I got an 84 733i awhile back and have been trying to get it ready for
wrecking by my now 15 year old son (okay, I should be a bit more
optimistic!) and have been having a pile of problems with the speedometer.
   The previous owner had replaced the instrument cluster, but it didn't
solve the problem, so it sat for a couple years.  I got the car, put in a
new sender and tried again.  Still no needle movement.  Started tracking
things down and found that the cluster the main board had a couple horribly
soldered in batteries, which had corroded away a few circuit traces.  I
tried to make out the routing as best I could and wired past the damaged
area.  Didn't work.  I got another cluster, put it in, and still no
response.  Using the replacement as a guide, I figured out how to fix the
traces properly, but at this point haven't made the repair yet.  The
replacement was intact, but had a pair of dead batteries, and the ones in
the "original" cluster charged up on the trip when I first took the car
home.  Unfortunately, no more clusters are available from my usual sources,
so trying another one is not a likely option.
   Will the batteries being dead have an effect on the speedometer?  I
doubt it, but since the wiring diagram provided by BMW is so vague about
what's inside the cluster, I'm not positive.
   If the sender _and_ cluster is good, what else could be at fault here?
I actually had my oscilloscope in the car, ensuring that the cluster got a
good signal, but at the time I was using it, I didn't have one with good
wiring to confirm that I was looking at the right signal at the speedometer
module itself.  I'll be spending some more time here before too long working
on it, but if anyone has some guidance about where else I should look it
would be greatly appreciated.