Getting ready to do some alternator/voltmeter rewiring and want to make sure I understand all of my options. 1986 460 with a 2g alt currently.

My focus right now is the LG/R wire. It's my understanding that this is the alternator excite wire and is the only wire retained (for excite purposes still) from the factory harness when doing a 3g swap.

**Let's remove the LG/R wire from the alternator for the next couple of questions and pretend it isn't in the 2g plug - so just a loose wire down by the alternator.**

Is that wire a good keyed-hot 12v source? From the EVTM it looks like it's hot on RUN but I might not be looking at all of the scenarios.

If so, is the LG/R a good candidate to both 1) trigger a relay for the carb choke and 2) trigger a relay for the hot (red) wire to the now-converted voltmeter? Will it be hot when needed and dead when it should be for both of those functions to work correctly.

While truck wiring isn't necessarily new to me, doing it 'right' where I am proud to look at it afterwards is.

 

No. Use that wire only for the alternator. You will get voltages on that wire from the alternator regulator. That wire really should be modified if you want it to be wired like the 3G was in it's original application. In it's original application there was a dash light wired in series with this wire. But it must work ok without this bulb in the circuit if people are using it like that.

 

Haven't really looked into alternator wiring before so I will fully admit that my understanding may not be correct.

You are saying the voltage on that wire comes FROM the alternator?
So if the wire isn't attached to the alt it won't have any voltage on it? I thought the excite wire (still under the assumption that is what this wire is) sent voltage TO the alt in order to kick start it charging.

I say it's the only one used in the 3g swap based on this post: https://www.ford-trucks.com/forums/8...-pictures.html. If I am reading that incorrectly please call me out on it.

For what I am wanting to do, the LG/R wire would [theoretically] work perfect for what I want based purely on location and availability. If possible, I'd like to know everything about that wire before it's either ruled out or accepted. - that's why I may push for more info/beat a dead horse even after it seems like it's a done deal.

For my own curiosity - when you say it should be modified in the 3g swap, are you meaning just adding the light into the circuit or are there more changes that should be done?

Note: did some more looking and the LG/R wire may (lots of mays and ifs) not work for the carb choke if it's hot when key-on vs run. Still trying to decipher the ignition switch diagram.
Edit to my Note above: guess even if it is hot on key-on vs run I could hook up an oil pressure switch (as mentioned on Gary's site) after the relay but before choke itself.

 

The diagram below shows how it was originally wired. When the alternator is not turning, and the key is turned and puts voltage on the dash light, the light bulb lights up, the lightgreen on the alternator side is close to ground. When the engine starts the alternator recognizes this small voltage from the other end of the lightbulb and that brings it online to charge or like you said "excites" the alternator. When this happens, and the alternator does have output, the voltage on the lightgreen/red from the alternator regulator raises up high. This turns the light off in the dash, since it now has close to 12v on the alternator side, and 12v on the ignition switch side.

Throw a belt, the alternator quits charging, the lightgreen/red on the alternator side drops voltage, and the light in the dash will come on to tell you there is a problem. Broken belt, faulty alternator, loose wire, engine stalls, etc will all turn the light on.

 

Thanks! But isn't that the equivalent of saying that without the lightbulb in the circuit the LG/R will always be 12v when the key is on? Either from the ign switch (if alt not turning) or from the alt (if turning)?

Side note: You are right about it sounding like the LG/R should be modified to have a light in the circuit during the 3g swap instead of straight connecting it.
Pulling from: http://www.prestolite.com/literature...tor_wiring.pdf

To go back to the root question, let's assume the alternator isn't in the truck at all but the truck is magically running without it. The LG/R wire is just dangling down there now. According to the diagram (per the Hot In Run label) and the quote from prestolite (and given that there is no light in the circuit), doesn't that confirm that the wire will be hot with the key in run? I am getting that it might not be 12v when the light is in the circuit but won't it be 12v if the light isn't present?

I just want to make it clear: While I would really like the LG/R wire to work for what I want, I am not trying to argue until I convince myself I am correct and disregard all wisdom given. I am just wanting to fully understand why it won't work if that is the case.

 

I saw your other thread. If you do not hook it up to the alternator regulator, you could use it for a hot wire for small loads. If you hook it to the alternator I would not use it as a hot feed wire.

 

That's great! Thanks. Yeah, I am trying to learn when to use relays and it was my understanding that I wouldn't want to put more than already is on the ignition so am hoping it will be ok to use as a relay trigger.