Bouncing Speedometer

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joose
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Bouncing Speedometer

Post by joose »

Hi All,

I have had this problem on my ghia for a while. The speedometer needle bounces up and down while driving at slower speeds (i.e. less than 35mph). I have also noticed that it over-reads by about 10mph across the range, so when the speedometer indicates 45mph, I am actually only doing 35mph (measured with GPS). Does anybody know if these two problems are related, and what could be causing them? I noticed once when I was fiddling with electrics behind the dash that I unintentionally fixed it for a short time (it was accurate and didnt bounce) so I thought it might just be that the cable was loose. I then unscrewed the cable from the speedometer, put some oil down the cable to lube it up, and then reinstalled it tightly (as tight as I could screw it by hand), but no joy - still does the same thing! any ideas? is it my cable, or the actual speedo?

thanks,
Gary


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Re: Bouncing Speedometer

Post by Muderick »

Hi Gary

This is normally quite an easy fix, once you know what to fix. applies to all cable driven VW speedoes I guess.

It could be one or a combo of 2 things. I have listed it below in order of probability:

1) You have a kink or bend in your speedo cable. What then happens is when the wheel, and in turn cable turns, the kink or bend casuses the cable to bounce, and this is more prominent at low speeds as it evens out at higher revolutions. Quick check, unscrew the cable from the back of the speedo. Jack up the front, spin the left wheel and check if the end of the cable spins around or jumps. If it jumps or bounces then you have a bend in the cable. Normally this happens at the wheel behind the dust cap. If its kinked, cut off the kinked part if close enough to the cap, or replace cable.

2) Lubricate cable on both ends

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joose
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Re: Bouncing Speedometer

Post by joose »

Muderick wrote:Hi Gary

This is normally quite an easy fix, once you know what to fix. applies to all cable driven VW speedoes I guess.

It could be one or a combo of 2 things. I have listed it below in order of probability:

1) You have a kink or bend in your speedo cable. What then happens is when the wheel, and in turn cable turns, the kink or bend casuses the cable to bounce, and this is more prominent at low speeds as it evens out at higher revolutions. Quick check, unscrew the cable from the back of the speedo. Jack up the front, spin the left wheel and check if the end of the cable spins around or jumps. If it jumps or bounces then you have a bend in the cable. Normally this happens at the wheel behind the dust cap. If its kinked, cut off the kinked part if close enough to the cap, or replace cable.
Muds
ah! that answers it then - my cable has been replaced by one that is far too long and it sits coiled up in the front boot! I guess you could call that a kink or bend! :P I will have to see how I can fix that then. thanks muds!
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Re: Bouncing Speedometer

Post by Muderick »

Yup!! :lol:

No probs...
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Re: Bouncing Speedometer

Post by Ron&Gill »

I think the 10% over-read is about normal. I don't think the kink and the overread are related.
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Re: Bouncing Speedometer

Post by fig »

The overread is most likely because you are using wheels/tyres with a smaller diameter than original.

The bouncing needle is probably caused by cable issues, as Muds said, but it can also be caused by dirt in the damper.

The damper is there to even out any inconsistencies in speed/acceleration, so you don't have a bouncing needle. It is simply a pair of brass cups, one inside the other and very close together. It works in much the same way as a torque convertor, except the transmission medium is air not ATF: the cable drives a simple gear set, which turns the one cup. The motion of the cup sets the air around it in motion and, because the gap between the two cups is so small, the moving air then starts the second cup moving. The second cup directly drives the speedo needle. Any sudden change in speed in the first cup will lead to momentary slippage because the air between the two doesn't transfer movement immediately, so the needle moves up and down smoothly instead of bouncing around. Any dust in between the two cups will tend to lessen the slippage and lead to a bouncing needle.
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joose
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Re: Bouncing Speedometer

Post by joose »

fig wrote:The overread is most likely because you are using wheels/tyres with a smaller diameter than original.

The bouncing needle is probably caused by cable issues, as Muds said, but it can also be caused by dirt in the damper.

The damper is there to even out any inconsistencies in speed/acceleration, so you don't have a bouncing needle. It is simply a pair of brass cups, one inside the other and very close together. It works in much the same way as a torque convertor, except the transmission medium is air not ATF: the cable drives a simple gear set, which turns the one cup. The motion of the cup sets the air around it in motion and, because the gap between the two cups is so small, the moving air then starts the second cup moving. The second cup directly drives the speedo needle. Any sudden change in speed in the first cup will lead to momentary slippage because the air between the two doesn't transfer movement immediately, so the needle moves up and down smoothly instead of bouncing around. Any dust in between the two cups will tend to lessen the slippage and lead to a bouncing needle.

thanks for the reply fig! Is the damper in the Speedometer itself?
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Re: Bouncing Speedometer

Post by joose »

Ron&Gill wrote:I think the 10% over-read is about normal. I don't think the kink and the overread are related.
hehe, 10% I could deal with, but its 10MPH! so when it reads 20mph, I am actually doing 10mph!

I do have aftermarket rims and tyres, but they are still 15", so they shouldnt be that much smaller that standard Beetle 15's. I know the profile on the standard beetle tyre is higher, so it will be a bigger wheel overall, but surely that shouldnt make such a big difference as 10mph?
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