Diode in 3-Wire Alternator Activation Wire

[FuzzyDriver]

Looking through 3-wire alternator wiring diagrams on the internet and doing (too much) research I see some that have a diode in series with pin #1. This is the wire that goes to the ignition switch to turn the alternator on. Most diagrams show the idiot light in series with this wire. Some show both a diode and the idiot light. Some show nothing between pin #1 and the ignition switch (just the wire). Some show a resistor. Some show that the person who drew the diagram has no idea what they are talking about.

Just to throw a monkey wrench into it, one diagram had a Zener diode. The reverse breakdown voltage of the Zener was not given.

Someone before me installed an adapter (PT2288 male » diode » PT2288 female). But why did they do that?

So...does anyone here know for sure whether or not we need a diode between pin #1 and the ignition switch on our Comets when we are upgrading to a 3-wire alternator? And, if so, why do we need it?

Maybe, MAYBE they were experiencing engine run-on with the alternator supplying the ignition via pin #1. If this is true, then I won't need it because I am running pin #1 with a relay (so there is no connection between the ignition and pin #1).

[vicegrip]

The only time I have seen where a diode may be needed is when using some type of MSD box as pointed out in the instructions.I did find that out to be true on one occasion when the motor continued to run with the switch off.I have never needed a diode on any OEM type wiring setups.

[FuzzyDriver]

OK, that makes sense. I hadn't thought about aftermarket electronic ignition systems since mine is stock and has the diode. Well, it HAD the diode - I'm leaving it off.

Any idea if a resistor to simulate the idiot light is necessary? One site out in the wild, wild web says you have to have a resistor or you'll damage your alternator.

And thanks for the reply!

[comethead]

Not sure if this helps but there’s a paragraph here on getting the warning light to work https://www.falconparts.com/ford-falcon ... or-d18.htm
It’s for use with their mini VR set up so not exactly what you need.
FWIW I’m running this unit on my 64 Falcon and so far it’s worked great .

Joe

[FuzzyDriver]

Thank you for your reply. I read the article you linked. I don't think it will help. As you know our Comets don't have idiot lights, and my alternator is a Delco 7127 (Don't blame me! I didn't install it! 8^O I'm just making sure it's going to work right after I rewire the whole car!)

Have a good one!