[Mitch//pa]

i use an external meter all the time when setting generator output and its a good idea not to trust any in dash guage until you can confirm its accurate with an ext meter. as mentioned digital multi meters tend to jump around (not give a steady reading) unless your using a good quality one but i have had success on some cheaper units you wont know till you try. put your meter in the amp mode and touch each lead to one of the junction box wing nuts on the firewall. this is essentially how the stock ammeter is wired into the circuit.