Jump to content
gusopenshaw

What does the knob on the instrument cluster...

11 posts / 1933 viewsLast Reply

Recommended Posts

(cylindrical, about a half-inch in diameter) do?

Does tightening it have any bearing on speedometer needle flutter?

Flutter is my Problem of the Day, after already lubricating the speedo cable. Would appreciate any thoughts.

Thanks.

Share this post


Link to post
Share on other sites

if you're talking about the 2 knobs on the back...they hold the cluster to the dash...there is no adjustment for fluttering speedo...

Share this post


Link to post
Share on other sites

The only knob on the instrument cluster is for tripmeter reset- you turn it to zero out the tripmeter.

Speedo needle "flutter" might be caused by a dry cable, but is more likely to be due to a worn out speedo mechanism.

Share this post


Link to post
Share on other sites

is going bad, you don't want to let it go too long. Seems they go bad quickly when they start fluttering. You don't want it to crap out on you, cuz the magnet could likely make contact with speedo needle shaft and snap off the needle, then you need to get a replacement speedo. Look up in the archives on how to fix your speedo.

Share this post


Link to post
Share on other sites
is going bad, you don't want to let it go too long. Seems they go bad quickly when they start fluttering. You don't want it to crap out on you, cuz the magnet could likely make contact with speedo needle shaft and snap off the needle, then you need to get a replacement speedo. Look up in the archives on how to fix your speedo.

what magnet....the speedo cable has a square end that fits into a square hole on the back of the speedo...the worse thing that could happen is the cable breaks....that being the case, simply pull the broken piece out of the speedo

the speedo is mechanically operated and has nothing to do with magnets

Share this post


Link to post
Share on other sites

The speedo does indeed use a magnetic field to spin the needle up. There is also a small, delicate spring to return the pointer to 0 and provide some linearity to the movement of the needle. I am not precisely sure what causes flutter, but I would guess weakness in the spring, or a problem with the needle pivot point or drive bushing. There is not much you can do if any of these parts go south (unless you are a watchmaker with a secret access to parts).

There are speedo repair shops that can fix and calibrate speedos. I bought a brand new 02 speedo for $25 from Walloth und Nesch (http://www.wallothnesch.com) a few years back during a sale. If you need an early style unit, you are probably SOL for a new unit.

What with the FX rate and W u. N's current price for a speedo, it might be cheaper to get yours fixed, or find a good, used replacement. Note that speedos are calibrated for the car (diff ratio...), so you should get one with the same numbers on the back as you have now... IIRC, look for a number like 1.38-1.39 on a speedo calibrated for a 3.91 diffs and 1.28 speedos for 3.64 diffs.

Share this post


Link to post
Share on other sites
The speedo does indeed use a magnetic field to spin the needle up. There is also a small, delicate spring to return the pointer to 0 and provide some linearity to the movement of the needle.

So that's how they do it! The next question is what exactlly do they do to calibrate these things? They are easy to remove and re-install, so not having the proper bench equipment should not be an obstacle to do a trial and error thing... Could it be as simple as to increase the windings around the magnet?

I have about 3 or 4 that don't work right!

Or perhaps changing the tension on the spring, that is allowing the magnets to pull more or less - nothing can be that difficult or secret...

Share this post


Link to post
Share on other sites

Michael, these things are not electronic, so there ain't no windings on the magnet. I "recalibrated" the speedo that was in my 02 when I bought it by stretching the spring a bit- just twisted the needle assembly toward the 120 MPH mark a bit until if felt a bit looser. This allowed it to spin up a bit more easily and read higher (it was about 10 MPH low at 60 MPH). I suppose you could do the reverse for the opposite problem.

I don't know how the speedo shops do it, but presumably they have replacement parts and instrumentation that allows them to precisely calibrate things.

Share this post


Link to post
Share on other sites
Michael, these things are not electronic, so there ain't no windings on the magnet. I "recalibrated" the speedo that was in my 02 when I bought it by stretching the spring a bit- just twisted the needle assembly toward the 120 MPH mark a bit until if felt a bit looser. This allowed it to spin up a bit more easily and read higher (it was about 10 MPH low at 60 MPH). I suppose you could do the reverse for the opposite problem.

I don't know how the speedo shops do it, but presumably they have replacement parts and instrumentation that allows them to precisely calibrate things.

today, just for kicks I opened one up to find out the mistery (I wish I had taken some pictures). The cable rotates a steel disk that has a circle clip around it. On the other side the needle is connected directly to a light aluminum cup that shrouds this disk. How the rotation of the steel disk makes this light aluminum disk move it's beyond me as there is plenty of clearance. It can't be magnetic, as aluminum is non-magnetic, small amount of friction perhaps, but that would wear out the aluminum cup, somehow the more massive steel disk produces air flow? not likely... these things intrigue me... smart Germans,,, maybe Italians or French, who invented this system....

Never mind: http://en.wikipedia.org/wiki/Speedometer

the question now is what changes that they go bad??

I can only think or two things: the return spring tension and the wear in the speedo cable square hole receptor.

Share this post


Link to post
Share on other sites

actually the connection between the speedometer cable and the needle is done through a physical property of aluminum called diamagnetism. Where steel has an attraction to a magnetic field aluminum has a resistance to the magnetic field flowing through it. As the wheel driven by the cable spin the magnets create a field that the aluminum cup tries to resist. The aluminum resist the magnetic field passing through it and this creates rotation of the cup and movement of the needle. As the driven wheel moves faster the aluminum cup continues to resist the passing of the magnetic field. Because there is more field trying to pass through the aluminum cup the needle moves around indicating higher speed. The flutter in the needle can be caused by the bushings that keep the driven wheel centered in the aluminum cup. As the wheel and the cup become more eccentric the magnetic field is stronger in certain areas of the orbit. This means that the resistance force of the aluminum varies depending on the gap between the wheel and the aluminum cup. If the situation is not corrected the cup and wheel make contact and the small coil spring is destroyed. When you see the spring you have to remember it doesn't wind up, it only creates a back force on the needle shaft that is attached to the aluminum cup. The spring is very weak but it is designed to overcome the diamagnetic property of the aluminum and allow the needle to represent speed based on the amount of resistance the field requires.

If you ever want to see this work in real life get a coin sized magnet and place it on an aluminum cookie sheet. Then tilt the cookie sheet up until the magnet starts to slide down the slope of the cookie sheet. The magnet will accelerate until it reaches the resistance of the aluminum to allow the magnetic field to flow through it. Most people think that aluminum just doesn't have the attraction that steel does and that isn't wrong, but aluminum also resist the flow of the magnetic field through it.

Thats why there is no gearing, just the speedo cable coupling directly to the back of the needle drive. Actually its a pretty cool way to do it, and as long as the wheel and the cup remain concentric the speedo work great.

Come to think of it you can probably change the drive ratio for the speedo by changing the strength of the magnets?????

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.