It is one of three things....
1. output speed sensor-$14.00
2. input speed sensor-$17.00
3.transmission control module- $175.00
I have one too, and it was doing the same thing. It took a while for me to figure it out.
If speedometer itself is working then gears inside the assembly are damaged.
The gears in the speedometer are stripped. If the odometer is working then you have input.
You drive slow and change gears too soon.
Replace the speedo head assembly. It is a sealed unit, with no servicable parts. If the speedometer is working, then the cable and the speedo drive gears are working. The failure must be inside of the speedo head assembly itself.
http:/www.tciauto.comTCI has drive gears, driven gears & calculators.
If you open the odometer, there are two small plastic gears. One is connected to the speedometer, the other to the odometer. The speedometer gear drives the odometer gear. In all likelihood, one of these gears is missing a few teeth. I have the same problem with my Ramcharger, same year.
If the odometer is not working but the speedometer is still indicating speed, the most likely fault is a set of small plastic gears inside the speedometer instrument itself. I have seen these gears available for sale at various BMW parts specialists. Removing the dash cluster for access to the speedometer is not difficult in a 1987 E30 car. With the cluster on the bench, you can repair the speedometer gears or replace the whole thing. Used speedometers are plentiful at on-line auction sites such as eBay.
More than likely there is a speedometer gear on your transmission that has a cable leading to the speedometer. These thiongs go out as they are usually driven from the gears in the tranny via plastic gears in the module. It is not hard to remove-one or two small bolts and it should slide right out. Their also very cheap to buy and replace. Good Luck!
Call the dealer, and ask for the different kinds of speedometer gears, you'll be lucky if you can still get one. They Usually go by color and teeth count. If I remember right, they are the typical cause.
Forward and reverse. It's an automatic
If the speedometer doesn't work either, then the cable broke or came loose. Otherwise, the gears in the odometer broke.
when you get bigger tires, or different rear end gears.