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MG MGB Technical - tach needle floats?

The other post on tachs brought me around to wanting to sort mine out. My tach works, but will "bounce or fluctuate" as I hit bumps in the road. Has anyone else ever had that or found a fix? Over all it is accurate, but I would like to see it hold steady. The car doesnt misfire or skip, so I think it can be isolated to the tach, or wiring itself.
Bryan

What year?

Mechanical vibration shouldn't really affect the tach needle as it is damped and balanced.

All tachs will suffer from a momentarily *dropping* needle if the 12v or ground connections are being shaken loose over a bump. Pre-72 cars would suffer an ignition hiccup as well if either of the sense wires were being affected as the coil feed runs through the tach pickup, but a 73 or later car with a loose sense wire will only affect the tach not the engine.
Paul Hunt

I dont know if this would be considered a hiccup or not. There does not seem to be any power loss when this occurs. It basically is constantly "floating". For ex. It is at a steady 2800 rpm, hit a bump it will go to 2700 then back up. More or less with every bump. I will check all connections to make sure I dont have a short or bad connection. It is in a 1970. How are they damped and balance, can that wear out?
Bryan

You could put a small 12v bulb in parallel with the tach power and earth and tape it somewhere visible. If that blinks off wnen the needle drops then its electrical connections , if not then you will have to get the tach rebuilt. Of course fitting this investigation may fix the problem.
Stan Best

The balance weight could possibly fall off but that would give a very non-linear response to engine speed. The damping is usually done by the former the turns are wound on to, so that is unlikely to have failed. Any dry joint or loose connection inside the tach could also cause needle jumping over a bump.
Paul Hunt

Thanks for all the suggestions. I will start with the light to see if I do have a power shortage and at the same time look for loose or dry connections.
Bryan

This thread was discussed between 26/06/2008 and 30/06/2008

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