[DeTomaso] Factory Shunt? was Re: Ammeter, was Re: Key difficult to turn

[Guido deTomaso]

Yes, interesting.  Thanks for all the input.

I notice double thin nuts on the NOS ammeter photo Mike provided, which adds 
further sheen of authenticity.  Do I also see washers between the plastic 
insulators and the first set of thin nuts?

A postulation is that only replacement ammeters from the parts organization 
had/have the shunt.  Customer has for whatever reason burned up an ammeter, give 
him more failure-resistant one as a replacement.  Tough to prove a negative 
though, 40 years later.

Curious how is it known that an internal shunt exists, without opening up the 
ammeter to find it.

Well, two completely different projects which happen to be physically adjacent; 
run a shunt or not, run some or all of the "red ignition wire load" off a third 
wire to the ammeter using relay(s).

Is there a popular relay choice between the ubiquitous "Bosch style square box" 
and the equally ubiquitous "Ford Starter Relay Form Factor"?.  Not sure a single 
30A square box is appropriate for all of the "red wire" current.

Thanks,

GD


 



________________________________
From: Roland Jaeckel <pantera874 at t-online.de>
To: MikeLDrew at aol.com; SOBill at aol.com
Cc: detomaso at realbig.com
Sent: Tue, October 16, 2012 8:45:01 AM
Subject: Re: [DeTomaso] Factory Shunt? was Re:  Ammeter, was Re:  Key difficult 
to turn

I have two new on the shelf. One 50 amps with external shunt and one 75 amps 
with internal shunt.

Roland Jaeckel
----- Original Message ----- From: <MikeLDrew at aol.com>
To: <SOBill at aol.com>
Cc: <detomaso at realbig.com>
Sent: Tuesday, October 16, 2012 5:39 PM
Subject: Re: [DeTomaso] Factory Shunt? was Re:  Ammeter, was Re:  Key difficult 
to turn



In a message dated 10/15/12 20 45 58, SOBill at aol.com writes:


> This quite interesting.
> 
>>> I think so too!  Having seen several ammeters with the same-looking
external shunt, I took it to be an OEM piece (especially since one came out of
a Ford box that way), but who can say for sure?
> 
>  >At http://www.panteraplace.com/Electrical/Taylor%20amp9.jpg is a picture
> of the solid brass bar which is internal to the ammeter and which connects
> the two external terminals together.
> 
>>> I presume both Pantera ammeters were set up the same way, but do we know
for sure?

I think the overwhelming majority came with 75 amp units, but the very
first cars (I think) came with 50 amp units.  I think the Mangusta uses a 50
amp ammeter.


> >This bar is about 0.25 inches wide and 0.0625 inches thick: a quite
> substantial electrical dead short between the ammeter terminals. If the 
ammeter
> terminals are properly mounted and there is no corrosion on the
> connections between the brass bar and the terminals no external shunt should 
be
> needed. If an external shunt has a significant effect, there is a problem 
>inside
> the ammeter.
> 
>>> I know that adding a shunt has long been advised to help with the
jumping-needle syndrome; depending on the size of the shunt, it would reduce the
deflection of the needle for a given amount of current flow, as some would
pass through the shunt instead of through the gauge.
> 
> > The current thru the internal brass bar creates a magnetic field
> which is sensed by a small magnet on the needle assembly and the needle moves 
>to
> indicate current flow.
> 
>  The reason the needle jumps around is because the only damping of the
> needle movement is the friction in the needle mounting pivots ...... not a
> consistent or reliable damping method after 40 years.
> 
> >>>I think the shunt goes a long way towards helping, but I think the real
culprit is the ancient mechanical voltage regulator.  Swapping for a modern
electronic regulator seems to cure the jumping-needle syndrome.

Mike (with a three-wire, internally regulated GM alternator, shunt, a
working ammeter, and no needle jump....)
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