Speedometer Not Working but Odometer Works

So the speedometer in my '84 hasn't worked since I bought it a few years ago. I've been doing a bunch of maintenance recently before I get it inspected and thought I'd tack a whack at fixing it. After doing as much troubleshooting as I can I'm still not exactly sure what's going on.

The odometer has been working just fine the whole time, so I don't suspect it's the cable. I checked the cable at both ends, and they look pretty good. I took the cable out of the housing and it seems to still have a little grease on it, and moves fairly smoothly when you twist it while in the housing. The square end doesn't look too rounded and seems to lock in fine when you insert it into the gauge.

I attached a drill to the wheel end of the cable while the square in was in the speedo and revved it up as high as it would go (it's a pretty lame drill though). The speedo needle moved smoothly up to 15MPH and hovered there while the drill was on, and came smoothly back down after turning off. I then attached the wheel end of the cable and left the square end loose while rolling the bike slowly through my driveway. The square end turned while the wheel was rolling, as it should.

After reinserting everything and making sure both ends were snugly in place, I took it out for a test ride. Speedo is still not functional. It does react to changes in speed, but the needle just bounces erratically between 5-15MPH as it always has. If I get above ~35MPH (haven't been on the highways yet so really haven't seen past 45MPH, waiting until inspection), the needle maxes out at 25MPH and stays put, no longer bouncing.

I think the only thing I've ruled out is the cable being bad. If I had a drill that could take the speedo up to ~70MPH, I would probably conclude the speedo drive is messed up, but for whatever reason manages to spin the cable while rolling the bike. It still seems possible that the gauge is messed up internally, I just haven't looked at it yet and really wouldn't know what to look for; was hoping you guys might. Whadaya'll think?

 

A couple of years ago I hit a deer with my 84 VF 750 F and among many other things, damaged my speedo. I was determined to fix it and finally did. What I found inside the speedo unit was the cable spins a magnet that turns inside a drum that makes the needle move. Their is no physical connection between the cable and the speedometer. The faster the magnet spins, the more force it generates on the drum to move the spring loaded needle. The relationship between the magnet and the drum is critical. I was able to move the drum assembly to the proper position and the speedo is now accurate to within a couple MPH. It did take some fussing and several tries, but it has been working great ever since. Good luck.

 

Damn! That sucks dude, glad I won't be removing any antler bits from mine. Well cool, I think I have a pretty good understanding of how they work now so I'll tear it apart and see what's going on. Definitely wouldn't surprise me if the coupling between the magnet and speed cup is wonky. Worst case it looks like a set of gauges is ~$50 on eBay. Thanks for the response!