Wiring Advice - Start Circuit

[Jeff10]

Hi Everyone,

What started out as a simple alternator replacement has gotten a little more complicated. I could use some help.

I am attaching a .pdf of two pages of the wiring schematic that I believe is correct for the '71. I have highlighted the wiring that's in question.

I have also put a red box around some other information and questions.

The questions are:

1. Presently the black-yellow wire that is shown on page 2 connecting with the alternator is instead connected to the battery +12 V. Is there any reason for doing that? If I don't connect the black-yellow wire to a 12V power source under the hood, I believe there will be +12 V being fed by the connection to the ammeter, as long as that wiring is still good. Is this correct thinking? Or, because this is a 1-wire alternator and the ammeter won't work, will I need to do something different?)

2. Does the pink resistor wire run through the firewall, or is it located under the dash in the cab? (Would that wire be inside the taped harness, or should it be exposed?) What's the best way to confirm that it's functioning?

3. The ballast resistor that was added by the previous owner is connected as shown in the schematic. I installed a MSD SS coil several years ago. The information with the coil said that a ballast resistor wasn't required. When I attempted to start the Bronco after installing the coil it wouldn't continue running. Putting the resistor back in the circuit enabled the truck to run. I figured at that time that there was an issue somewhere else. Now I'd like to make it right. I don't have a good understanding of the pink resistor wire (nor do I know if it is functioning).

Any thoughts would be appreciated. (If it's easier I can email the schematic. Shoot me a PM with your email address.)

Thanks,

Jeff

 

[70_Steve]

Much better wiring diagrams here!

seabiscuit68.tripod.com

1. Presently the black-yellow wire that is shown on page 2 connecting with the alternator is instead connected to the battery +12 V. Is there any reason for doing that? If I don't connect the black-yellow wire to a 12V power source under the hood, I believe there will be +12 V being fed by the connection to the ammeter, as long as that wiring is still good. Is this correct thinking? Or, because this is a 1-wire alternator and the ammeter won't work, will I need to do something different?)

The Black w/Yellow wire is one end of a "loop" that supplies power to the Bronco and recharges the battery. You marked the other end of the loop as the Black wire that ends at the Battery side of the solenoid. Stock wiring will have a Fuse Link on the Black wire at the solenoid. That wiring diagram shows that entire loop to be unprotected! A bad thing!!!

Attaching the Black w/Yellow to the Battery side of the solenoid effectively doubles the gauge wire supplying power to the inside of the Bronco. I've done almost the same thing, except I attached the Black and the Black w/Yellow wire to a 30 amp circuit breaker, with the other end of the circuit breaker attached to the Battery side of the solenoid.

You can remove and cap off the Black w/Yellow wire if you want, but that will leave only the Black wire to supply power to the Bronco.

In either case, the ammeter will not, and should not function. To have it function, the output of the alternator would have to go through that loop, and that loop is only rated for 60 amps. I suspect your 1-wire is rated for a bit more than that.

2. Does the pink resistor wire run through the firewall, or is it located under the dash in the cab? (Would that wire be inside the taped harness, or should it be exposed?) What's the best way to confirm that it's functioning?

Pink wire runs from the ignition switch to the inside of the bulkhead connector at the firewall. Mine ('70) was inside the harness. That resistor wire is only 1.6 ohms. Need a pretty good digital VOM to measure...

3. The ballast resistor that was added by the previous owner is connected as shown in the schematic. I installed a MSD SS coil several years ago. The information with the coil said that a ballast resistor wasn't required. When I attempted to start the Bronco after installing the coil it wouldn't continue running. Putting the resistor back in the circuit enabled the truck to run. I figured at that time that there was an issue somewhere else. Now I'd like to make it right. I don't have a good understanding of the pink resistor wire (nor do I know if it is functioning).

Not much to the resistor wire or it's function. It's there to limit the current to the coil. If When the engine is running, the power to the coil should come through the ballast resistor wire. When starting, the starter solenoid puts 12 volts, directly from the battery, on the Brown wire. Note that it connects to the Red w/Green wire shown on your schematic. This is to provide a little more current, and thus a hotter spark, for starting.

 

[Jeff10]

Hi Steve,

It's always great to hear from you! How are things in Arizona?

Much better wiring diagrams here!

seabiscuit68.tripod.com

That schematic is awesome. Thanks for posting. Definitely something I'll keep as reference.

