The Ford Barn

rockfla · 03-04-2025, 09:20 AM

Why does polarity matter to some Volt meters/testers....For example, why will some testers test 6.3V on a V8 system going from black to negative and red to positive ie ground to ground, hot to hot, but not the other way around? Its the same voltage either way, right?

I have a digital Power Probe single wire voltage test probe and I can touch the alligator clip end on the negative terminal of the battery and the probe/meter end on the positive and get 6.3V reading....Do the reverse and "nothing", a blank screen. OR I can connect the ground clip to the ground strap on the firewall to the motor and try and test the voltage at the coil with it and get NO reading, blank screen. WHY? The ground strap to the battery AND the ground strap to the motor are both grounded off the firewall.

03-04-2025, 09:20 AM	#1
rockfla Senior Member Join Date: Mar 2014 Location: Jacksonville FL Posts: 4,806	Question for the "electrical" Guru's Why does polarity matter to some Volt meters/testers....For example, why will some testers test 6.3V on a V8 system going from black to negative and red to positive ie ground to ground, hot to hot, but not the other way around? Its the same voltage either way, right? I have a digital Power Probe single wire voltage test probe and I can touch the alligator clip end on the negative terminal of the battery and the probe/meter end on the positive and get 6.3V reading....Do the reverse and "nothing", a blank screen. OR I can connect the ground clip to the ground strap on the firewall to the motor and try and test the voltage at the coil with it and get NO reading, blank screen. WHY? The ground strap to the battery AND the ground strap to the motor are both grounded off the firewall.