Firstly , does your C3 have cruise control ? If so there are two cables , a longer one from the auto trans to the cruise module and a shorter one from the module through the bulk head to the speedometer .I have a 74 C3 three speed auto, I need to replace the speedometer cable but unsure which one I should order, any help greatly appreciated.
Hmmm, there are cruise controls on a stalk, but I hadn't assumed that the cruise system was fitted, and I failed to ask on purches of the car( few weeks ago). The cable is not one piece though, there is a connector a short distance from the trans, which I split to have a look, I can see where the cable end and square keyway is worn.Firstly , does your C3 have cruise control ? If so there are two cables , a longer one from the auto trans to the cruise module and a shorter one from the module through the bulk head to the speedometer .
I ordered my cable from Corvette Kingdom . As Ross said , it could be a tightly routed or non lubricated cable . If the end of the cable is worn it may well be slipping in the square hole it locates in . It could also be the damper in the speedo failed which is designed to stop the needle bouncing . Eliminate cable issues first as that is a cheaper fix as speedometers are expensive and can be hard to come by .I'm away at the moment, but will check the set up when I get back. My speedo flicks up and down, almost pulsing. So there is no possibility of working out what speed you doing. I just split the cable at the knurled connection to see what state the cable was in. What cable did you order for your C3, the single non cruise option??