https://www.ipdusa.com/products/4793...110397-1239928

Don't forget to remove the light socket from the dash.

Downshift manually D to 2.

 

I just ground a slot in the old solenoid and it works the same way. A quick fix.