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

MikeLDrew at aol.com MikeLDrew at aol.com
Tue Oct 16 11:39:45 EDT 2012


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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