Revs limited to 3,300, speedometer stopped working

[tkeenan]

Yesterday my speedometer finally stopped working. At the very same time, my revs were limited to 3,300 rpms (without some major lurching). It was a slow drive home.

Tach works fine, gas gauge flits back and forth a bit, speedometer over the past year would stop working for a second or two, but come right back.
87 with a '95 US S50.

I would assume this is some type of limp mode derived from the ECU receiving no speed signal.

[tkeenan]

I'll go through these tests, but still curious about the speedometer failure being simultaneous with the limited revs.

Those gauges both rely on a speed reading from the speedo sensor in the diff.

Be methodical in analysing the problem. Do you have a multimeter to measure resistance and voltage? If not, buy one. It makes tracing electrical problems MUCH simpler.

Download the wiring diagrams for your car from this link: http://shark.armchair.mb.ca/~dave/BMW/e30/

Study page: "6210-0 INSTRUMENT CLUSTER".

First test whether the speedo sensor is working. Jack up the rear wheels, unplug the sensor, measure resistance across the terminals of the sensor while slowly turning the rear wheels. The sensor is basically just a switch that opens and closes. You should see resistance alternate between zero and infinite as the diff is turning. If that works, plug the wiring connectors back on to the sensor and move up to the instrument cluster.

Look at your wiring diagram. The two wires from the diff-mounted speedo sensor come directly via a Brown/Red (BR/RD) wire to Pin-12 and a Brown (BR) wire to Pin-8, both on the C2 connector. That's the WHITE 26-pin connector on the back of your instrument cluster.

Carefully unclip and unplug that C2 connector, then measure the resistance across those two pins while again slowly turning the rear wheels. Unless your multimeter probes are thin enough, you may need to stick a paper clip into each pin connector. You should see the same thing, resistance switching from zero to infinite as the switch in the speedo sensor opens and closes. If not, then you know you have an issue with the wires running from the sensor to here.

If that works properly, then check for power and ground. The wiring diagram shows that power comes to the speedometer whenever the ignition switch is in the Accessory, Run, or Start position, via Fuse-12, flowing on a Violet (VI) coloured wire to Pin-13 of that same C2 connector. Use your mulitimeter on the DC-voltage setting to confirm there is 12v showing on that pin when the ignition switch is on. If not, then check Fuse-12.

Now check the resistance between any good ground and the ground-pin, which is the Pin-16 Brown (BR) wire on the C2 connector. You should have zero resistance.

If all those simple to check things prove okay on the C2 connector wiring - good 12v, good ground, good signal from the speedo sensor - and your speedometer is still not working, then you know for sure the issue is internal to the instrument cluster.

If that is the case, then jump directly to this thread and follow the links from there. http://www.r3vlimited.com/board/showthread.php?t=220619

Good luck, and let us know what you find.

[Ferdinand]

I'll go through these tests, but still curious about the speedometer failure being simultaneous with the limited revs.

It is curious. It makes sense it needs to know engine RPM, but why does the ECU care about vehicle speed? The Service Interval Indicator keeps track of mileage from the speedo output, but why does the ECU care?

Anyhow, there is a speed output signal from the speedometer to the ECU from Pin-10 of Connector-2 (C2) on the back of the instrument cluster. C2 is the White 26-pin connector, C1 is Blue, C3 is Yellow.

The wiring diagram shows the speedo sensor in the diff is just a switch that opens/closes counting pulses. That wiring is easy to check, as described in the post you quoted above. If that checks out, then you'll need to pull the instrument cluster and examine the solder joints on the pin connectors and follow all the traces on the printed circuit board leading to/from the speedometer connections, as described in this post: http://www.r3vlimited.com/board/show...5&postcount=26

[tkeenan]

Finally got around to checking the wiring from diff speed sensor all the way to speedometer, etc. - everything checked out.
Determining it is a cluster issue, I decided not to mess with this and send the cluster to Bavarian Restoration.