The speedo is driven magnetically.
The speedo does in fact work with eddy currents from a spinning metal disk that deflects the needle without actually touching it. The needle is attached to a "cup", and the metal disk spins inside of this "cup" and deflects the needle more or less with speed of rotation.
Eventually the little bronze bushing that connects the speedo cable to that "cup" works itself out slightly, and causes erratic speed readings and jumpy needle deflection.
I would bet that if you disassemble the speedo, clean, gently re-seat everything, and reassemble, you will have a working speedo. The odometer cable is the same size as the speedo cable. You will need to pop it out of the back of the speedo when you remove the speedo. Just by twisting the cable in the speedo with your fingers, you can get a deflection of 20-30 mph, so I would guess your bushing is in need of re-seating.
Lots of good pictures here to help visualize this, and a great video by an 02 guru on how to work on the speedo.
Hi All, My speedo stopped working during a highway-speed jaunt a few weeks ago (stuck @ 80 KMH). Thanks to mthomson's write-up on dash removal, I summoned up the courage to take this on. Michael confirmed that he actually got his speedo out without pulling the whole dash, so I gave it a go...
www.e9coupe.com
Firstly, you will need to loosen the nuts holding the instrument cluster housing so that you can lift the housing up from the dash a fw cm, else you'll never get the gauges out.
When removing the gauges, remove only the easiest to reach mounting clamps. Just loosen the upper clamps so that the gauges can slide out. You can later reinstall the lower clamps easily with the gauges positioned, but pushed back just prior to re-installation.
Easiest to start by removing the fuel gauge cluster, as this gives you plenty of room to work on everything else. You could bag it and tie it out of the way without disconnecting everything.
Pay attention to how your odometer cable is dressed through all of the other cables from the access door to the speedo. It is crowded in there, and can go several ways, so make note of the best way.
Yes, remove the 2 knurled nuts, flip the speedo 180 degrees, and use all your zen skills to find a path down and out.
As Stevehose says, when removing the speedo, note that there is a vertical sheet metal frame located conveniently behind the speedo to block its exit. The speedo has to exit to the left as you're facing it. I recommend to push in the gauge and rotate the face/body of the speedo to the right and work the tip of the speedo cable mount past that sheet metal to the left, then rotate the face back forward. You can then summon all of your zen skills to gently rock and rotate it to find a way for the mounting flanges to get through all of the surrounding cables and metal.
I found that unseating the tacho [easy(ish) access from below] and pushing it in a bit gave me easier access to manipulate the speedo.
While you're in there, I would strongly urge you to replace the odometer drive gear, as it is surely worn. Mine worked fine before I disassembled my speedo, but it did not work when I reassembled, as I apparently disturbed it just enough for it to start missing on the worn spot. Had I known, I would have happily paid the $25 bucks to
not do this exercise twice!
Cut the old gear off with a razor knife, and press the new one on.
You don't have to remove the oil from the transmission to do this, there will be a tiny amount of oil on top of the o-ring but that's it.
