It won't upshift into the 4th ('overdrive') gear.
I don't think it is a solenoid or wiring problem.

Please help me troubleshoot & repair my wife's '91 740 wagon's automatic transmission. We live in a rural area with steep winding roads & it has not been slipping or hard-shifting or anything obvious; it just doesn't shift into 4th anymore. The overdrive did refuse to engage/disengage once last year & once a few years ago when the fluid level was quite low.

Although I see no brake-lining material or other crud in the fluid, I'm thinking that the passageway that is controlled by the OD solenoid/valve might be blocked or gummed up?

I hope that there is some safe, simple solution unknown to me...
like... is it okay to shoot compressed air into the valve body?
Or some kind of chemical treatment + flushing?

The fluid level is a little overfilled. I resoldered & then replaced the OD relay, & replaced the damaged-looking solenoid. Current is reaching the solenoid & heard the valve clicking (just like when I bench-tested it).

No fourth gear. What could it be? Thanks.