68 GTS Speedometer doesn't work
I had the transmission rebuilt about 3 years ago and that's when it stopped working
i wonder if the speedometer gear holder, or whatever it is called, is not clocked properly
That's where I was going, and the rebuild and it stopped working at the same time really points me in that direction.
The way the transmission speedo adaptor works the following way.
There is a shaft in the trans with a worm gear on it. The speedo gear mates up to it. If the speedo gear has a lot of teeth, say 38, the diameter of the gear will be large. Now say you have a 26 tooth gear. It will be smaller diameter. If the centerline of both gears stays the same, the small gear will not touch the worm gear. The speedo gear is not centered in the adaptor, that way you can rotate the adaptor to move the center of the speedo gear, closer or farther from the worm gear. What Furyus2 was talking about is that the adaptor might be (as an the example) in the 38 tooth position but have a 26 tooth gear. In that condition the smaller gear would not be touching the worm gear in the trans.
(BTW if you do have your gear out for any reason get its tooth count and record it for later use, with that, a speedometer reading and a GPS reading you can determine what gear you would need if let's say you changed your rear gear ratio or rear tire size or both)
When you say "do not pull the inner cable out from the speedo head" are you talking the end of the cable at the transmission
The outer cable is screwed (or later years snapped) into the speedo head and trans adaptor. Inside the outer cable is the coiled cable that actually rotates when the output shaft of the transmission rotates. The inner cable typically can be pulled out of the outer cable, (I have exp with some old Ford's that the inner is not retained by anything so you can pull it out from either end of the outer cable) if you pull the inner cable out an inch or so it will no longer be engaged in the speedo. Not to worry, usually you can get it back in without removing the outer cable.