Speedo Testing

[speedfreak]

So, I have an issue with my speedo... Works great to about 50mph, then looses its mind and just flies back and forth. Is this something anyone has dealt with before?

When I went to test it, I was very surprised to find that using a drill on the end of the cable would not get the speedo to register any speed. I assume the drive gear rotates at a higher rate that the drill, but I couldn't even get a blink on it. Is there any other way to test it, other than riding?

[Alan H]

Make sure the drill is running the carrect way. I use a small lathe to run speedo/tacho after rebuild, and they work at slow speeds.
Sounds like either a faulty cable, or perhaps the damping oil has gone awol.

[Coyote]

Test drill must run in reverse.. A regular electric drill motor will give you about 40 MPH.

[speedfreak]

Tried it again, both directions, and got nada. Cable tests good.

What's the damping oil you speak of Alan? I have no knowledge of the internals of these things - so any insight is very much appreciated (particularly so before I tear it apart and damage someting permanently...).

[yeadon_m]

SF,

Just to prove to yourself that you're testing validly, a question and a suggestion. Question, does your tacho work? if yes, test it in the same way. Can you make the needle swing to a few thousand rpm? if yes, your speedo is broken. If not, your speedo test is

Cheers,
Mike

[Alan H]

Tried it again, both directions, and got nada. Cable tests good.

What's the damping oil you speak of Alan? I have no knowledge of the internals of these things - so any insight is very much appreciated (particularly so before I tear it apart and damage someting permanently...).

Your original post suggested that the speedo was OK at low speeds then not so good over 50. It could be the damping oil inside (which stops large swings of the needle) has leaked/been drunk by devils etc. If the speedo doesn't work at all, then that's something else altogether. If you use a piece of speedo cable inner (or a nail filed square) with one end in an electric drill and the square end in the speedo, that's a good test that it works at all. If it has stopped working completely, then either get it sorted by someone who knows about that sort of thing (people on here do it) or have a look yourself. Downside of doing it yourself is that if it was repairable and you knack it, then it costs more.
Also make sure that the cable spins when it's not in the speedo and the front wheel is turning. It could even be the drive unit on the wheel. Mike's previous post suggests that you try the rev counter with the drill too. Good test.

[Coyote]

If you are using a cordless unit, there is not enough speed to operate the gauge. Must be a normal ELECTRIC drill -- in reverse. Tach testing is in forward direction.

[speedfreak]

So I tested the tach - works. Tested the speedo again with a wired drill instead of cordless - same deal, no motion.

Now I'm going to ask what could be a dumb question. Does the speedo have to be powered?? I thought not (just the bulbs for backlight), but perhaps I'm missing something obvious.

If not, then it looks like the speedo is toast. It was working at slow speed, but I guess I finished it off while testing. Grrrr....

[ConnerVT]

No power required.

[Coyote]

Alan suggested using a nail as the test driver. You might try that. It takes the cable out of the picture. If it works with the nail, then you know your cable is broken inside. Grabs sometimes and sometimes not. That's what it sounds like to me.

[speedfreak]

Didn't think it was powered, but but at this point I'm second guessing myself...

It's a new cable, but I'll give the nail trick a try. Brought it to work today and the speedo is working, up to about 50mph still. Perhaps the cable is the culprit...