Hi Richard/SIA,
If the amp meter in your TF truck is the same as the one in my '50 AD, it maxes out at 40A. I found that I needed the shunt with a 63A alternator. On cold startup the initial charge way exceeds 40A.
The .33 ohm 300W resistor array pushes the full scale deflection of the amp meter to around 70A -the reason this isn't a precise figure is that the Chevy amp meter internal resistance and therefore its current reading varies with temperature, the shunt resistors have a relatively flat temperature coefficient.
There isn't any kind of coil internal to my amp meter, it's the so called "Moving Iron" type that relies on the current flowing through the amp meter's metal frame which creates the magnetic field to move the needle by attraction or repulsion.
Here's a US supplier of the 100W 1 ohm resistors you need.
http://www.caddock.com/Online_catalog/power/Power.htmlHopefully they'll be able to sell you retail size quantities.
Stuart.
