It is worth repeating. I keep coming back to this story of vulnerability in Henry Ford's basic electric system that could cause you a lot of grief. It is a subtle and sometimes baffling occurrence that will keep you scratching your head for a long time. Your entire lighting system could suddenly extinguish as you find yourself driving in the dark some night, baffled because your car is still running fine and you may be able see enough in the moonlight to make it home -- or not. Having reached your destination you may observe a burning smell coming from under the hood. Again you ignore the cause and head to bed because it's late.
The next morning you get in your car to start it and find the starter is turning nicely but the engine will not kick over. More head scratching.
If you are an accomplished mechanic and you are still baffled I can fully understand that. If you are not familiar with “things under the hood” then you get the car serviced by a qualified individual. Knowing that there is plenty of voltage at the battery the first thing that person will do is check the distributor or ignition coil and discover there is no current supply there or to anywhere else for that matter. By doing more diagnostics it will be discovered the ammeter is not passing current. The flaw in the Model A electrical system is not so much a design deficiency as an application issue. Ammeters were traditionally installed using thumb nuts. Believe it or not you can still buy them (PN A-10864). This has to be the worst application I have ever encountered, it should be relegated to the making of model cars not Model A Fords.
In any car, as in the Model A, the generator current feeds the battery naturally through an ammeter. All reasonable loads, with the exception of the starter current, are connected on the generator side of this ammeter. The thumb nuts are critical because they are tightened by thumbs are therefore not necessarily tight enough which can cause heating to develop under load at the ammeter terminations with the inevitable serious results of an open circuit. The generator, having no voltage regulation and relying on the battery alone to maintain a modicum voltage range, is now only connected to vulnerable components such as lamps that can be damaged by a sudden voltage rise. This high voltage is also impressed internally to the generator field coils which inevitably cause the generator to heat up and if the problem persists will damage the generator.
Replace those thumb nuts with ones that you can tighten using a proper wrench.
Happy Model A'ing.
Yvon Picot
