The speedometer head is the last thing to suspect. The exception would be if the odometer still works.
First suspect the cable. They can snap or the square end at the transmission can round off inside the gear. You may be able to reach behind the instrument cluster to disconnect the cable, then watch or feel if it turns when you move the truck. It's often easier to disconnect the cable at the transmission, then spin it with your fingers while a helper watches the speedometer. That's also the time to inspect the end to see if it's rounded off.
The gear inside the transmission could be stripped too but that's not as common as a snapped cable. If the odometer does work, that would prove the cable and gears are okay. Usually when the speedometer head fails it gives a lot of warning first. The needle will typically bounce around a lot and the assembly will make noise. On rare occasions the head can lock up and cause the cable to snap, so if you find the cable is badly twisted, check the replacement for free movement with your fingers. A locked-up factory cruise control servo can do that too. They were run by the speedometer cable.
Monday, November 26th, 2018 AT 9:37 AM