740 Overdrive

[Giddy_Geddy]

Hi, I recently bought a '91 740, and I noticed that it fails to engage the overdrive. It'll shift into third gear but then never shift into fourth, so I'll be doing 60mph at 4K rpms. If I press the button on the shifter, the orange arrow will turn on and then off when I press it again, but overdrive will not kick on. From what I've read, the overdrive relay is the problem when the arrow fails to turn on, but mine is functional. Any information would be helpful!