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In the past, the speedometer on my stock '75 would work intermittently. Even when it wasn't working, it would read something, never zero. I decided to get under the car a few weeks ago and "fix it".
I unscrewed the cable where it goes into the transmission and re-inserted it. Now it doesn't work at all. Need to find out what I did wrong. Is it not tight enough? Not able to use a wrench. Have to use plyers to tighten it back. There's a U-shaped piece right where the cable screws in that might be out of place since I messed with it. Is there a certain position this needs to be in? Why is it there? What does it do?
I had the exact same problem on my 71 AT - was afraid I needed to rebuild the speedometer itself because of strange noise coming from there. The cable looked fine.
However, I chose to install a new cable as a first step and -- bingo. Now I got a smooth going speedometer.
So, my advice is for you to take the simple step of installing a new cable and see if that helps you.