Speedometer Cable Question

[Black_and_White_F100]

I am curious to know what this particular device is and what it's purpose is. The big red arrow is pointing to the part in question. BTW this is on a '71 F100 Sport Custom 302 w/automatic transmission.

[FORDification]

The short answer: It's part of your emission control setup. Not necessary.

The longer answer: It's a speed sensor, part what was initially called the Distributor Modulator System (Dist-O-Vac) setup, later revised in '72 and changed to T.R.S. (Transmission-Regulated Spark). Essentially it regulates the amount of vacuum advance to the distributor, based on vehicle speed and ambient temperature.

If you want to do some reading on this setup, you can check out the Dist-O-Vac training manual here:

http://www.fordification.com/library/ma ... temId=2324

...or you can even watch a factory training filmstrip on the T.R.S system at this link:

http://www.fordification.com/library/ma ... temId=6314

...and view the accompanying 14-page handbook for the T.R.S. system here:

http://www.fordification.com/library/ma ... temId=6323

[Black_and_White_F100]

Thank you for that very clear explanation and also for the links you attached.