Re: Alternator Wiring

[Tim Buja <buja@domain.elided>]

NickArg wrote:
> On the end of the wiring harness that connects to the alternator
> there is a white block with 2 thick brown wires and 1 thinner
> brown/yellow wire. This plugs straight into the back of the
> alternator no problem. However I also have a single brown wire with
> a standard connector which is currently not connected. A previous
> owner, dumb or maybe not so dumb, had taped this wire to the harness
> to keep it out of the way. 
...
> Where should wire this go? Anywhere? or should just I tape it up
> again? I can't see anywhere to connect it to on the alternator.

Here's a brief primer on alternator wiring.  I'm assuming that you
have a later type alternator with an internal regulator, not the
early type with the control box on the right wheel arch.

The two thick brown wires are the alternator output.  The thin
brown/yellow wire in the same connector is the IGN light feed to the
alternator.  This wire provides the start-up excitation power to the
regulator until the rotor is turning fast enough for the alternator
to power itself.  If the alternator can't produce enough power to
power itself (bad diodes, shorted windings, etc.), the battery will
supply the regulator excitation current through the IGN light on the
dash, which will light up to tell you that something has gone wrong.
The IGN lamp usually needs to be about 2 watts or greater so that it
will pass a sufficient amount of current to feed the regulator.  If
the bulb is too small, it will light but the regulator won't get
sufficient current and the alternator will not produce any output.

All alternators need some voltage feedback to the regulator to
properly charge the battery.  Some alternators sense the voltage at
the alternator terminals (machine sensing), while others sense it at
a remote point to overcome any voltage drops caused by high
alternator output current (battery sensing).  Battery sensing
alternator regulators usually have a backup machine sensing lead
just in case the battery sensing lead fails.

The alternator that originally came on your Stag was a battery sensing
unit that used the thin brown lead that you found taped to your
harness.  It is supposed to be connected to a single male spade
terminal on the back side of the alternator.  If you take the black
plastic back cover off your alternator, you should see this connector.
Hold the alternator with the back facing you so the diode pack is at
the top and the regulator is at the bottom.  Look just above the
regulator to find the two metal straps that connect to the brushes.
The male lead should be to the right of the upper brush strap, and
should have a lead going to the regulator.  (Sorry, I can't remember
the color since I'm doing this all from memory.)  There should be a
square or rectangular cutout in the black plastic cover corresponding
with the location of the spade connector.

          nut
       diode pack

    ==brush strap==  look here

    ==brush strap==

       regulator

I've heard that Lucas has superseded the four wire battery & machine
sensing regulator with two and/or three wire machine sensing
regulators that only sense the voltage at the diode pack.  If you
don't have a male spade connector on your alternator, I would guess
that the previous owner had the alternator rebuilt and the shop used
a machine sensing regulator.  If so, you no longer need the thin
brown lead.  For safety's sake, I would tape up the connector, and
then tape the thin brown lead to the wiring harness to keep it out
of harm's way. This lead is connected to the battery without a fuse
and if it finds a way to touch the chassis, you'll have a really
nasty electrical problem.

Tim Buja - Rockford, Illinois USA - 80 TR8, 73 Stag, 72 TR6