Tachometer Acting Up

[rzgkane]

har har.

I found a guy in the bay area who will fix it for somewhere b/w 50-100 bucks. You think it's worth pulling it apart and risk breaking it worse or should I just have him fix it?

Have a professional do it. I suspect there's very little inside that is user serviceable.

[G-Man]

Check out the posts on speedo repair in the forum. The mechanism is the same only the tacho' is simpler.

Essentially there is an input bearing which which drys out with time causing the 'stick slip' problem which manifests itself as the crazy needle syndrome. Inside the instrument there is a two part system consisting of a rotating magnet and an aluminium drum. This must be clean with an air gap (no oil) between the two parts. The magnet drives the aluminium drum agains a hair spring and this then works the needle.


CL77 Speedo return spring by graham.curtis, on Flickr

The only difficult specialist job is getting the bezel off. Work very slowly and carefully with an old flat screwdriver.


Removing speedometer Bezel by graham.curtis, on Flickr

G

[bart3]

Did you ever get any resolution to the problem? I'm having the exact same issue, unfortunately none of the advice offered up has corrected it. Thanks.

[conbs]

Study the picture that G-man (Thanks Graham) put up in his earlier post where he is holding the guts of the speedo. There is a lube point/"bearing" down inside the two cups that fit together (right under the spring) and that is most likely where the problem is.

The bottom/outside cup is hooked directly to the input that the cable slides into. The input/shaft it is on and bottom/outside cup all spin at the same rpms as the cable. The top/inner cup is hooked onto the shaft that the speedo needle fits on. Another lube point in the brass bushing right where that shaft goes through the that top plate

Think about how the needle moves - very little. When a speedo works normally the rpms of the spinning cable and input shaft and bottom/outside cup have to change to a very small movement of the top/inner cup on the needle shaft. To do that there is a third lube point/bearing down inside the two cups. It is a "point" on the end of the needle shaft that rests in a little "V" in the bottom/outside cup. Think of a watch movement. The bottom/outside cup is magnetic. As it spins the magnetism pulls on the top/inner cup and that cup would end up spinning at the same speed as the bottom/outside cup, except for the bearing down inside. The little watch spring pushes back against the magnetic force of the rapidly spinning outside cup. Where they reach equilibrium so points your needle. As your bike speeds up the magnetic cup spins faster and pushes harder against the spring and the needle moves clockwise ever so slightly.

When there is a problem in the little "V" point bearing, it creates extra friction that overcomes the force of the spring so your needle starts to spin wildly. It won't take much. The whirring/buzzing noise comes from the little "V" point bearing when rust/dirt/wear make it so the two parts are not spinning smoothly.

Unfortunately, getting at all of this requires that the speedo be taken apart. Most guys are very intimidated by a speedo and how it works. I was until I took one apart and realized how amazingly simple they are. There are a couple of areas where you need to be careful.

1. When you are prying on the chrome bezel to remove it, BE CAREFUL AND GO SLOW. It is made of brass and will split. On a Dream speedo the biggest problem is the corners. I know from experience. But, anywhere that gets a nick on the edge is a potential problem. Just go slowly and bend it out a little each time and keep working your way around and you will get it. Same with re-assembly. Go slow.

2. When you take the internal mechanism apart be careful with that little watch spring or you could bend it. I didn't try this, but I think you could slide the needle back on a little so it holds the needle shaft in that top plate. The two shafts/cups should just slide apart.

It is no wonder that guys are intimidated by how a speedo operates. It it not obvious and really seems like magic. But, after I took my first one apart and saw how simple they are I was amazed that they work at all, let alone pretty accurately. All of this is to encourage you that you probably can fix your own speedo if you are at all mechanically inclined - and patient.

The speedo repair guys recommend synthetic oil in the 3 lube points and I think you would want to use a synthetic grease on the odometer gears, but white grease would probably work on the odometer okay, too. Clean everything (carb cleaner will eat the numbers off your face plate). I think it is a good idea to use EvapoRust (Harbor Freight) to remove all rust from everything after all grease/oil is removed. Lube everything and consider a new face plate decal. That's it. Obviously, you want to make sure that "V" point bearing is very clean and lubed. Don't use so much oil that it starts to interfere with the two cups spinning at dramatically different speeds.

If you do take a run at your own repair and figure out you are in over your head, you can still box it up and send it out for repair (you will actually have saved them some time). But, if you do get it fixed, you will have a giant smile every time you look down at that perfectly operating speedo. And, your friends who are still intimidated by speedo's will think you are some kind of genius.

Hope this helps. Best wishes on your project.