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Unread 06-02-2014, 10:55 AM   #1
foggybottombob
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1984 CJ7 
 
Join Date: May 2008
Location: Monument Colorado
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Speedometer needle swings

My 84 CJ7 Speedometer needle swings back and forth in a 10mph arc. Is this a common problem? Anything I can do to damp this oscillation? My current speed is, at best, a guess.

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Unread 06-02-2014, 11:05 AM   #2
atlshaun
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1983 CJ8 
 
Join Date: Oct 2013
Location: Atlanta, GA
Posts: 64
I have the same issue. I have no clue how fast I am driving.
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Unread 06-02-2014, 11:24 AM   #3
RenoF250
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1985 CJ7 
 
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Have you tried pulling out the cable and cleaning and lubing it? They can get sticky and then make the needle jump. Aside from that probably a new speedo.
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Unread 06-02-2014, 11:25 AM   #4
foggybottombob
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I suppose that brake cleaner down the speedo cable tube is the place to start.
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Unread 06-06-2014, 01:54 PM   #5
foggybottombob
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I swapped the speedo gear in the Dana 300 today and could not get the speedo cable cap to screw back onto the gear holder that fits in the side of the D300. I finally realized that the inner cable is pushing before the cable cap engages the threads on the gear holder. I think the internal cable is too long and this is why I'm getting the sticky inner cable. The inner cable is binding against the sides of the outer cable since it is being compressed when I get the cable cap screwed down. So I think I need to cut a bit off the end of the inner cable to fix this. I'll try to fix this when I can get an afternoon that it's not raining.
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Unread 06-06-2014, 02:09 PM   #6
Mike Romain
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Sounds like one of the universal cables that was cut too long. When mine went jumpy like that, the outer casing had tagged the exhaust pipe melting the inner liner.
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Unread 06-06-2014, 06:03 PM   #7
5-90
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Quote:
Originally Posted by foggybottombob View Post
My 84 CJ7 Speedometer needle swings back and forth in a 10mph arc. Is this a common problem? Anything I can do to damp this oscillation? My current speed is, at best, a guess.
This is typical for a mechanical speedometer. More modern speedos were progressively dampened, but there was little that could be done due to the nature of how the reading is transmitted (my 88XJ does the same thing.)

This was virtually eliminated with electronically-driven speedometers.

The problem lies in the fact that you have a constantly-spinning element (driven by the cable from the transfer case,) which is a yoke that surrounds the armature that drives the needle. The two are coupled using a weak magnetic field - there is no direct coupling between the two. Therefore, there will be some "uncoupling" happening, as the lack of a direct connection comes into play.

(Electronically-driven speedometers are generally simply "analogue frequency counters" - driven by a pulsed signal from a similar location, interpreted by the ECM, and that signal used to drive the speedo. Early electronic speedos were driven directly by the signal. In either case, you could "reprogramme" the interpreter to account for changes in final drive gearing or tyre size, often more accurately than swapping gears in old mech speedo setups.)

Suggest either looking for a speedo head with more modern construction, or finding a "conversion" setup where you can install an electronic speedo in place of the OEM mechanical (both should be available.) The problem is that you can't really dampen the mechanical speedo overmuch, because the magnetic field coupling the moving parts is necessarily weak. Therefore, overdapening the needle will result in a "false low" reading - and "instrument error" isn't much of a positive defense in traffic court...
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Unread 06-06-2014, 06:51 PM   #8
foggybottombob
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I cut 1/4" off the inner cable and now the speedometer works like it should. It still bounces a little but it's useable. The inner cable was definitely too long. I used a cutoff wheel on a dremel tool to shorten the inner cable.
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