Speed and Cable questions

[tparker]

My Speedometer never worked back in the day. It looks fine as does the gear in the transmission. I can't recall if it was actually connected or not, since its been over a decade since I removed it. Assuming it is the speedo itself, what are things to look at to determine if it is broken? Are they repairable? Is there a way to bench test? Hook up the cable end to a drill or something?

Also, how do speedos actually work?

Thanks
Tom

[Cuda Cody]

I don't know the answer, but this "may" help.

[tparker]

  I don't know how to answer that. Is that a thing or am I being pranked? As long as it isn't my muffler bearings I am probably good to go. 

I didn't know that existed. I'll check it out.

Thanks

[blown motor]

You could try spinning the cable with a drill. I'm not sure which way to spin it so go slowly at first. Make sure the gauge end of the cable is properly seated in the gauge. The steel cable inside the sheath just slides onto a small square shaft that  drives the needle.
I've had a couple speedometers that didn't work right and I sent them to Redline Gauges for a rebuild. Not cheap but they work perfect now and I don't fit a minute regret spending the money.

[pschlosser]

The cable looks like one long spring, so it has some "give" as it spins.  When the cable starts to wear out, or there is friction within the cable, the rotation at the speedometer-end can vary abruptly, causing the needle to jump periodically.

When the needle jumps around, especially at lower speeds, it means the cable needs to be greased.  If greasing doesn't fix it, then replace the cable.

The cable spins with the output shaft of the transmission.  The cable is indirectly connected with the needle in the speedometer.  The cable turns this round metal dish, inside the speedometer.  Incasing the "input" dish is another larger "output" dish (or vice versa).  The output dish is connected to the needle.

The dishes have magnets on them.  As the input dish spins, it grabs the output dish momentarily to cause some movement as those magnets grab each other.  A spring pulls on the needle and output dish returning it back to zero. So the input dish is always fighting against the spring.

When the input dish spins slowly, it grabs the output dish for only a moment, before the spring pulls it loose again.  The faster the input dish spins, the further it can move the output dish before the spring pulls it back.

With the speedometer out, you can observe how this works.  Spin the cable slowly, and the needle moves a little.  Spin the cable quickly, and the needle moves more.

With the speedometer out of the dash, a small screwdriver inserted into the hexagonal/square fitting can move the needle some.

If your speedometer doesn't seem to work in the car, and tests okay by spinning the input shaft/dish, then confirm your cable spins as the drive shaft is spinning.   Sometimes the cables can actually break.

Repairing the speedometer yourself is not easy to do, IMO.  It is an art, it's delicate, and may need recalibration.  Sending it to a service provider for repair/restoration often exceeds the cost of another used speedometer in good working and cosmetic order.