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speedo problems

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I am on my second speedo in my 91 5.0 LX. The one I replaced it with has started doing the same thing the first one was doing. The needle jumps around and quivers. I drove the speedo (both of them) outside the car with a drill and they did the same thing. The speedo cable and the gears are new, there isn't anything obvious, looking at the speedos.

ol dirty doug:
i had that problem in my 68 mustang, and it turned out to be a damaged cable, it was slightly ground thru from speedbumps, so i put a new one in and in doing so i kinked it. it ended up with a bend inside the internal cable so when it spun it scraped the outter shroud. replaced it again and it was good to go.

I appreciate the response, I will check the cable just to be sure but...... as I said in the initial thread the speedos do the same thing out of the car.

You have a plastic gear (cog) inside the speedo that gets gnarled and will act like that.  Get a new speedo and use some graphite inside your speedo cable. 

I appreciate your attempts to help,but there are no gears inside this speedo.... the cable plugs into the back of the speedo and turns a shaft that turns 1 to 1 with a magnet that turns inside a bell that is attached to the needle... as the magnet spins inside the bell it causes the bell to turn causing the needle to move, the faster the magnet spins the farther the bell and needle turn. I have worked on older speedos that have gears inside them but this is not one of them.


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