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first disconnect the cables at each end.

place a small screwdriver in the end of the drive and either kick the bike over or rotate the wheel. See if the screwdriver rotates. You might need to do it several times to make sure that the driver is in fact in the correct position.

If it does rotate, then the drive is ok.

Refit to the drive and stick a small flag of tape on the dial drive end of the cable so that when you do the same test, it's easy to see the square drive rotate.

Either it will, or won't but either way you'll find the problem
 

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You can test the gauges by putting a small screwdrive in the square drive socket and spin it with your fingers. If the needle moves, you can go to an electric drill to see if it moves uniformly.

If your cables are seized, rusted, packed with crud or broken and jammed up, they could have destroyed the gear in the speedo drive and the little sheetmetal drive in the cam for the tach.

There are multiple failure points on both the tach and speedo systems. So asking what's wrong will only result in a list of potential causing. And it could be more than one at play. For example, your cable housing could be crushed and this caused the drive core to chafe. It started to unravel and catch on the housing. When it would catch it would put stress on the drive gear teeth. Eventually the cable snags and seizes and the overstressed teeth fail. So you now need a new cable/core assembly and a new drive gear assembly. That's why my first order of business for a newly purchased bike is inspecting the speedo and tach cables, cleaning and lubing them and replacing if necessary before they do more damage to related parts.

To beat this a bit more, I had a cable that would snag, wind up like a spring and then release. Speedo would go from zero to max in an instant. Although that speedo still works, I'm sure it did not do it any good. I did stop along the roadside and disconnected the cable from the speedo. I should have pulled the core out too, but it was probably too late by then anyway.

regards,
Rob
 
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