Speedometer needle bounce

[Sixt8Chrgr]

I took out what I think was the original speedometer cable and the needle did not bounce, read the correct speed etc. Installed a new repro crap cable and the needle bounces, does not keep the correct speed. Wheels and tires all the same size, transmission not rebuilt, etc. I expect the cable from the repro part is junk. I was going to slide the good cable out of the old housing and install in the new housing. Anyone see an issue with this?

[b5blue]

Routing is not kinked or bending and ends are fully connected? Was the new cable greased? Worth a try to swap out I'd say.

[Sixt8Chrgr]

I will double confirm. But the repro does not read the correct MPH it is fast.

[c00nhunterjoe]

I dont see how the cable chamges the accuracy of the speedo. They are direct drive. Bouncing- yes. But accuracy is in the speedo itself and trans gear. Fwiw, ive never seen one that was accurate at all speeds.

[Sixt8Chrgr]

My original one was very accurate, and did not bounce. My thought is that the tension in the spring that drives the gear is not the same in my repro as compared to the original. Can't lose anything I don't thing. Doesn't the inside just slide out and I can slide the original one in the plastic sleeve?

[Nacho-RT74]

After part out couple of speedos i can say there are several reasons why the speedo can bounce

1-wire dragging with cover
2-aluminium cup and magnet dragging cause is not centered
3-magnet coupler ( which is teflon ) got cracked and is loosen from brass drive ( which is splined ) into the housing, and sometimes loose the rpms jumping out the splines. This failure can make also drag the magnet with aluminium cup making to close the gap between them leaving the brass drive further out.

The return spring has nothing to do with a jumping reading, it just works or not as far the bushing for the needle drive is not cracked. Can be a bit far from an accurate reading though, but can be adjusted.

[Sixt8Chrgr]

Nacho,
When I say "spring" I mean the piece that drives the speedometer inside the plastic casing.

[XH29N0G]

Just joining this discussion.  If the change of cable resulted in the change of behavior, I would look there.  The only possibility I can think of is some sort of binding and releasing of the cable on rotation.  I am assuming the teeth on the gear in the transmission are fine because they are fairly robust on the OE transmissions.  I am assuming the speedo is operating properly.   Without changing the gear on the trans, you should not change the speed

Yes there are hair springs in the speedo and yes if the needle or other parts of the assembly binds the needle can  bounce, and yes if the hair spring is wrong or the magnetization changed, the readout can change, but if the change occurred with one change, a cable, then I would look for the cable as the culprit.  IF it does not spin evenly, I believe it can cause a bounce.

[Nacho-RT74]

There is just one spring on speedo and is the helicoidal one which gets the return function and the intertia to keep working againts the speedo cable rotation.

Believe or not a speedo cable change can not broke but change the piece deepness inserted into the splined brass driver IF this piece was already cracked. The brass driveshaft gets a slight axial play inside which can push out a bit this piece when making the speedo cable change. Is not that it got broken on that moment, but accidentally moved out when it was already cracked.

This piece once it gets slightly out of the brass driveshaft can dragg phisically the magnet assembly witn  the aluminium  cup.

Being there donde that!

[Nacho-RT74]

I have dealt with several issues searching for a kilo speedo, getting them accidentally broken or finding damaged ones. Check this thread. Posting the last page which takes you to the cracked piece I meant which with an axial play can drag phisically the speedo aluminium cup. A change on the speedo wire can get you a symptom of a piece what it was broken previously

http://wichargerguy.proboards.com/thread/19119/patiente-pays-brock-thing-help?page=4

NOT SAYING that's the problem but it could be very well the one.

[Sixt8Chrgr]

I am going to double check and make sure there is no bind. I am going to remove the cable from the tranny and see if I can spin the cable freely. Will let you know. Thanks for the help!

[b5blue]

Yea be certain the thing is plugged all the way into the back of the speedo. It can be close and almost locked but seem to be in all the way.