The Ford Barn - View Single Post - OT: Question about auto electrical test device

rotorwrench · 08-07-2025, 03:49 PM

Polarity sensitive items need to be connected in like polarity. Positive to positive and negative to negative. Which is power and which is ground is not part of the equation. Only the polarity is relevant. Connecting it the wrong polarity will likely damage it.

Ignition systems are affected by polarity in the way the primary coil is wired. A coil will function with the wrong polarity but the spark efficiency will just be somewhat less. This is why a positive ground system has to have the power connected to the negative side of the coil if so marked. The breaker points are connected to the positive side so in effect, it is properly connected in these systems. A lot of old coils were not marked as positive or negative in the Positive ground era. They were marked SW or IGN for power and DIS for the distributor breaker connection. This is why it's a good idea to have a polarity tester for the old pre 1956 Ford products.

As far as amps are concerned, the ampere meter will register as a discharge amp flow if connected backwards but it won't hurt the ammeter. On a meter that only shows one way then it may not show flow at all if backwards. A generator is not an alternator but both put out a DC voltage. The generator output is DC but the alternator has to be rectified from AC to DC.

08-07-2025, 03:49 PM	#7
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 17,410	Re: OT: Question about auto electrical test device Polarity sensitive items need to be connected in like polarity. Positive to positive and negative to negative. Which is power and which is ground is not part of the equation. Only the polarity is relevant. Connecting it the wrong polarity will likely damage it. Ignition systems are affected by polarity in the way the primary coil is wired. A coil will function with the wrong polarity but the spark efficiency will just be somewhat less. This is why a positive ground system has to have the power connected to the negative side of the coil if so marked. The breaker points are connected to the positive side so in effect, it is properly connected in these systems. A lot of old coils were not marked as positive or negative in the Positive ground era. They were marked SW or IGN for power and DIS for the distributor breaker connection. This is why it's a good idea to have a polarity tester for the old pre 1956 Ford products. As far as amps are concerned, the ampere meter will register as a discharge amp flow if connected backwards but it won't hurt the ammeter. On a meter that only shows one way then it may not show flow at all if backwards. A generator is not an alternator but both put out a DC voltage. The generator output is DC but the alternator has to be rectified from AC to DC.