Stock Ignition Coil Resistor

[stanz]

I have an early 74 Bronco with a 302 - it has the points distributor. My question is whether the stock coil uses an external resistor or not... I don’t see one and wasn’t sure if one was integrated into the harness.

As I’m looking at replacement coils, even NOS ones, they almost always state an external resistor is required. Trying to understand if I already have one and don’t see it or if I’ll need to add one.

 

[Steve83]

No, and yes. Copper wire in the 70s wasn't as pure as it is today, so it had more resistance. So a long, small-gauge wire could be used as a resistor, and that's what 16 R/Gn is in this diagram:

(phone app link)


In some trucks, it has pinkish (old dirty) insulation near the ignition switch, like in previous years.

(phone app link)

 

[Broncobowsher]

Yes, there is a resistor wire. It is a special wire buried under the dash. More than just a small gauge copper wire. You don't cut or splice into it, ever. Even the insulation is special.

If you set the engine so the points are closed, turn the ignition on, and measure the voltage on the positive post of the coil it will only be 8 or 9 volts. That is the resistor wire doing its drop. (if you check with the points open there is no load so you will see battery voltage on both posts of the coil). Resistors only drop voltage under load.

 

[Pa PITT]

Bronco bowsher that's the best description of the resistance wire I've ever heard .
But I still don't totally understand it.
I had a chevy truck .. That had a bad Resistance wire ... When we finally figured it out. It took a section of 34.6 inches of that wire.
... But when we got it fixed it was correct.
... I'd High 5 YOU FOR THAT ANSWER. Good Job young man.

 

[stanz]

Thanks all! This is just what I needed, very clear, very thorough!

Part II... I’m thinking of dropping the points and going with this distributor (https://www.summitracing.com/parts/...Sm9AombpR7rBIe2tVQRUAFRxZioVAGvBoCzd4QAvD_BwE) and this coil(https://www.summitracing.com/parts/...M1DzrXdcWGAkC-IV-fNKa4e2uCbA91SBoCnQYQAvD_BwE)...

Would I want to go full 12v to the coil (leaving the stock wiring in tact but using it to trigger a relay for the 12v)? Or would I just use the stock wiring as is with the pink resistor wire?

 

[Broncobowsher]

Use the "I" wire at the voltage regulator. It is hot in run without the resistor. I would run a wire off the voltage regulator and splice it into the stock coil+ wire. This way you are sure to keep the voltage at start and you don't have any spare open ended wires.

My other option would be to cap the stock coil+ wire. Disconnect the stock "I" wire on the starter solenoid. Make a new wire off the "I" terminals of both the voltage regulator and the starter solenoid and run that to the ignition.

 

[DirtDonk]

Pretty sure that's not the case. The I wire is AFTER the resistor wire, and the I post on relay is only hot with 12v in START. So it should have the same voltage while the engine is running. The Brown wire only joins up with the Red w/blue wire at the firewall connector inside the engine compartment.

So unless the starter relay has failed internally and the I post has power all the time, it's not a good source for 12v in RUN.

Paul

 

[Broncobowsher]

The branch to the voltage regulator happens before the resistor wire. Thus the "I" on the regulator gets full 12V when in run

The "I" on the starter solenoid is after the ballast resistor. The resistor bypass. Gives the ignition a little extra shot of voltage to cover for the lower voltage due to the load of the starter.

I've seen enough cases of the ignition switch not making reliable contact on the run terminal (coil and voltage regulator) while in start to know that it can be an issue. For normal use it is a non-issue since the resistor bypass ("I" on solenoid to coil+) will give the ignition the power it needs.

 

[Steve83]

... the "I" on the regulator gets full 12V when in run

No, the starter relay I terminal is only +12V when the key is in START (relay & starter energized). In RUN, the I terminal is at the same reduced voltage as the ignition module.

Use the "I" wire at the voltage regulator. It is hot in run without the resistor.

Not necessarily; especially with the original mechanical VR shown in the diagrams above, in which I is not even wired. But S (904 Gn/R) would serve this purpose since it's actually part of the coil feed (16 R/Gn). You can see they're connected at the ig.sw.

