OD on '89 740 AT Turbo

Not at all. There is a switch a relay and a solenoid and that is basically it. The switch operates the relay which selects the solenoid and the light. As you have an auto gearbox there must be a fault in the light wiring (probably at rear of instrument panel) if the box works as it should. I assume that the instrument panel has not been changed over at any time as a mismatch can occur between car wiring and panel.

All the best, Peter.

If the arrow is on then the problem is electrical. I assume you did push the OD button in attempt to turn the arrow off? It's possible the button or more likely the wiring to it is broken.

We pulled the #12 fuse and saw that it was broken, so I'll be replacing that with a new 15amp. We've got a spare 25amp in there right now, and that made the "UP" arrow light on the dash go off, at least. I still don't have OD, though and my engine revs between 1-2K rpm at idle from a vac hose leak somewhere.