12v feed

[cruzin71]

Hi all, so if I understand this correctly, the only way to get a key'd hot 12v feed in my 71 is to either tap into the only 1 wire at the ignition switch that has 12v with key on or hook up a Pertronix relay because it can operate on slightly lower voltage to allow 12v to be tapped from solenoid?

 

[midlife]

There's a red/yellow wire that has a female bullet at the gang of plugs at the firewall (NSS/BU, engine gauge feed, etc.). I believe the molded connector is white. This line isw RUN-only and was used as a carb emissions device feed.

 

[mrgmhale]

Attached is a schematic snippet that ought to help you. It is from a 1973 manual, but there are only a few minor changes from 1971, and the 1971 schematic does not show that level of detail in the Mustang circuitry. One of the differences in 1973 is the Seatbelt Interlock is shown as part of the NSS/BUL. Most cars had those Interlocks disabled by manufacturers when they proved to be a huge PITA to deal with. For your 1971 you can ignore that Interlock circuit (circuit 32A/B).

My GoTo for circuitry schematics is Forel Publications:
https://www.forelpublishing.com/clickbank/index.html#Top
Their 71 and earlier schematics are not as complete as 72, and especially 73 schematics. The price for their PDF files is so low it was worth it to me to get 71, 72, and 73. But it is the 73 manual I use most, even for 71 and 72 vehicles, as there are only a few small changes (Throttle Position Solenoid, Electric Chick circuit in 73, aforementioned MSS/BUL/Interlock in 73, and emission control vacuum switches also in 1973 on some vehicles).

One place I get Switched Power from under the hood is the power circuit for the wiper motor. It is not only handy to access, it is protected by a fairly large amperage fuse. I use it mostly for headlight and/or electric cooling fan trigger power on our two 73 Mustangs. The same wiper motor wiring is relevant for 71 and 72 Mustangs. For 1971, 1972 and 1973 Circuit #640 (Red with Yellow Hash) for the Throttle Position Solenoid became available. I have a YouTube video showing where that wiring is located:

 

[Hemikiller]

The throttle position solenoid feed wire (Red w/yellow hash) is hot in both run and start. It's present on all 71-73 cars, and is powered through the 14A fuse in position 4 on the fuse box.

 

[jpaz]

I use the TPS wire for my electric choke and it works fine.

 

[Stanglover]

The throttle position solenoid feed wire (Red w/yellow hash) is hot in both run and start. It's present on all 71-73 cars, and is powered through the 14A fuse in position 4 on the fuse box.

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Ah! I always wondered what the arrowed plug socket was for on my 71. It may have been connected to the original carb, which I have and has no TPS on it (nor tag either for that matter) Thanks for that info.

 

[Stanglover]

On mine, I cut the pink resistor wire out from the under dash socket and replaced it with a 12 gauge (I think) wire directly to power the Pertronix Ignitor II. In hind sight, I ought to have used the relay, which would have been simpler. These cars are a total pita to work on under the dash, even with the seat out.

 

[mrgmhale]

The throttle position solenoid feed wire (Red w/yellow hash) is hot in both run and start. It's present on all 71-73 cars, and is powered through the 14A fuse in position 4 on the fuse box.

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I missed seeing it on the 1971 Schematics or Mustangs. I will make note of it on the 1971 Schematics I have on file. Thank you.

 

[cruzin71]

Thanks for all the help, I did locate the red/yellow wire. Thanks again

 

[Ron Tanzi]

I use the TPS wire for my electric choke and it works fine.

That is what I use as well and it works perfect.

 

[midlife]

Whew. I'm vindicated!! I knew it was there...

 

[mrgmhale]

The throttle position solenoid feed wire (Red w/yellow hash) is hot in both run and start. It's present on all 71-73 cars, and is powered through the 14A fuse in position 4 on the fuse box.

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I scoured the 1971 Colorized Wiring Schematic from Forel Publications, and finally found where Circuit #640 is shown to feed the carburetor Throttle Position Solenoid, at least for 6 cylinder engines per the schematic. I saw no reference to v-8 engines, but can't believe the same harness would not be used. Thank you, HemiKiller. I have annotated my 1971 schematic file to show that circuit for the TPS more clearly. It is a nice way to tap into 12 volt Run power.

 

[mrgmhale]

I use the TPS wire for my electric choke and it works fine.

The electric choke circuit (#4) was introduced in 1973, and runs from the Alternator Stator Terminal with a White wire with a Black Stripe. It is used for factory electric chokes, and puts out 1/2 of the Alternators normal voltage, and in AC current, not DC. Pretty every aftermarket carburetor I have seen with an electric choke specifies their electric choke needs to be fed with a 12 volt DC circuit. In that case, circuit #640 is a great place to tap into that kind of power. I only recently became aware of Circuit #640 having been included in the 1971 Mustang harness. But, sure enough it is in the 1971 schematics.

An aside, we have a 73 Mach 1 with a transplanted 351W (not C) engine using a Holley 750 CFM carburetor (engine is not stock). The prior owner continued to use the Stator Circuit #4 to power the Holley electric choke. When warming up the carb tends to runs too rich as the choke is not opening fast enough to prevent fuel fouling the plugs until it is eventually fully opened. Luckily, the plugs eventually clear up once the choke is fully off, with a few revs to 2,500+ RPM. I suspect if I were to switch to Circuit #640, as opposed to continuing to use the current Circuit #4, the 12 volt current will cause the choke to open far earlier, and the fuel fouling when warming up will no longer be an issue. That is a project for this new car Season (2022). Easy enough to do.

