Speedo "stuck"

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So the speedometer on my car is stuck at the 75mph mark. The thing is the engine & trans are out of the car, so I think it's safe to assume the problem is with the speedo head LOL

How hard of a fix is this? I plan on pulling the instrument cluster out at some point this winter/spring, I'm just not sure how how big of a deal this will be to fix. Looking for thoughts, suggestions, fixes, etc. Thanks!
 
Could be the speedo cable bound up or broken in the sheath. Disconnect it from the back of the speedo and check it first. Might save you from pulling the instrument cluster if you don't need to.
 
Hmmm I suppose. How hard is it to disconnect from the back? Can I do that by reaching up under the dash? The car is not at my house so I can't go check. Does it unscrew or unbolt?
 
There is no physical contact between the cable and the speedo head, so no it isn't (can't be) the cable.
It's the speedo head.
 
Yep. I stand corrected. I have my gauge cluster sitting on the workbench. I now see there is no way a stuck cable would hold the speedo at a certain speed. Now I can't figure out how the speedo works since there doesn't seem to be any mechanical coupling between the cable and speedo needle.
 
It's a very close tolerance magnetic coupling.
A pair of little bent metal fingers driven by the cable spins around the perimeter of a metal cupped "disk" which is attached to the needle. If the bearing (nylon?) wears out the tolerance closes and the fingers contact the metal disk. Also dirt can easily be between them giving the infamous vibrating needle. It if touches you have a locked up unit independent of the cable.
(My needle was wobbling today when I first started driving between 60 and 70. Cold parts our first cold day. It got better when it warmed up.)
And when it does lock up under speed you get a trashed needle.
On my square later model ones, I bench test them with a piece of speedo cable end in a drill motor.
Only thing to do with yours is bite the bullet and take it apart and look.
I use LPS1 greaseless to lubricate them. Doesn't attract and hold dirt.
 
If the speedo head magnetic drive parts contact each other and seize up it trashes the cable.
Twists it all to hell. :D

When you get the unit out of the cluster look at the back where the cable normally attaches and notice the little clock spring lookin assembly.
It's the calibration adjustment.
Betcha it's not attached to the body of the unit any longer and is not retuning the needle to zero.
The last one I fixed like that I estimated where it looked like it might have been and epoxied the spring unit back and got it within 5 mph. :D

Oh, and it's totally acceptable to hold the drum the needle is attached to and lightly twist the needle if it needs moved, removed or adjusted.
It's just lightly pressed into the drive unit.
 
Hmm thanks OK well it will probably be a spring project but I will plan on removing it and taking it apart to check it out. If anyone has a thread with pics they could refer me to on what's inside there please share the link. For now I guess I won't really worry about it since getting the engine running is a bigger priority.
 
adriver said:
It's a very close tolerance magnetic coupling.
A pair of little bent metal fingers driven by the cable spins around the perimeter of a metal cupped "disk" which is attached to the needle. If the bearing (nylon?) wears out the tolerance closes and the fingers contact the metal disk. Also dirt can easily be between them giving the infamous vibrating needle. It if touches you have a locked up unit independent of the cable.
(My needle was wobbling today when I first started driving between 60 and 70. Cold parts our first cold day. It got better when it warmed up.)
And when it does lock up under speed you get a trashed needle.
On my square later model ones, I bench test them with a piece of speedo cable end in a drill motor.
Only thing to do with yours is bite the bullet and take it apart and look.
I use LPS1 greaseless to lubricate them. Doesn't attract and hold dirt.
Click to expand...

...and this is what it will look like when it goes out at speed!!

IMG_3064.JPG
 
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