There's something to be said for that. The alternator light goes inline in the exciter wire so when the ignition is on but the alternator is not working it sees 12v on the battery side and 0v on the alternator side and illuminates. When the alternator is running properly there is 12v on both sides and the bulb will turn off. If the alternator is on its last legs and or loaded down the light will be dim and may go out if you rev it. The diode is to prevent back feeding. It acts as a check valve that only permits current flow in the battery to alternator direction. Without it the ignition may not turn off the car (depending on how it's wired) and even if it does there can be some nasty current spikes and whatnot at shutdown which may pop the fuse for that circuit. Edit: Illustration from our own forum: https://www.jalopyjournal.com/forum/threads/alternator-idiot-light-necessary.1195329/#post-13605725
What gauge wire should be use for the diode shunt wire?.... { from battery ( diode shunt in middle) then wire to alternator.}??? my alternator in 78amp, I'm using 8ga wire from alternator + I am using 2 of the diodes that you said to use
enough to handle 40a [assuming the ammeter an handle 35a] An 8 gauge wire typically has an amp rating of 40a.
I'm just asking..could you bridge the posts of the ammeter with the diode shunt like a piggy back add on ? would that work in the same way ? like I said just asking.
That is a different question to how you worded it above The shunt wires can be added directly to the posts on the ammeter [it is the same circuit] and the shunt only needs to be 8 gauge. BUT one caution. The main harness wires [Battery to Ammeter, Ammeter to Alternator] would need to be able to handle 78a [4 gauge] You could "piggy back" upgrade 2 x 8 gauge wires to the Ammeter OR simply connect the shunt direct from the Alternator to the Battery [which is literally the same thing]
Thanks for that information. That was my plan all along..But I thought I would ask first. I could not see why it would'nt work. Thanks again.
Update on what I decided to do. I swapped out the alternator for an early 60’s 30 amp Ford generator and with Charlie Stephen’s advice, used a piece of 10 gauge wire as a shunt on the back of the amp gage. I also used 10 gauge wire from the voltage regulator, through the amp gage to the bat post on the starter. I am not concerned on accuracy and just need needle movement. I may install a volt gage, which I will hide, as I don’t like the multi gage clutter on the dash.
A good place for that extra gauge might be around the steering column (common part from Model A suppliers) or in the kick panel upholstery on the right side of the car. Charlie Stephens
More advice please. I installed a shunt wire as per Charlie’s advice and no reading on the gage, shortened it and thinner gauge, very little movement on the needle. Removed it all together, full deflection. My question is, it’s a 20 amp gage, 30 amp generator, will this be an issue thinking that there must be some tolerance on the gage itself.
I'm replying to this since you seem like you might be the guy to help in my situation. I do like the idea of the diodes and understand the concept well enough. However, I am currently (pun intended) wiring a '50 Ford with an inductive ammeter. It does not have a number scale, just charge and discharge. I am using a regular Delco 10SI 63amp alternator. Because it's an inductive pickup, there is obviously no voltage drop across the guage like a normal ammeter. So, in this case, will just running parallel wires do the job of dampening the needle movement? Or will it even be affected by higher amperage? The only reason I'm concerned with it at all is just to avoid mechanical damage to the guage if it pegs. Im not worried in the least about accuracy since it's just charge/discharge indication.
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