 

[Hemikiller]

Electric chokes began to be using on the Mustang in the '73 model year, and were then only a supplemental device, not a standalone system. All 71-73 chokes were heated via a hot air stove in the intake or exhaust manifold. The heater hose was also used as a supplemental heating strategy to limit choke "on" time and reduce emissions. The 73 electric choke had an internal thermistor that activated it only above 60°F, to open the choke quickly in warm conditions.

The stator wire ( white w/black stripe) only provides around 6V with the engine running, so it may not work satisfactorily with a 12V choke. Another strategy is to use the stator to energize a relay to power the choke with a full 12V.

 

[Stanglover]

Well guys, now you've got me thinking where the 670 Holley carb (now replaced with a new 670 Holley carb) that was on my car when I bought it, got its choke 12V feed from. I have always "assumed" it was off the correct keyed 12V circuit. It does seem to work fine, no noticeable problems. I have the original carb off the car, but there is no TPS on it. That of course could have been removed before I got it with the car.
Spring is finally here and now time to get the old girl running again, so I will have to go diggin and see what's what.
Interesting thread here guys, thanks.

 

[mrgmhale]

Electric chokes began to be using on the Mustang in the '73 model year, and were then only a supplemental device, not a standalone system. All 71-73 chokes were heated via a hot air stove in the intake or exhaust manifold. The heater hose was also used as a supplemental heating strategy to limit choke "on" time and reduce emissions. The 73 electric choke had an internal thermistor that activated it only above 60°F, to open the choke quickly in warm conditions.

The stator wire ( white w/black stripe) only provides around 6V with the engine running, so it may not work satisfactorily with a 12V choke. Another strategy is to use the stator to energize a relay to power the choke with a full 12V.

HemiKiller,

I need to look at our unmolested 73 Mustang Vert's 302 2v 2150 carb. I do not recall seeing an exhaust manifold heater tube on it, just the electric choke. But, at the time I was working in that area I was only interested in the electric choke as the choke was not coming to a fully opened position for a long, too long, period of time, and the plugs were loading up. I did a quick check on the electric heater element and found it was getting current, but no AC amps flowing through the circuit when cold. A new cap fixed the issue. I will look at some photos and videos to see if I accidentally got the choke area of the carb in frame. I believe I may have with at least one YouTube video I posted a while ago.

No matter, there are so few folks who understand about the 6v AC at the Stator terminal, vs 12v+ DC at the output terminal of the alternators. It is refreshing to see your posts in general. And it is nice to see affirmation of things I have seen with and learned about, these First Generation pony cars over the past many decades. I love working on the pony cars. Every time I work on, tinker with, or drive one I am 19 again. And lucky me, it turns out Lynda (wife) is a MustangShelby Chick! She found both of out 73 Mustangs, and was very helpful in me navigating the waters when we purchased the 69 GT500, 97 Del Sol SI (another one she found), and the 2020 GT500. And she is the videographer for most of the How To YouTube videos posted under my channel (Gilbert Hale).

Have you seen, and downloaded the various emission control engine vacuum schematics I have posted in other threads in this forum? If not, they are attached here. The 67-72 calibrations were put out by The Mustang Barn. I, with their permission, assembled all of their PDF files into a single PDF, group by year and sorted by engine. They made the PDFs available for no fee, and I did the same with the consolidated PDF file. Feel free to share with anyone you deem needing it.

The other file is a snippet from the 1973 Mustang Shop File, Volume 6 where finally we have some good calibrations made available. I added 3-15A and 3-16A for the 73 351C 4v (man and auto tranny) calibrations. I do not recall where I got those from off hand. I should have included that in the two affected pages. No matter, they are embedded in the snippet. I may have some annotations in the snippet, but I do not recall seeing errors that I felt needed to be corrected in that snippet. In the 73 electrical schematics I have pointed out a few errors, and added some enhancements that I and others have found. If that interests you email at
[email protected]
and ask for my copy of the Forel Publications 1973 electrical schematic. I have purchased a few copies of that publication, thus have no problem making it available to you, as I am certain you will benefit from the items I enhanced and annotated...

Of course, anyone else reading this message is also welcome to download and use the attached files also. I do not mean to hijack the thread in order to try to have a secretive message posted for HemiKiller only... I see plenty of other folks also with a high level of familiarity and great comments/answers/suggestions "out here," not just HemiKiller and (dare I say?) myself. it is just that his comments re: the electric choke and Stator terminal struck me as being really out there in the land of rarely seen details on these older vehicles. It is refreshing...

 

[mrgmhale]

Well, it turns out I did a YouTube video on the 73 Mustang's 302 2v 2150 carb, focusing on the electric choke system, and there it was. In living color, evidence that HemiKiller is absolutely correct the choke system having the electric choke added as a supplemental choke cap heating device. In the first few frames, and several other sections as well, it is very clear that there is a filtered air inlet tube leading down to the passenger side exhaust manifold choke heating stove area, and another asbestos wrapped hot air tube connected to the choke warm air inlet.

Now the Ford oem use of the 6v Stator AC current for their carbs, and aftermarket cars requiring/needing the full battery voltage DC current for their carbs, makes more sense as they do not (tend to) have hot air tubing that we see on oem Ford carbs.