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Discussion Starter #1
I recently purchased a 67 XR7 GT w/ 390 auto, and I'm tinkering with all the little stuff as I go. Next up on the list: The speedometer bounces wildly from 0 to 70 at speeds under about 45 mph. get above 45 and it's smooth and accurate. Odometer is spot on w/ mile markers on e-way, so I'm assuming the cable is good. Any ideas before I tear into this one?
 

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Contributing Sr Motorhead
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The cable is probably bad - either binding or rusty inside the outer sheath. Or the cable could have gotten kinked when someone did a exhaust change or trans service. Or you can have the dreaded melted cable from being too close to the exhaust! If the speedo is accurate, that pretty much rules out the other cause - the speedometer internals have flying magnets that sometimes come loose with age.
 

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Not sure on the cable. I replaced the speedo in my car to change over to a tach. I replaced the wholoe cluster with one I bought thru ebay. My old speedo worked fine but the replacement speedo bounes around at slower speeds then levels off. Not sure of the speeds, been awhile since the car was one the road!
 

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I might also try just lubricating it. I took my dash apart to have the plastic chrome redone and when I put it back together it bounced. So I took it apart again, lubed it thoroughly, and now it's fine. Also, I noticed that the speedometer gear was cracked, so I changed that, which I am sure helped quite a bit. Amazingly, I got the gear at my friendly local Ford dealer!
 

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Discussion Starter #6
Thanks, after I inspect the cable for damage to the sheath, I'll try lubricating the cable and inspecting the gears. Have to pull the dash to swap the temp guage anyway, so I'll be close by. Looks like cascade has an NOS cable if I end up having to replace the whole thing. The car is all original except for paint, so I'm trying not to use anything reproduction if I don't have to.
 

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I thought speedo cable for sure when I had the same issue with my newly finished resto - until the speedo seized at 70 MPH on the way home from Carlisle! Changed it out with another I had and works perfectly. From what I could tell of the troubled one, the bearing of the speedo can wear to the point that you get a little mechanical interference in the coupling mechanism (which normally has only air) - making the needle jump. The speedo works on eddy currents from magnets and (normally) there is no mechanical linkage between the needle and the drum the cable spins.

Here's an informative link on the topic I found on a search:

http://www.explainthatstuff.com/how-speedometer-works.html

Regards,

Bob
 

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That's what I meant about 'flying magnets'. The spinning cable hooks up to a shaft with rotor on the end of it. The magnetic attraction is what moves the speedometer needle.
 

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Make sure the cable is not kinked and is routed away from exhaust pipes.
 

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about the install ,i purchased a new one and was trying to install it today.
now my car is finished , engine installed and the exhaust also
now because i dont have much space left between the exhaust and transmission i could not locate the hole where the speedometer must be connected to..

anyone can describe the exact location to connect ?

and how to install the speedometer in the dash ?

greets !!
 
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