My 740 '91 sedan has the Yazaki instrument cluster which is identical to that I've seen in an acquaintances' 940.
(remove the cluster via two slits in either side of the inside cluster shell).

The odo and speedo were unreliable at best but mostly not even working at all.
I read up on this issue type and bought the gears to replace and studied the words about capacitor replacement and have not yet bought.
I did not work too quickly at all.

Next job, replace the w.pump, cam timing belt , tensioner, seals.
decided to replace the accessory belts - p.steer, alternator, aircon.
(So I found the alternator belt v.loose and v.worn and ) replaced it and tightened into tolerances.

BONGO!!
The speedo / odo are back up and working !!
I can now tell what speed I'm doing. The odometer is working.

It seems that the alternator fix did the trick ?
'çoz NOTHING else was done and it's been 6 days now and everything is just lemon sweet and I can go back to checking fuel consumption.

I'm in Perth Western Australia where COVID is something the whole world has and we ain't seen it since May2020. ZIP! NONE!
Clean greenest warmest freshest place in the world. and summer is coming on strong.

Doors are bolted so you can't get in.