Originally Posted by foggybottombob
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...