speedometer repair

My speedometer needle does not go above 12 mph. Odometer works. Car is '39 Mercury. Any suggestions on how to fix it? I like to try to fix this stuff by myself.

Speedometer operation is pretty simple. At least in theory. The needle has a spring to return it to zero. And it has an aluminum disk or cup and covered by that cup is a bar magnet connected to the the speedometer cable.

As the magnet rotates it sets up eddy currents in the aluminum which have the effect of trying to rotate the aluminum cup with a force proportional to the rate the magnet rotates. That force moves the needle until the force from the return spring is enough to balance it out.

Things that can go wrong: Dirt or damage to the needle pivot points, magnet weakens with age, spring ages or breaks.

However as simple as they are they are also very delicate and my experience has been that when I try to fix one it just means a higher bill from the professional to repair the damage I've done in addition to the original fault. Unless you are used to working on delicate instruments, like watches or mechanical clocks, and have the appropriate tools I'd leave it to a pro.

Thanks for the explanation. I had an idea and have played with several... will soon be replacing the head in my Corvair speedometer with a donor... but never understood how the magnet turned the aluminum.

Turns out cleaning it up was the easy part.

I'm sure that PLY 33's answer is correct. I'm trying to learn stuff, but I really don't want to break the speedo that I have so I'll probably surrender it to a pro. (Unless someone else tells me some simple thing to do ( ????? )

It turned out that mine had two gaskets between the head and the casing, causing the bottom to rub on the front.

While I had it apart I noticed there is a little lever you can move to put more or less tension on the spring that holds against the magnet. It would be nice if there was an access hole somewhere to nudge it, but at least on the Corvair there isn't, and pulling it isn't that easy. I now have it working, but it registers about 100 kph instead of 94 kph. About as accurate as my new Grand Cherokee, but if I ever pull it again I'll take a smudge of pressure off that spring.

ply33

Thank you for the explaination of the speedo operation. Never knew that before. Good to know!

Speedometer wobble is usually caused by a cable that needs lubrication. From under the car, disconnect the cable from the transmission and pull out the inner cable. Inspect for wear, kinks or rust. If it is in good condition grease it and put it back. You can buy a tube of speedometer cable grease from any auto parts store. Coat the cable with grease but do not grease the top 6 inches or 1 foot. You do not want grease to work its way into the speedometer.

Put the freshly greased cable back in. Reconnect to the transmission. This should cure the wobble or jumpiness. Also check the cable is not kinked or bent, it should hang freely in gentle curves.

You may have to replace the whole cable but only if it is broken, rusted, or kinked.

I have also seen a couple of my speedos to that. First was a broken cable. Second was the die-cast body swelling with age and tightening on the shaft inside the speedo. This would also result in a broken cable. The first could have been the same thing...