Bouncing Speedo

This is normally caused by the cable being sticky.

There is a cog in the gearbox which turns the cable which goes to the back of the speedo. This is a cable in a braided tube which in theory is free to rotate but in practice is filled with tar after a certain amount of time.

What you then get is the cable winding up, one end stuck, until the tension is high, the cable then twists abruptly releasing the tension, but then sticks again. Each time the cable frees up, it bounces the needle up, then it sticks, and the needle drops back, averaging about the middle where the speed is.

There's two solutions if this is the problem, oil the cable by dripping medium light oil down the cable and driving about to work it into the whole length of the cable (the gearbox will turn the cable when going along and the oil will then mix with the tar), or just putting a new cable in (which will mean another thirty years before you need to put another one in, but which involves getting at the end which connects into the gearbox, easy enough if you have the right sort of fingers.)

HTH

Just a cautionary note: The oil will screw up the cable and flood the back of the speedo, which is why grease was used originally. It depends on the helix of the flexible part of the cable, but it does happen, and nothing is worse (after a bouncing needle) to have a dead speedo.

Best to let the cable run with the oil in for a while and then let it drain out at the gearbox end if you can get the retainer off. A light grade detergent oil os better than a heavier oil BTW.

This warning used to be a regular in the old weekly car magazines in the 1950's and 1960's (shows how old I am getting!)

Peter

Check the cable. It's a standard fault when the cable's about to go, it starts fraying and the loose ends make it jerk.

In message , Michael Howard writes