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Posted by: ericread May 7 2010, 08:19 AM

For the past month, whenever I go above about 35 MPH, my speedometer begins to whine and the needle jumps around to a very high speed, far in excess of the speed I am traveling.

I first disassembled my speedometer.




There are basically two parts.

The part on the left connects the speedo with the cable, and causes a magnetic disc to spin. The part on the right has the speedometer dial and includes a metal ring of which the magnet spins inside. My understanding is that the magnet and the metal ring should never touch. The magnetic field from the magnet causes the ring to be advanced, which in turn causes the dial to advance.




Here we see the magnetic disc that spins from direct connection to the speedometer cable.



To the left of the speedometer dial, we can see the outside of the metal ring the magnet spins inside.



This is a picture of the metal ring.




Upon closer inspection, you can see some scoring on the upper right side of the inside ring. It appears that the spinning magnet is rubbing against the metal ring, which would explain the issues I have been experiencing. If I understand the theory of operation correctly, there should be no metal to metal contact here.





There are a two adjusting screws on the back of the magnet assembly. The flathead screw would seemingly adjust the magnet forward and back. Which direction should I go??? And the second adjustment is a very small screw with a retaining nut near the center of the moveable assembly. Will adjusting this screw move the magnet closer/farther from the metal ring? And what effect would this have on the speedometer reading?


Any thoughts?

Posted by: ericread May 7 2010, 12:52 PM

Come on there - don't be shy. Somebody out there must be a speedometer expert...

Posted by: McMark May 7 2010, 01:55 PM

Did your odometer work?

Posted by: ericread May 7 2010, 02:09 PM

As the first sentance of my post mentioned: "For the past month, whenever I go above about 35 MPH, my speedometer begins to whine and the needle jumps around to a very high speed, far in excess of the speed I am traveling."

So no, it was not working correctly.

I understand the large screw allows the magnetic wheel to be adjusted forward and backwards, and that maybe I can make the adjustment there. But what does the small screw with the locking bolt do? Does it adjust the depth of play, or is it merely a centering rod?

Eric

Posted by: McMark May 7 2010, 03:25 PM

I'm talking about the little numbers that count your miles. Not your speed.

Posted by: john rogers May 7 2010, 03:29 PM

I'd say take it to a good speedo shop as fixing these seems to be beyond mere mortals! Oh yeah, I still have the e-brake handle if you want it.

Posted by: ericread May 7 2010, 03:35 PM

Take it to a speed shop? WHat fun is that???

I'll give you a call this weekend about a time that's good for you for the e-barke handle.

My thanks.

Posted by: Dr Evil May 7 2010, 03:48 PM

Get another one. It is not the magnet depth that is the issue. The bushing that the cup is retained by seems like it is allowing axial play. You need to make sure the cup can not move axially, only radially. Do not mess with the magnet depth or it will be completely unreliable. You can see in one pic where the cup is lopsided.

Posted by: charliew May 8 2010, 04:54 PM

I agree one of the bushings has worn and the cup and the magnet are rubing a very small amount. A jack leg repair (thats a old ncr term for guys with silver solder repairing cash registers without good parts) might be to grind a small amount off the magnet to reduce the od. And then put a little clock oil on the shaft and bushing. The next thing that will go wrong is the drive gear will loose it's mesh with the driven gear and the odometer will quit working. Really speedos are pretty cheap. Just keep the old one for posterity if you want to keep up with actual mileage. You might also file the alignment holes a little and shift the cup to center it better to give a little more room if it is just off centered more one way than the other.

Posted by: ericread May 9 2010, 11:15 AM

Thanks for the advice everybody. I jst purchased a speedo on eBay for $25. I wonder if it'll work???

But this was the first time I'd seen the inside of a speedo. Kinda cool.

Posted by: JRust May 9 2010, 12:17 PM

I've got some spares. You want me to bring a few along to WCR just in case?

Posted by: ericread May 11 2010, 09:27 AM

Thanks for the offer. Unfortunately I have continuing education classes at PWC in downtown Los Angeles on Saturday, so the WCR is out for me this year.

Eric Read