> > Just a thought ... when you're running down the road (which
> presumably is
> > when you need bright lights the most), the highest voltage is not at the
> > battery, but rather at the alternator output terminal.
>
> This is an interesting point. I recently came across an article
> (which, of course, I can't find right now) that points out that
> a three-wire alternator is better at keeping the system voltage high,
> if you go to the trouble to connect the sense wire at the main junction
> point of the harness (say, the ammeter input) instead of just jumpering
> it at the alternator. This lets the alternator's regulator sense
> the voltage in the system, after any voltage drop through the wiring
> harness.
It actually should be as close to the battery as practical, the starter
solenoid is a good place. There normally is a "safety bypass" inside the
alternator, that prevents the output from going sky high if for some reason
the sense wire gets disconnected.
> Without this, the voltage at the main junction will always be a
> touch low. I'd never really considered this possibility. Probably doesn't
> matter so much for our smaller cars with relatively short wiring
> harnesses,
> but it's apparently a big deal in older Detroit iron.
Having it jumpered at the alternator will limit the output current of the
alternator to some extent, meaning it takes longer to charge the battery
after the car is started. I've only looked at one TR6, but with a heavily
discharged battery, there was over a volt of drop between the alternator
terminal and the battery terminal. With the sense voltage regulated to 14.4
(say), that means the output will begin to taper off as soon as the battery
reaches 13.4.
One volt at 40 amps is only 25 milli-ohms, which includes the wires from the
alternator to the ammeter (on earlier TR6), ammeter to the junction block, 4
connections and the ammeter itself. And, if you have a heavy load for
whatever reason (those 100 watt headlamps and driving lamps for example),
then the battery voltage may never come high enough to get a full charge.
Randall
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