E30 - Speedometer has failed

'91 318is, 120k miles

Speedometer has been quitting intermitently, but has finally quit
altogether, with the oddometer failing at the same time (I presume this is
obvious.. but ...). Checked the connection at the erar differential and all
seems clean and in order. Do I need to disassemble my dash, or is there
something else to check first? TIA

Matt

> '91 318is, 120k miles

Nice :-)

> Speedometer has been quitting intermitently, but has finally quit
> altogether, with the oddometer failing at the same time (I presume this is
> obvious.. but ...). Checked the connection at the erar differential and all
> seems clean and in order. Do I need to disassemble my dash, or is there
> something else to check first? TIA

It's probably the diff sensor. maybe the wiring is corroded. Try
remaking the connections.

--
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M Warren wrote:
> '91 318is, 120k miles
>
> Speedometer has been quitting intermitently, but has finally quit
> altogether, with the oddometer failing at the same time (I presume this is
> obvious.. but ...).

No, it is *not* obvious ... but it is a very valuable diagnostic. If
*only* your speedometer had failed, it would indicate the plastic gears
in the dash were broken. However, if both have failed simultaneously,
it is almost undoubtedly the sensor, or the wiring to it, in the rear,
since both instruments (and your OBC, 'economy meter', and cruise
control, if so equipped) share that signal.

> Checked the connection at the erar differential and all
> seems clean and in order. Do I need to disassemble my dash, or is there
> something else to check first? TIA

Back under with you! If the wiring is all *good* (do a continuity
check - especially if they have ever been 'baked' by an exhaust leak
back there), then replace your sensor unit. That will probably be it.
--
C.R. Krieger
(Been there; broke that)

I would take a look at the speed sensor in the diff before I took the
dashboard apart.

The speed sensor gear is typically mounted on an eccentric to accomodate
installing different gear sizes to account for changes in the ring and
pinion gears. If your car had 3.07:1 gears, and you wanted to change to
4.10:1 gears, then you would need a new speed sensor gear so your
speedometer and odometer remained in proper calibration. Since this would
require speed sensor gears with different tooth counts, then the speed
sensor gears would necessarily be different sizes, and would need to be
mounted differently in order to maintian the proper gear mesh.

The eccentric is generally held in with a single screw that clamps the edge
and prevents it from moving. If the screw is a bit lose, then the gears on
the speed sensor might fail to mesh properly, and the speedo would stop
working, or might not work smoothly. It's possible that the speedometer
would work fine until the teeth (plastic -- nylon really) wear off, then
suffer a catastrophic failure all at once.

In any case, I'd pull the speed sensor from the diff and give it an
inspection. You may find worn teeth, or you might find that the device
grinds and growls while you turn it with your fingers. Either way, you
should find the problem. When you re-install the sensor, you will want to
spin it in the mounting hole to mesh the gears tightly. Since the teeth are
plastic, you'll want to get a good mesh without mashing the gears together
too tightly.





"M Warren" > wrote in message
...
> '91 318is, 120k miles
>
> Speedometer has been quitting intermitently, but has finally quit
> altogether, with the oddometer failing at the same time (I presume this is
> obvious.. but ...). Checked the connection at the erar differential and
> all
> seems clean and in order. Do I need to disassemble my dash, or is there
> something else to check first? TIA
>
> Matt
>
>

While the wires can certainly be a problem, I'd suspect the simple mechanics
of the sensor's gears meshing with those inside the diff. My guess is the
diff is fine, and the speed sensor itself is toast. It could be the wires,
but I think other wires in the area would be similarly aflicted, and if the
wires LOOK okay, they probably are.

My money is on the sensor itself, it's either failed or the mount screw is
loose and it moved.




"C.R. Krieger" > wrote in message
ups.com...
>
> M Warren wrote:
>> '91 318is, 120k miles
>>
>> Speedometer has been quitting intermitently, but has finally quit
>> altogether, with the oddometer failing at the same time (I presume this
>> is
>> obvious.. but ...).
>
> No, it is *not* obvious ... but it is a very valuable diagnostic. If
> *only* your speedometer had failed, it would indicate the plastic gears
> in the dash were broken. However, if both have failed simultaneously,
> it is almost undoubtedly the sensor, or the wiring to it, in the rear,
> since both instruments (and your OBC, 'economy meter', and cruise
> control, if so equipped) share that signal.
>
>> Checked the connection at the erar differential and all
>> seems clean and in order. Do I need to disassemble my dash, or is there
>> something else to check first? TIA
>
> Back under with you! If the wiring is all *good* (do a continuity
> check - especially if they have ever been 'baked' by an exhaust leak
> back there), then replace your sensor unit. That will probably be it.
> --
> C.R. Krieger
> (Been there; broke that)
>

