84 733i speedometer woes

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.

carl mciver wrote:
>
> Will the batteries being dead have an effect on the speedometer?

I think they can. Bad SI (Service Indicator) batteries screw up a
bunch of stuff in there. As well, I understand that driving won't
recharge them by itself. You need to charge them before use. This is
why a lot of people swapping in replacements mount them remotely and
accessibly.

> If the sender _and_ cluster is good, what else could be at fault here?

Well, I'm not sure you've determined the sender is good. Then, there's
all that wiring in there. Check for continuity from the sender
connector to the other end of the wires at the speedo head. If there's
a break, look under the car for brittle or burnt (exhaust leaks will
cook 'em) places where the wire might be broken. If you have
continuity, try replacing the sender in the diff. It's easy and
relatively cheap. If that fails, you're down to the cluster ...
--
C.R. Krieger

"E28 Guy©" > wrote in message
oups.com...
| carl mciver wrote:
| >
| > Will the batteries being dead have an effect on the speedometer?
|
| I think they can. Bad SI (Service Indicator) batteries screw up a
| bunch of stuff in there. As well, I understand that driving won't
| recharge them by itself. You need to charge them before use. This is
| why a lot of people swapping in replacements mount them remotely and
| accessibly.
|
| > If the sender _and_ cluster is good, what else could be at fault
here?
|
| Well, I'm not sure you've determined the sender is good. Then, there's
| all that wiring in there. Check for continuity from the sender
| connector to the other end of the wires at the speedo head. If there's
| a break, look under the car for brittle or burnt (exhaust leaks will
| cook 'em) places where the wire might be broken. If you have
| continuity, try replacing the sender in the diff. It's easy and
| relatively cheap. If that fails, you're down to the cluster ...
| --
| C.R. Krieger


The sender was the first thing I changed out, and was disappointed to
find it no better. The previous owner had replaced the cluster, and when I
brought it home from his place the batteries charged up on the trip, which
was nice. The ones in the second cluster aren't dead as a doornail, so
maybe they will charge up also. Don't know if I'll have the opportunity to
drive it that long, though.
The wiring to the sender had all been checked to be good before I
cracked open the cluster and found the corroded main board, so I tried to
repair it as best I could figure out. Repairing circuit boards is a
previous career, but I usually had more information to go on. Using the
second board as a pattern I'm going to redo my patch job, but if I have a
couple other possible places to look I'd feel a lot better, since I'm not
100% confident that this will solve the problem. There's such a lack of
decent information in the wiring diagrams that the wiring shown for inside
the cluster that I'm starting to doubt it's accuracy.