Electronic Speedometer has stopped working. What to investigate? 72k on truck. Needle will rise ok but not come back down with decreasing speed. Seems to stop at 35mph when you are sittimg still. What is the "green control box" mounted on a cover below the steering column?

Sounds like the speedometer is faulty. If the speed sensor were the problem, the odometer would not work correctly, the transmission would not shift correctly, and the check engine light would be on.

Phylspilot, my 86 Spider never displayed less than 10 mph. If you look closely, you should see a tiny pin that the needle touches at about 10 mph. That prevents it from ever falling to zero. The needle will not move up until the speed exceeds 10. From my experience, that is normal. In your case, not displaying >20 is NOT normal.

The problem could be a few different things. Since the odometer appears to be working ok (I assume you checked that it racks up the miles at a rate commensurate with your approx speed), the speed needle may be sticking. I had the opposite problem with mine - speed needle okay, and odometer messed up.

I recommend pulling the speedometer housing out of the dash and checking to see if anything is causing the needle to bind. If it isn't, then check the fine spring where the needle is attached to ensure the spring is okay. You should be able to move the needle with your finger smoothly and have it return gently to the start position (in your case 12 mph).

This is a tricky operation. If you remove the needle from the stem, carefully place it back on the stem. Do this without moving the stem. Just reinsert the speed needle on the stem exactly at the small pin that holds the pin at about 10 mph.

Check the entire speedometer housing for cracks. Any crack could be enough to prevent proper operation. (That was the source of my odometer issue.)

Have fun putting it all back together.

if the speedo is not sending a speed the trans will go into back up mode and the trans will not shift normally check the speed sensor

The speedometer is electronic and controlled by a micro processor, the needle is operated by a special DC electric motor called a "stepper" motor, this little motor can cause this problem as well as the Vehicle speed sensor on the transmission (see photo below), either part can cause the problem, the cruise not working would lead me to believe the speed sensor is defective, but you should have a Check Engine light on and a fault code for the sensor, do you have either? Autozone and others do the fault code test for free, ask for an OBD2 fault test. Here is the picture of the sensor I mentioned.

Bad speed sensor on the trans if its an elctronic sensor if its mechanical then it would be a bad speedometer gear or broken cable. Sounds like its electronic so its a bad speed sensor parts a lil spendy but mostly its a pain in the *** to get to

Sounds like the cable is dry and needs lubricating, sometimes you can take off the gearbox end and pour in some oil.

If this is a mechanical speedometer, the return spring that pulls the needle to zero may be broken--fix is to replace the speedometer as they are sealed like a can. Check the wrecking yards for a used replacement. Hope this helps!

No it it electrical,you need to have someone replace the speed sensor on the transmission.

the plastic gear on speed sensor is broken , replace , this is on transm. rear case

the speedo is electronic, not cable, this problem is caused by a sticking stepper motor that moves the needle, a stepper motor is a miniature electric motor that can move and hold one position, this was a common problem at the dealer I worked at, u will need to pull the speedo and send it out for repair, I use the following link for this service.
http://www.dnaspeedometers.com/