I had a similar situation develop on my '84 318i and was able to keep it
going for another year just by giving the instrument cluster a sharp rap
with my knuckle whenever the speedo quit. This will also let you know if
the problem is in the dash or elsewhere. All of the instruments in the
cluster are connected to a backplane by plug on connectors which can become
intermittent due to oxidation. You can obtain a more permanent fix by
disassembling the cluster and cleaning the connectors. I have also heard
people say that they repaired theirs by resoldering connections on the
backplane. Removing the cluster is not really a big deal. My bentley
manual said that the steering wheel had to be removed in order to remove the
cluster but I found that this was not true. Remove two screws at the top of
the cluster that are common to the glare shield. Remove two thumbwheel type
nuts from the backside of the bottom of the cluster that are accessable by
removing the trim panel that is under the dash. Rotate the cluster aft and
down to gain access to two wiring harness connectors and you're done. I had
mine out several times as different gauges failed and could get it out in 15
minutes after some practice. Another thing you should consider is that's
it's probably time to replace the batteries on your service interval circuit
board as long as you have the cluster out. You can get the solder tab
batteries at any radio shack. Instructions for doing this are available on
the E30 website. Don't forget to disconnect the battery before you start.


"M Warren" > wrote in message
...
> '91 318is, 120k miles
>
> Speedometer has been quitting intermitently, but has finally quit
> altogether, with the oddometer failing at the same time (I presume this is
> obvious.. but ...). Checked the connection at the erar differential and
> all
> seems clean and in order. Do I need to disassemble my dash, or is there
> something else to check first? TIA
>
> Matt
>
>

"Jack" > wrote:

>I had a similar situation develop on my '84 318i and was able to keep it
>going for another year just by giving the instrument cluster a sharp rap
>with my knuckle whenever the speedo quit. This will also let you know if
>the problem is in the dash or elsewhere. All of the instruments in the
- snip


I've got one of the *tap to use* speedo's..... had the instuments out
a few times, cleaned all the contacts etc.. still comes back after a
few months.
I can live with it...

keith

J Strickland wrote:
> While the wires can certainly be a problem, I'd suspect the simple mechan=
ics
> of the sensor's gears meshing with those inside the diff. My guess is the
> diff is fine, and the speed sensor itself is toast. It could be the wires,
> but I think other wires in the area would be similarly aflicted, and if t=
he
> wires LOOK okay, they probably are.

OK; forgive me for being an E28 Guy=AE, but does the E30 *not* share the
same type of speed sensor as the E28? If so, there *are* no gears, as
such, back there; it's a magnetic sensor reading a toothed wheel and
there's no mechanical contact at all. They just fail. You spend the
$10 or so for a new one and stick it in.

That said, the excellent BMW tech who educated me about these told me
it's quite common for the wires to get 'baked' by a now-forgotten
exhaust leak. They are now more likely to break inside, so a
continuity check, at least in any area where it's close to the exhaust
system, is usually a good idea.
--
C=2ER. Krieger

"C.R. Krieger" > wrote in message
oups.com...

J Strickland wrote:
> While the wires can certainly be a problem, I'd suspect the simple
> mechanics
> of the sensor's gears meshing with those inside the diff. My guess is the
> diff is fine, and the speed sensor itself is toast. It could be the wires,
> but I think other wires in the area would be similarly aflicted, and if
> the
> wires LOOK okay, they probably are.

OK; forgive me for being an E28 Guy®, but does the E30 *not* share the
same type of speed sensor as the E28? If so, there *are* no gears, as
such, back there; it's a magnetic sensor reading a toothed wheel and
there's no mechanical contact at all. They just fail. You spend the
$10 or so for a new one and stick it in.

That said, the excellent BMW tech who educated me about these told me
it's quite common for the wires to get 'baked' by a now-forgotten
exhaust leak. They are now more likely to break inside, so a
continuity check, at least in any area where it's close to the exhaust
system, is usually a good idea.


Well, okay then. I would suspect a mechanical failure before a wiring
problem, but I also assume that while one is unplugging the wires to look at
the mechanicals, one would notice the baked wires and change course in mid
stream. I haven't had the speed sensor out of my BMW, but I have had several
speed sensors out, and operating by mechanical means -- gears -- or by
magnetic pick up presents pretty much the same repair experience, except
that gears are admittedly easier to look at and identify a problem.

Hello,

Whilst these other gents sort out their differences let us take a
minute to think this through.

Have any other gauges acted funny.? What about your service lights, are
they on constantly? If so then I would say chasing the sensor might not
be what you need to do. I think you might have the dreaded SI board
failure caused by leaking batteries.

The batteries leak acid down onto the board and eats the electrical
traces off the board. I have repaired many and re-soldered new
batteries with good results. Others simply buy a new board.

Here is a link for you to ponder...
http://e30m3performance.com/maintena...d/SI_Board.htm.

Best of Luck!
DieInterim