Differences in these speedometers

[Tony_nc]

The speedometer in my 73' doesn't work and I was going to replace it with the one out of my 71'. What is the box that the arrow is pointing at? What if it isn't used?

[JLPurcell]

One has the shunt directly attached and other has been removed or is not located directly on the speed-o head. jlp

[EScanlon]

Interesting...

At a wild a$$ guess, and as far as I know the 73's did NOT come with Cruise Control from the factory, if that could be part of the starter interlock mechanism / seat belt warning system.

That red wire seems to go to a bulb. If it is a bulb, remove it and shine a flashlight through the hole in the back and see what "lights" up on the face of the speedo. If it's a Seat Belt Warning Avatar, there you go, if not, it would be interesting to know what it is.

If on the other hand it isn't a bulb, that would be interesting to investigate as well.

FWIW

E

[Tony_nc]

I had to look up shunt (genius). Not sure about that but interested in why you first thought that.

I took pictures of the red wire. It seems like a ground but not sure why its red.

[JLPurcell]

In the parts diagram the only part shown in this location is labeled as a "shunt". It indicates that it also contains a fuse so below is the only definition which makes since in this application. I am not sure this is the use but it does make some sense. It could be a constant voltage stabilizer common method back then to stabilize the voltage for the gauges. Without seeing where all the wires are going it would be hard to make any further educated guesses. Does anyone else have an idea?

"Shunts as circuit protection

When a circuit must be protected from overvoltage and there are failure modes in the power supply that can produce such overvoltages, the circuit may be protected by a device commonly called a crowbar circuit. When this device detects an overvoltage it causes a short circuit between the power supply and its return. This will cause both an immediate drop in voltage (protecting the device) and an instantaneous high current which is expected to open a current sensitive device (such as a fuse or circuit breaker). This device is called a crowbar as it is likened to dropping an actual crowbar across a set of bus bars (exposed electrical conductors)."

[Tony_nc]

Ok so I have figured out the speedo differences. When I spin the back of the speedo the needle moves (makes me assume its good). Then I hook up the cable to the speedo and down to the trans. I lubed the cable and it seems ok. I pulled the gear pinion out of the trans and the teeth look good. Now when I hook the cable up to the gear and spin it (not installed of course) the needle moves. So why doesnt my speedo work and whats the next direction to go?

Thanks