Well, I am just now catching up on some prior posts. It looks like a viable solution was found, but of course I feel compelled to offer some more info in case anyone else reading the posts in this thread has questions re: why the 1973 electric choke is wired "differently," and want more info re: where to find the Circuit #640 Red with Yellow Hash wire is located, and another alternative switched wire circuit power source is located. And, of course, I will begin with the stock 1973 carburetor choke cap wiring. If you are using a 1973 carburetor with the Ford electric choke cap, the info re: using the Alternator Stator terminal to power the electric choke cap is what you want to pay attention to. For anyone using an aftermarket carburetor with an electric choke cap, you will want to become familiar with the stock Stator terminal wiring, but read on for how and why the aftermarket carburetors are different - and how to wire those.
I am pretty sure the electric choke cap became available in 1973. I re-re-reviewed the 1972 wiring schematic and show no Circuit #4 (alternator stator) feeding anything other than the voltage regulator.
Anyway, (apparently) beginning 1973 Ford began to use an electric choke cap, where the power for their electric chokes came from the Alternator Stator Terminal (Circuit # 4, White with Black Stripe wire coloration). The Stator circuit is unlike any other circuit's current in the vehicle other than the alternator internally. First, its voltage is 1/2 of the alternator's output voltage. Second, it outputs AC
power, not DC
power. Also, it only outputs current when the alternator is actually producing power. In this way the Ignition Switch can be in the Run position, with the engine not running, and the Stator terminal will not produce output power. But, if the engine is running, and the alternator is operating correctly, the Stator terminal will output 1/2 alternator voltage in AC current.
The Stator current is enough to let the Ford electric choke cap to get warm and open the choke timely with a Ford carburetor. But, for aftermarket carburetors their requirement is to provide 12 volts of battery (DC) power to their electric chokes. Using the Stator current will work, kind of. But the aftermarket electric choke cap may not warm up fast enough to open the choke as soon as the aftermarket carburetor is designed to open. It is not be the end of the world if the Stator circuit is used incorrectly," as eventually the choke will open. But, by then you may have some fuel fouled spark plugs that will need to get cleaned off with some spirited romping and revving. Because the electric choke terminal is so easy to find and use on 1973 and later vehicles I see why it is so many engines with aftermarket carburetors end up getting connected like that.
Surprisingly, when I see an aftermarket carb choke heater connected to the Stator circuit, I do not hear anyone complaining about the carb running too rich when the engine is cold or warming up. So there is that to consider, under of the heading of, "If it ain't broke don't fix it." But, of course, I would take the time to ask a person with an aftermarket carb wired like that how the engine runs cold and during warm-up. If all is well, I tend to leave things alone. If the plugs are loading up with excess fuel I advise of an alternative wiring for the electric choke (after checking to make certain the choke cap is otherwise working).
If you are using a 1973 Autolite carburetor, the following does not pertain to you, but it may be good to be familiar with anyway.
For 1972 and earlier Mustangs with no Stator wiring to the choke cap, and even on other vehicles, I often see the aftermarket carb electric choke cap being powered by the Starter Relay's "I" (Ignition bypass) terminal. Although, like the Stator circuit used for aftermarket carbs, that wiring "may" work "well enough," it is not how I personally like to do things. First of all, only when the engine is being cranked over is the "I" terminal going to be getting "full battery voltage." But, that does not mean it will be getting 12 volts, as during cranking the battery voltage drops to anywhere from 9.6 - 10.5 or so volts. Once the engine is running, and the ignition key is in the Run position, the voltage for the "I" terminal is only 5 - 6 volts (or higher, but less than battery voltage), as that circuit is then being fed its voltage from "the far side" of the primary ignition circuit resistance wire (or ballast resistor for other systems). For a choke cap being designed to run with full battery voltage (12+ volts), 5-6 volts is really not going to do the trick, although like the Stator circuit powered choke caps it may end up working out eventually where the choke does open up after a while. Also, even if there is enough current available for the ignition coil to operate with the nominal drain of the electric choke cap being fed the primary ignition circuit's reduced voltage, it is less amperage being made to the ignition coil. It may all work just fine like that, but I prefer to leave the primary ignition circuit alone.
So, that then begs the question, "From where shall we get the battery switched power from," for the electric choke cap on an aftermarket carburetor? And TheRktmn offers an excellent solution with the Throttle Position Solenoid (TPS, Circuit # 640 Red Yellow Hash color) - it is my preferred option also. Using that circuit offers a few advantages. The TPS circuit is available on all 71 - 73 wiring harnesses, unless it has been cut off by someone. It is easy to find, easy to tap into, and definitely offers battery voltage only when the Ignition Switch is in the Run (and maybe also Accessory, but I am pretty sure Run only) position. But...
As easy as it is to "normally" find the TPS wiring on a 71-73 Mustang, that is not always the case. As it turns out, when we acquired our 1973 Mach 1, a prior owner had the Holley 735 CFM carburetor's electric choke cap (definitely aftermarket carb) to the Starter Relay's "I" terminal, which actually worked fairly well, although during warm up I would get some fuel fouled plugs until the choke did eventually open fully. When I first had the vehicle I suspected what had happened, but I just kept the revs up when the engine was cold or warming up, and in time the plugs cleared up and everything was fine. No serious harm, and I did not feel compelled to alter the choke wiring as I had other dragons to slay on that vehicle. But, in Car Season, 2022, that changed. I decided to rewire the electric choke "correctly," or at least as correctly as I could (one never knows what will unfold on cars this old).
I decided the rewiring of the carb electric choke cap would be a nice subject for a YouTube video., So, of course I recorded what I did, step by step. In the YouTube video's description I have some text re: the oem carburetor choke heating system, along with some additional YouTube links relating to this subject. But, the thrust of the primary video is how I rewired the electric circuit "more correctly." In the case of our 73 Mach 1, when I tried to find the TPS circuit and its plug, it was missing
. I am assuming a prior owner saw no need for that circuit with the Holley 735 CFM carburetor, and likely just cut it out from the underhood wiring harness to clean up the wiring a bit. It happens. I could have migrated back closer to the firewall and pulled the Red Yellow Hash wire out of the loom and spliced into it. But, I decided to take a different approach, one that would be more helpful for folks on older Mustangs that did not have an electric choke circuit.
The alternative 12 volt switched power source easily accessed under the hood of a Mustang is the Circuit #63 power wire that feeds the Windshield Wiper Motor. What makes this a nice place to tap into is it is easy to find, has plenty of amperage to handle the Wiper Motor as well as the electric choke, and is also easy to get to. The Circuit #63 wire is solid Red in insulation color. I opted to splice into that circuit on the "motor side" of the plug where the motor wiring is connected to the underhood wiring harness, as opposed to altering the underhood harness itself. That is a personal preference, where I like to leave the vehicle's main wiring harnesses untouched.
Relevant YouTube videos:
In the link below a Motorcraft 2150 2v carburetor is shown with info re: setting its idle speed and idle air/fuel mixture. From the opening frames and through much of the video the cool air hose and tubing from the passenger rear of the upper carburetor, and warm air return tube wrapped in asbestos cloth leading to the choke housing can be clearly seen. The electric choke is also visible.
The following link is a video showing where the TPS is located, as well as the Circuit #640
wiring harness lead location:
Another video that references the carburetor choke cap, and showing how to adjust the choke cap for leaner or richer choking is at the following link: