It may not even be an odometer gear. Odometers used in E30s have an ac powered motor on them that turns the numbers, and those motors CAN burn up. I wouldnt go as far as to replace such a motor, just because when i took mine apart to modify the backlighting with red led strips the motors are soldered onto the board for the speedo. They look like a royal PITA to replace. Anyway, the red LED backlighting on my cluster looks AMAZING. So if your feeling creative when you replace your speedo, put in red LED strips. Theyre cheap, and the mod is quite simple. They were like 5 or 6 bucks shipped from ebay. And all you have to do is hook up the cluster since it isnt labeled, test the lighting leads with a DVOM to find out which side is positive, and then solder the LED strips onto the leads for the twist in lights. I also put red christmas light LEDs into the sockets for the lightbulbs with resisters so the temp gage and the fuel gage light up a little better.
EDIT: i didnt realize yours is an e21. It may not be electronic on these models. Sorry for any confusion, but the LED mod will still work on them.
Last edited by m20enginebuilder; 04-09-2012 at 08:31 PM.