 

[DirtDonk]

Sorry, I completely glossed over the "voltage regulator" connection when you mentioned the "I" wire. Since there is no "I" in a Bronco voltage regulator circuitry I literally jumped to a conclusion and immediately thought "starter relay" instead.

As Steve said though, there isn't even an I circuit, so you probably meant the "S" circuit for the voltage regulator (Green w/red)?

So yes, using the voltage regulator's S wire (especially if you no longer use it) is an option. If it's still in use for a voltage regulator, I wonder how much current the ignition pulls, and if it would be healthy for that wire and the switch?
Does Holley (edit: nevermind, no Holley in this discussion!) give any indication of what gauge wire they recommend? Maybe a fuse size as a rating?
Guessing the stock wire is 16ga but I don't know that for sure.

Paul

 

[stanz]

So in short, I’d be looking for 12v switched power for the coil on the setup I’m interested in... and do not want to have the resistor wire in the feed (thereby reducing the voltage). Correct?

I got a little lost in the string above on what’s a good source to tap into. Trying to do minimal cutting/splicling to the factory wiring and maintain a stock factory look.

 

[tody]

Why not use a simple Pertronix unit? Eliminates the points, and you can keep the rest of the setup, even the coil. A lot cheaper, anyway.

Thanks all! This is just what I needed, very clear, very thorough!

Part II... I’m thinking of dropping the points and going with this distributor (https://www.summitracing.com/parts/...Sm9AombpR7rBIe2tVQRUAFRxZioVAGvBoCzd4QAvD_BwE) and this coil(https://www.summitracing.com/parts/...M1DzrXdcWGAkC-IV-fNKa4e2uCbA91SBoCnQYQAvD_BwE)...

Would I want to go full 12v to the coil (leaving the stock wiring in tact but using it to trigger a relay for the 12v)? Or would I just use the stock wiring as is with the pink resistor wire?

 

[DirtDonk]

So in short, I’d be looking for 12v switched power for the coil on the setup I’m interested in... and do not want to have the resistor wire in the feed (thereby reducing the voltage). Correct?

Yes, correct.
But you want only one of two circuits controlling it. Those are either the old Red w/green ignition wire, OR the old Green w/red voltage regulator wire.
The reason for this is that those are the only two circuits that are on the key in ON only, and not when in the ACC position.
This way you can still listen to the radio or do other diagnostic stuff without the ignition activated and possibly overheating.

I got a little lost in the string above on what’s a good source to tap into. Trying to do minimal cutting/splicling to the factory wiring and maintain a stock factory look.

Sorry about the Holley comments. I was getting mixed up with another thread where the discussion was about the Holley ignition system to go with the Sniper EFI.

Paul

 

[stanz]

Thanks Paul! Very helpful. On the regulator, I get that the I terminal is Ignition. When you say voltage regulator wire, is that the S or F terminal?

 

[Broncobowsher]

I must have been confused with the letters on the voltage regulator. I know "F" is field. But I guess I mixed up what the other ones were. "S" is switched? I knew them at one time.

 

[stanz]

I must have been confused with the letters on the voltage regulator. I know "F" is field. But I guess I mixed up what the other ones were. "S" is switched? I knew them at one time.

@paul Which terminal were you thinking?

 

[Steve83]

I'm not sure what the VR terminal letters stand for. In the context of an alternator, S usually means "Stator", but that can't be what it is on the VR. A is usually the always-hot sensing wire from the B+, but IDK why Ford uses "A".

 

[DirtDonk]

Correct in the rest of the world. F is field, S is stator, A is armature, and I is indicator (or ignition).
IN the rest of the Ford world the I wire from the dash indicator light is also the 12v switched circuit that energizes the regulator.
But in our world of ammeter-equipped Fords (all of them that I know of) the F is still field, the S is "switched" and the A is simply changed to mean "always hot" just for ease of memorizing.
They don't mean that officially of course, and the I is always left empty/blank/unused. I just changed the wording to make it easier to remember.

No indicator present, so the S wire is now the switched 12v from the key (the same Green w/red trace that would be on the I terminal on other Fords).

Paul

 

[stanz]

So the S wire I what I want to tap to power the coil?

 

[DirtDonk]

Yes. The "S" on the voltage regulator (Green w/red wire) is 12v with the key in the ON position only.

Paul