Over this summer, I am rewiring my 120. in the course of this
project, I am replacing the generator charging system with an
alternator. The alternator has an output of about 60 amperes.
This is more than double the output of the genny. The stock
ammeter reads to + or - 30amps. I have already had the new wiring
harness upgraded by increasing the wire guage in all of the
charging circuit wiring.
My question is about the Ammeter. I believe that a wire (shunt?)
can be run across the two terminals of the ammeter to decrease the
current running through the ammeter. Is this correct, and if so,
how do I choose the correct wire (shunt) size?
Roger–
Roger, 1954 120 DHC BRG, S678300
Tamarac, South Fla., United States
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In reply to a message from Velocette sent Thu 11 May 2006:
If you put a shunt across the ammeter, two things will happen.
First, current will be shared between the shunt path and the
ammeter path in proportion to their relative resistances. Second,
the ammeter will only display a reading for the current which
passes through the ammeter. Unless you calibrate the total current
to the ammeter reading, you will not have a good idea of charging
current totals.
Maybe there is a shunt developed for this purpose. I use calibrated
low resistance shunts in high current applications in my laboratory
but am unaware of any developed for your ammeter. Or maybe others
will tell you to live with the ammeter pegging itself occasionally.
Or maybe there is an ammeter to which you might change. Or maybe
you should limit the alternator output. Don’t count on the
regulator working properly at the higher currents unless others
with such experience say it works. Good luck!–
Roger McWilliams
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why not replace the ammeter with a voltmeter. That wold give a better
indiation of battery condition anyway. Didn’t later British cars use
the same basic instrument…the same size??
the amp gauge is of little use with an altenator as you are controlling the
voltage only. the amp output is wired into the system and you really don’t
have much control over the amps.
Rog and all interested in changing over to alternators.
You need not worry about the alternator’s output capability unless you
attempt to charge a bad or near flat battery through the original 30 Amp
instrument.
Don’t forget that the cars battery has the ability to deliver hundreds of
amps but since the car’s accessories only need a few of them (load), the
30 Amp instrument suffices. Since you won’t be adding much to that load
(electric fan, fog lites) the meter will hold up fine. It may
occasionally pin on charge but will soon settle down, depending on the
battery’s condition when the alternator regulation circuit takes over
You could put a 40 amp ckt breaker in series with it to protect the
system overall. If you shunt the ammeter you will (depending the shunt)
have to estimate the actual readings (which were that not accurate
anyway).
A 2in. piece of solid copper #12 from screw term to screw term should
about double its capability (60 amps) and half its readings.