Positive Coil Circuit Power Rating

basstrix · Nov 30, 2024

I'm looking for the power rating for the circuit that powers the positive coil terminal. I looked through the schematics and it appears the power is supplied from the battery, to the starter relay, through the heavy yellow lead, up to the start switch connector. The yellow lead is protected by a fusible link. I wasn't able to trace any further to determine if there was a specific fuse protecting that circuit or if its power came through the yellow lead.

After having switched to Duraspark I, this terminal is unused and I'm considering using it to power an electric choke. The choke coil calls for 10A fuse protection.

midlife · Nov 30, 2024

Fusible link terminates about 9" from ring connector to an OEM splice. I believe you're possibly referring to fusible link on the alternator harness? No, in either case, that power is always hot and may drain your battery. You want a RUN-only line. There's a red/yellow wire near the gang of plugs on the headlight harness, or it is part of the engine gauge feed harness. You can tap into that wire for your electric choke. You can also use the STAT terminal on the alternator for an electric choke.

basstrix · Nov 30, 2024

midlife said:
Fusible link terminates about 9" from ring connector to an OEM splice. I believe you're possibly referring to fusible link on the alternator harness? No, in either case, that power is always hot and may drain your battery. You want a RUN-only line. There's a red/yellow wire near the gang of plugs on the headlight harness, or it is part of the engine gauge feed harness. You can tap into that wire for your electric choke. You can also use the STAT terminal on the alternator for an electric choke.

The schematic I looked at showed a yellow wire at the ignition switch. That wire traced back to a fusible link and then to the battery pole of the starter relay. Did I interpret the schematic incorrectly?

Edit: Here's the diagram I'm referring to.

The question I have is, what is the OEM + coil circuit rated for?

midlife · Nov 30, 2024

Sorry...I misinterpreted your original statement. That is BATT power, and it fed by the fusible link and if that blows while the car is running, the alternator output can feed the ignition switch. Since you asked about the power going to the + side of the coil, that line is switched to power at the ignition switch. What line is it that you believe you can tap into for your choke? I'm lost as to the specific terminal that isn't being used...

basstrix · Dec 1, 2024

The + coil terminal is what I'm asking about (power rating). When I switched over to Durspark I, that terminal was left unused. The choke coil calls for 10A. I would think an ignition coil would draw somewhere around there. I no longer have the factory tach in that circuit....went with a Rocketman conversion tach.

TheRktmn · Dec 1, 2024

Use the old primary circuit to fire a relay that turns a power feed from the battery on/off.

basstrix · Dec 1, 2024

An internet search says the ignition coil draws 3-4 amps. I measured the choke coil and it's drawing 1-2A.

midlife · Dec 1, 2024

Typically for a 16 gauge wire, expect something on the order of 10 amps max. DC.

Don C · Dec 1, 2024

basstrix said:
I'm looking for the power rating for the circuit that powers the positive coil terminal. I looked through the schematics and it appears the power is supplied from the battery, to the starter relay, through the heavy yellow lead, up to the start switch connector. The yellow lead is protected by a fusible link. I wasn't able to trace any further to determine if there was a specific fuse protecting that circuit or if its power came through the yellow lead.

After having switched to Duraspark I, this terminal is unused and I'm considering using it to power an electric choke. The choke coil calls for 10A fuse protection.

The stock coil with the 1.4 ohm resistor wire and 1.4 ohm primary resistance coil draws about 4.3 amps with the engine not running, ignition on, and points closed. When the engine is running it draws about 2.4 amps. The wire to the coil is not suitable for powering the choke, as the portion under the dash has a built-in resistance. I'm like Midlife, not sure which terminal you are referring to that is unused, the terminal on the ignition switch, or the push on angle connector that originally connected to the coil.

I also agree that the red/yellow hash wire that is hanging on the engine side of the firewall and has a white connector would be the best for providing 12 volts to the choke. However, if you turn the key to run and don't immediately start the engine that wire will be hot and cause the choke to open before you start cranking the engine, or if the engine doesn't start immediately the choke will start opening. That is why Ford used the stator terminal to power the choke, it only provides power to the choke when the engine is running.

The Ford choke has a resistance of about 9 ohms, which means it has a current draw of about 1.4 amps when connected to 12 volts.

Hemikiller · Dec 2, 2024

I'm using the Red w/Yellow hash for the coil on my 71 Country Squire. Don is spot on, it's hot with the ignition, so you need to be mindful of the choke starting to open immediately. Other than that, it functions perfectly. That circuit has a 14A fuse.

basstrix · Dec 3, 2024

There have been a couple of comments that the factory coil power lead is unsuitable to power an electric choke. For the sake of understanding, why is that so? I understand it's a resistor wire...which means it will provide a reduced voltage....probably around 9 volts. My understanding of basic electricity says it may take longer to warm the resistive element in the choke and may not get as hot. I would think that ambient conditions would play a part in how much heat (equates to power) the choke needs to open, no?

basstrix · Dec 3, 2024

Hemikiller said:
I'm using the Red w/Yellow hash for the coil on my 71 Country Squire. Don is spot on, it's hot with the ignition, so you need to be mindful of the choke starting to open immediately. Other than that, it functions perfectly. That circuit has a 14A fuse.

Is the Red/yellow hash the power to Idle Stop Solenoid? If yes, I'm using that lead to power the Duraspark-I ignition.

Hemikiller · Dec 3, 2024

basstrix said:
Is the Red/yellow hash the power to Idle Stop Solenoid? If yes, I'm using that lead to power the Duraspark-I ignition.

Yep, that's it.

Positive Coil Circuit Power Rating

Help Support 7173Mustangs.com:

basstrix

Well-known member

midlife

Shorts checker

basstrix

Well-known member

midlife

Shorts checker

basstrix

Well-known member

TheRktmn

Well-known member

basstrix

Well-known member

midlife

Shorts checker

Don C

Fords Forever

Hemikiller

Well-known member

basstrix

Well-known member

basstrix

Well-known member

Hemikiller

Well-known member

Similar threads

Latest posts

Positive Coil Circuit Power Rating

Help Support 7173Mustangs.com:

Well-known member

Shorts checker

Well-known member

Shorts checker

Well-known member

Well-known member

Well-known member

Shorts checker

Fords Forever

Well-known member

Well-known member

Well-known member

Well-known member

Similar threads

Join the conversation!

Join today and get all the highlights of this community direct to your inbox. It's FREE!