Instrument Voltage Regulator
Just like that. Thank you very much. Any thoughts on why my speedo isn't working? I understand the premise behind how it is supposed to work but I can't find any way to test that the speedo cable is doing its job.
Well it can be frustrating, there are several possibilities:
the cable is broken inside the housing
the gear (which is plastic) is damaged, either the gear teeth or the square socket which mates with the cable
the gear is installed wrong--notice the plug which holds the gear in the transmission has index marks--it is "eccentric" so rotating the plug allows bigger or smaller gears to be installed
Or the trouble may be right in the speedo--some sort of internal damage to the instrument.
Probably the best way is to put a "small load" on the cable to be sure the gear is not "stripping" or that the square cable drive at the transmission gear is not slipping. You can disconnect the dash end, either drive the car a short distance, or run the car in gear with the (carefully) front wheels blocked and rears jacked up, then feel the cable end with your fingers, and squeeze gently with your fingers or a rag to see if it still turns.
If you replace the cable, you can buy a "universal kit" that replaces just the drive cable itself, not the outer jacket. Be careful with this, because if the jacket is old, or has poor lubrication, or has been damaged by the failed cable, the jacket will be rough inside, and will quickly wear out the new drive cable.