Speedometer doesn't work

[Steve L]

Last year, my car was hit in the right rear quarter panel and my trailing arm was damaged. The impact pulled the right axle out of the differential. The car is back together, but my speedometer doesn't work. The sensor is in the right rear of the differential, so that seems to be the right place to begin diagnosis.

My Bentley manual has almost no information regarding schematics of the wiring system from the speedo sensor to the instrument cluster, but I think (maybe) the circuit runs thru the main computer. It has no information regarding the inside of the final drive.

So far, I have pulled the sensor and it appears to be fine. The wires to the sensor look intact. I think that I may have damage in my the trigger for the speedo sensor that must be inside the differential, but I would like to confirm that I have current & continuity to the sensor first.

Does anyone know where there is a schematic of the wiring from the speedo sensor to the speedometer? I assume that with the ignition on, I should get 12 volts thru the red wire to the sensor. There is a second grey or brown wire to the sensor. What is that, to ground?

Does anyone know how to reach and check the trigger to the speedo sensor in the differential? Can it be seen by pulling the axle flange off of the right side?

I welcome anyone's knowledge about this.

[HerbE30]

I'm not too sure, but I would pull the sensor out and wave a piece of metal between the two parts of the sensor. While keeping an eye on your multimeter.

Pull the sensor, and the two bolts that allow the sensor to mount. Shine a flashlight in there and you will see the part that passes between the sensor. However if parts were missing, I believe that would create an improper reading.

Long story short, try the SI board first, the issue is probably there.

[edit] just realized this is not a 30.....?

[Steve L]

SI Board???

Thank you for giving me a start. What is the SI board, where is it located and what do I do to "check it out"?

[HerbE30]

http://bavrest.com/technical-articles/bmw-e30-si-board

Basically the board that you cluster uses. This article has a lot more information than I can quick recall.

"I would first start by replacing fuse 10, 12, and 21 in the fuse box with 7.5A. Then check the condition of the conductors in the speed sensor connector at the rear differential. Also check that there is no wire breaks.

At the white harness connector behind the gauge there is a brown and brown/red wire. The brown wire should show continuity with ground at all times. The brown/red wire will show a pulsed signal that increases with wheel speed. You can measure it with a test light or meter set to continuity or ac volts.

If signal is present and power is present, either the solder joints on the main cluster board are bad or on the speedo circuit board. Lastly, if that all checks out, there could be other electrical component failures on the speedo circuit board. " This is a little trouble shooting I seen another member Greg/// post.

More info about the car could help? Like year and that looks like a 6 bolt diff?

[Steve L]

HerbE30:
You are awesome!! I have already pulled the sensor and checked continuity between the terminals with my mulit tester. It shows continuity between the terminals with increased ohm resistance when I pass metal between the magnetic prongs. So, I assume that the sensor is good.

Next, I will follow your directions and check the wiring. Perhaps I could spin the tires and perform the multi tester/ohm test on the sensor with it installed in the differential.

It's still below freezing where I live with a lot of snow on the ground that will keep me from moving this car from my storage building to my shop. But I also plan to remove the cover plate on the differential to inspect the trigger. A very likely possibility is that the trigger for the speedo sensor was damaged when the axle got yanked out from the accident. Pulling the cover will allow me to see more clearly.

[HerbE30]

Thank you! Happy to help Steve!

With the way the wires run under the body, theres a little 'zip clip' I would check around there, last time I was under it the insulation is starting to rub off.

I'm still gonna bet it's in the cluster/board. :p Good luck!

[Steve L]

I don't know how I would maintain '80's BMWs without a forum like this one. I've spent 40 years working on '60's era pony cars, but the electronics on these newer cars is so much more complex. Thank you Herb30 and other forum wizards.

I still think that because my speedo was working prior to the accident that yanked the right axle out of the differential, that the problem has to be in the area around where the damage occurred. It will warm up within the next few weeks and I plan to put the battery back in my E30 and perform more diagnosis. I will get back to the forum when I have more information. But now I have a plan to proceed.