The REAL reason that GM started using voltmeters was because the design (yes, even the shunt resistor design) was that they had a tendency to literally burn up as they got older - setting fire to the wiring harness and the cab.
The ammeter design has to do an in-series type of circuit. This type of circuit demands that you use a low resistance in order to work properly. The problem with low resistance loads is that they devour power at an astounding rate. I can prove it mathematically.
P = VI, or Power = Voltage * Current (I).
and V = IR, or Voltage = Current (I) * Resistance.
but also I = V/R.
If you back substitute into the equations:
P = V (V/R) = V (squared)/R
Now lets say that the power resistor shorts out a little bit, and the resistance drops. The alternator is going to try to keep that voltage at 14 volts or thereabouts - it will keep pumping out that power as long as it is able, even in a short circuit condition.
Initially, the power dissapated across that resistor is low, because the resistance is - lets say - 100 ohms
P = V (squared)/R = 14*14/100 = 1.96 watts.
Now the resistor shorts out, for whatever reason. The resistance is now 1 ohm.
P = V (squared)/R = 14*14/1 = 196 watts
That resistor isn't going to last long disappating nearly 200 watts.
This is what in fact happened to a number of GM designs before the 70's when they started putting fusable links in their circuits and started using voltmeters instead of ammeters. They were basically designing out a fire hazard in their wiring diagrams.
Voltmeters read a circuit in parallel, so they use the highest resistance possible in order to get a good reading. When the gauge fails, it tends to fail in the open circuit position, rather than a short circuit condition. This pretty much eliminates the fire hazard in their early designs.
Whether GM admits it or not (and I'm sure they won't admit liability to a fire hazard) this is why they changed the circuit design.
