Wacko Speedo

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Joined
Dec 4, 2005
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27
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177
1988 FJ62:

weird problem with speedo: when it is cold out, and at freeway speeds (55+) the needle will sometimes go ape**** and start making a whirring sound and pegs at the highest limit. This lasts for about 30 seconds to a minute, then stops, not to return for....? Whenever.....another day. Only the needle is affected, as it goes wild, the odometer and trip odometer seem to roll over normally.

Have already replaced the speedo cable at great expense and effort, seemed to solve the problem for a few days. With the return of cooler weather it has started again.

While the speedo was out I attached to a hand drill and "bench tested"- everything seemed in order. Connection between cable and speedo was in good
shape, not worn or anything.

Any ideas?
 
I had a similar problem with a different vehicle. I took out the speedometer and disassembled it cleaned every thing with WD-40 then used some dry graphite powder to lube everything up. I think it was the cleaning more than the lubrication which fixed my problem though.
The speedo works like this.
There is a magnetic cup which is spun by the speedometer cable (which you replaced) the cup slides over a small metal flywheel inside the speedometer case. This flywheel is attached to the speedo needle via a shaft and is held at 0mph by a coil spring when your vehicle is not moving.
There should be no contact between the rotating cup and the flywheel, but it is and should be a very close fit.
As the cup and cable spin faster the magnetic energy creates more force which fights the spring trying to bring the needle/flywheel combo back down to 0 .

My theory is that when it is cold outside/inside the rotating cup contracts ever so slightly and if there is some grit or gunk in between these two moving parts (cup/flywheel) there may be some contact sending your speedo needle into a frenzy as it tries to spin around as fast as the rotating cup. You should disconnect your speedo cable until you can address the issue inside the speedometer.
HTH
[pacer at gmail.com] if you need further explanation.
-Aaron
 
Yep, you need a "rebuild" on your speedo.

Or it will break the needle off.
 
Wow, Pacer, someone who actually knows how a speedo works!

So here's what happened to me...

I was getting nasty screeching noises from my dash, which coincided with my speedo doing odd little jumps (not as bad as JB's) . It sounded like I had a nest of baby birds in my dash. Very annoying, and no one wanted to ride with me....

So, I takes off the instrument cluster, and spray silicone lubricant down the end of the cable. I actually did this with the truck moving (don't ask me how) so that the lubricant would work its way down in the cable and I could add more in the "cup". Oh yeah, and I did remove the little rubber boot at the end of the cable so that I could spray into the cable jacket better.

So while the cluster was off and sitting on my pass seat, I decided to spray a little silicone lubricant on the back of the speedo.

BAD IDEA.

Once everything was re-assembled, I achieved the goal of no more squeaks from my speedo cable, but the speedo now registers low (like 25 when I know I'm going around 40) or NOT AT ALL.

Oh, but the odometer still works.

Why did I mess up my speedo by spraying a little silicone lube in the back? And how do I fix it?
 

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