speedometer not working

[nrasac]

I have a 81 c10 with a 72 350 engine. Has a th350 automatics transmission w-cruise control. Speedometer has not been for about 6 months. It stays steady below zero and once in a while it bounces when road is bumps. It has two speedometer cables.

Best way to diagnose. I read other posts on the subject but am still confused. When using a drill to check if speedometer turns, do I use the drill on the inlet that goes directly into transmission. I still have speedometer cable attached to truck so I do not how difficult it is to remove but looks easy.


Thanks , on my last matter I finally replaced temp gauge with mechanical and truck is running fine, the old gauge was off by 35 degrees. Thanks for the help.