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erratic speedometer when stepping on the brake

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I just bought a '74 automatic and driving it home from Portland, OR to Placerville, CA. While driving the speedometer operates normally and accurately registers the speed; however, when you step on the brake pedal the needle will immediately drop to zero depending on pedal pressure. I am pretty sure I am not going from 65mph to zero in an instant. With less pedal pressure the needle will bob around at a much lower speeds. I am not entirely sure what's going on here. My first thought is that some part of the brake system is pinching the speedometer cable? It's not critical but it's a little disconcerting.


Any insight is appreciated. 


Thanks, Cort

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Hmmm, if it was me, I'd inspect the cable first.  I'd reach under the dash, and twist the knurled cable connecter.  Then I'd put the car up on stands and follow the cable from the engine bay to the gearbox input.  I'd test the input connection to make sure that it's firmly in place.  If all those connections feel right and there is nothing pinching the cable, I'd suspect the guts of the speedo have come loose.  With the inertia of braking it could effect the delicate balance of the instrument inside the gauge.  That's my thoughts.



73 Inka Tii #2762958

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Thanks so much for the insight and I believe you are right that the inertia of just slowing the spinning movement of the speedo cable is causing the speedo to erratically read. I don't believe that my initial thought is correct as the cable is in constant mechanical movement in relation to the drive gear of the transmission. It would be impossible to stop the movement of the cable without severing it which hasn't happened. I will replace the speedometer and see what happens.


Thanks for insight...Cort

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