How do I find a diagram for troubleshooting speedometer that is not working?

Your vehicle may be equipped with a one piece or 2 piece speedometer cable. On the two piece cable, the solenoid switch mounted in between the two cables may fail internally, thus the drive cable will turn, but the speedometer side will not. You can check this by removing the solenoid and turning the cable nipple with a small screwdriver. The other component to consider is the speedometer pinion gear in the transmission tailshaft. You can remove adapter with one 7/16" wrench. Place a pan underneath to catch any fluid that may drip out.