1987 Ford L8000 Road ranger 10 speed. 7.8L

Speedometer is not working, need to know how to test it and find out why its not working.

 

it is most likely electric driven. some one may have unplugged the sensor at the back of the trans to kill the odometer miles from going higher while driving it.
If you need to reset for different gear ratios and tire height there is a link in the tech folder thread

in post #2
https://www.ford-trucks.com/forums/8...ch-folder.html

Check your grounds and all other wires.
A factory service manual is very handy for this type of problem.
Sometimes you can find them on Ebay cheap.

 

I am curious about this too. I just found out the speedo on our 95 L9000 is acting funny. You can be sitting still and it will read 5, 10, 15 or 20 mph all while rolling over the odo. Once you move, it reads normal. Like broncoman said, we need to track down a service manual.