There is typically a difference between meters, but it's fractional - being off by two volts indicates a problem with the on-board system and it's probably a defective display - defective in what it is reading, trust the off-board meter ahead of the on-board. It's kinda like the volt meter in cars and trucks - typically they only display a high and a low number and the indicator is somewhere in between. Both my F150 and Mercury Gran Marquis have similar gauges. The Merc seems to read low and the F150 high - With a multimeter, they both read about 14 volts at idle - the day my alternator failed and the battery went dead, you couldn't tell it by the gauge. Damn thing just died when the voltage got down below 10 volts. On the F150 it reads low 8 volts and high 16 volts - 12 would be 1/2 way between, so normal is just above 1/2 and the engine dies just below 1/2. Very helpful?
Shop Owner and Mechanic with over 50 years experience
Last edited by latebird; 07-07-2019 at 11:56 AM.