First let me appologize for that dirty engine, almost embarassed to post that!
I've posted in a few threads that I don't even run a choke but got to thinking since it's there I might as well use it. lol. High idle while closing the garage door is mainly what I'm after, it actually runs decent without the choke. Or I guess I could get a new remote for the door opener. lol. Anyway does anyone know what connector I am probing in the pic? I was thinking it would be for the factory choke. This is on a 1977 F-150. But the side I'm probing in the pic is a male connector and is hot with the key in the run position. The other terminal in that connector is female and shows 7.45 volts DC when the truck is running. I check for AC voltage thinking this must be coming from the stator in the alternator and my meter shows out of limits which I don't get. Its a Fluke 88 meter, high dollar. So do I want the choke hooked to a keyed 12 volts or do I want it hooked to 7.45 volts DC which is available only when the engine is running?

 

I believe the 12 volts is hot-in-RUN, which would have been used for the factory idle stop solenoid. The idle stop solenoid would have been mounted to the original carburetor and maintains the idle speed when the engine is running, but as soon as the engine is shut off, the solenoid retracts and lets the throttle plates shut (almost). It was used to prevent dieseling which was a common problem on "smog motors."

The 7.45 V signal (assuming you measured this with the meter set to DC while the engine is running) would be the stator signal of the alternator. This was used for the factory choke. It's convenient for the factory choke because it's only hot while the engine is running.

My engine bay is really dirty as well - but it looks like the BLUE wire is the hot-in-RUN 12 volt keyed power, and the WHITE (maybe YELLOW)? wire is the stator signal. Again the colors may be off and I couldn't tell if they have stripes because of how dirty my engine bay is, so don't quote me on that. This was on my '79 - from your picture it looks like the colors might be a little bit different. To be sure, you should check the voltage under different key positions.

I have an Edelbrock 1403 on mine with an electric choke, and it calls for 12 volts keyed power, so I just have it connected to the BLUE wire. You just have to be careful not to leave the key in RUN with the engine shut off - otherwise the choke will be burning power with the engine shut off and may open when you don't want it to. Ultimately you should connect it however your carburetor's manual says - whatever you do, just make sure it's fused.

I checked my stator voltage with the engine running on my '79 and it was 2.6 V. This was with my Fluke 179, which is a true RMS meter. The only reason I can think that it would say out of range on yours is because I don't believe the Fluke 88 is a true RMS meter, so it might be getting confused when it tries to display the RMS voltage (the stator voltage is the neutral point in a 3-phase signal, so it's pretty complex).

A non-true RMS meter assumes that the AC signal is relatively simple, and displays what it "thinks" the RMS value is by multiplying the signal amplitude by a scaling factor. For example, the AC signal coming from the wall outlet is actually (very roughly) a sine wave going between -170 and 170 V. A non-true RMS meter will correctly show the RMS voltage as 120 V RMS, and so will a true RMS meter. However, a true RMS meter will actually integrate the waveform to find the actual RMS voltage, hence the term "true RMS." As such, a true-RMS meter is the correct tool to use for complex AC waveforms. Of course there are restrictions with true RMS meters as well.

 

Ah, thanks! I didn't even think of the idle stop solenoid. When I got the truck it had a 351m with Edelbrock 4 barrel intake and Holley 600cfm carb so that solenoid was long gone.
Yes I got 7.45v DC with engine running. But I'll hook it to the 12v keyed then, thats where Demon says to hook it. I thought that might only be because finding a keyed 12v is a lot easier to explain than hooking it up the other way. lol. Maybe not the best hook up, just the easiest. The 12v keyed power is only on in the run position. Its not hot in accesories so I shouldn't have any problem with the choke opening when I dont want it to.
So you are running your choke off the connector I have pictured above correct? Basically the wire that originally powered the idle solenoid. Is it fused from the factory? I can check my manual at home but I don't have it handy here at work. If it is not fused where did you add one? Just a holder between that connector and the choke? It doesnt seem like that would look very nice. lol. Unless the air cleaner hides it.
I'll also look into the capability of my meter. Thanks for the help!

 

The connector in your picture is in the same spot and the same shape as mine, but since I can't make out the wire colors in your picture, I can't be certain it's the same. I'm also not sure if there are any relevant year-to-year differences between 77 and 79. However, I went and checked on mine, and on my connector, the BLUE wire is hot-in-RUN and has a male bullet connector inside the pigtail. The WHITE with BLACK stripe is the stator signal and has a female bullet connector inside the pigtail.

The idle stop solenoid should be fused, and this is what I depend on in my setup. On mine, the idle stop solenoid (BLUE wire) is protected with a 7.5 amp fuse in the fuse panel. It's labeled "ENG. SOL." for engine solenoid. You can see if this is the case on yours by removing the fuse and checking if you lose power with the fuse removed.

 

I always ran my Holley and Edelbrock carbs off of the stator wire and they work fine.