[6pack] Alternators and sensing wire (or lack of)

[Foster, Stan]

I recently fitted  (but have not yet tested) a Bosch 55A alternator, ex Ford
fiesta (it is a new unit). I researched this project and visited several web
sites that describe this conversion and found that these alternators seem to
differ and the suggested installations differ. One of those differences is
with the connections. The Bosch alternator is plug compatible with the
two-wire modular connector on the Lucas alternator. Depending on the web site
you either insulate and tuck away the unconnected brown "sensing wire" or you
plug it into an appropriate connector on the Bosch unit, just north of the
modular connector.

The unit I purchased from Autozone looks the same as the other 55A Bosch
alternators illustrated on the various TR6 project sites except that on the
back of the alternator there is a small black component (maybe 1.5 inch long,
1 inch high, .5 inch deep) that component has one wire and it is plugged into
the spade terminal that other sites show as the spot for the sensing wire. So
now I'm wondering:

What the heck is a sensing wire anyway ?
What happens if I don't connect it ?
What is the black thing on the back of my alternator that is plugged into the
sensing wire terminal ?

Stan