The Black w/Yellow wire is one end of a "loop" that supplies power to the Bronco and recharges the battery. You marked the other end of the loop as the Black wire that ends at the Battery side of the solenoid. Stock wiring will have a Fuse Link on the Black wire at the solenoid. That wiring diagram shows that entire loop to be unprotected! A bad thing!!!

The existing wiring actually has both the black and the black-yellow attached to the primary battery side of the existing isolator. There is no fuse link. (The isolator is being eliminated.) I appreciate your comment about being unprotected.

Attaching the Black w/Yellow to the Battery side of the solenoid effectively doubles the gauge wire supplying power to the inside of the Bronco. I've done almost the same thing, except I attached the Black and the Black w/Yellow wire to a 30 amp circuit breaker, with the other end of the circuit breaker attached to the Battery side of the solenoid.

I'll follow your advice about using a breaker. I believe you're just saying attach one side of the breaker to the black and black-yellow wires and the other side to the battery side of the solenoid. Is there an advantage to wiring to the battery side of the solenoid rather than to side post of the battery?

You can remove and cap off the Black w/Yellow wire if you want, but that will leave only the Black wire to supply power to the Bronco.

As long as there's nothing wrong with wiring it with the breaker as you mentioned, we'll feed it from both ends. (Not something I would have tried on my own; but, I'm good with your explanation.)

In either case, the ammeter will not, and should not function. To have it function, the output of the alternator would have to go through that loop, and that loop is only rated for 60 amps. I suspect your 1-wire is rated for a bit more than that.

True statement... the alternator is 130 A.

Pink wire runs from the ignition switch to the inside of the bulkhead connector at the firewall. Mine ('70) was inside the harness. That resistor wire is only 1.6 ohms. Need a pretty good digital VOM to measure...

I'm trying to understand why a separate ballast resistor was added. Unless the resistor wire is bad, what would be the point?

Not much to the resistor wire or it's function. It's there to limit the current to the coil. If When the engine is running, the power to the coil should come through the ballast resistor wire. When starting, the starter solenoid puts 12 volts, directly from the battery, on the Brown wire. Note that it connects to the Red w/Green wire shown on your schematic. This is to provide a little more current, and thus a hotter spark, for starting.

I will look for an answer to this question on my own... What is the reason for limiting the current to the coil?

Thanks for such a detailed reply. There's always something to be learned.

Take Care,

Jeff

 

[70_Steve]

Hi Steve,

It's always great to hear from you! How are things in Arizona?

Things are fine here, if not just a little hot. This week is seeing about a 20° bump in temps. Only hit 112° today and calling for 116° tomorrow. I had some work to do on my heater. It's not a priority now...

Is there an advantage to wiring to the battery side of the solenoid rather than to side post of the battery?

No, not really. The stock wiring connected to the solenoid, so that's where I connected mine. Although I just installed a PMGR starter, and with all the wires on the Bat+ side of the solenoid I ran out of room. I had to install an additional +12V post to accommodate all the connections.

I forget the group# of the battery I'm running but it has top and side posts. I read that the top posts are rated for more current than the side posts, so I have the winch wired to the top posts, and the side posts run the Bronco.

I'm trying to understand why a separate ballast resistor was added. Unless the resistor wire is bad, what would be the point?

I assume this was the handy-work of the PO... No telling if the modification was done based on sound troubleshooting and investigation, or lack of knowledge, or you call it!!!

Most coils need an external ballast resistor of some type. The Bronco just happens to use a resistor wire, which is not all that uncommon, and can be found as a replacement part today.

I will look for an answer to this question on my own... What is the reason for limiting the current to the coil?

My understanding is that a standard coil isn't designed to run on the current flow from a full 12 volt source. The coil, as any resistor would, dissipates that energy in the form of heat. Without the current limiting resistor, after the engine runs for a while the coil will overheat and get damaged.

Some aftermarket coils state they need no ballast resistor, but I've read that the coil simply has the ballast resistor inside the case.

The coil can stand full 12 volts for a short period of time. As I mentioned, the Brown wire on the I terminal of the solenoid provides that full 12 volts during starting. When the solenoid deenergizes, the solenoid no longer provides 12 volts to that terminal, and all the current for the coil comes through the ballast resistor wire.

 

[DirtDonk]

To expand slightly on what Steve already said, the old coils were a hand-me-down from the old 6v electrical system days. So when they were installed without change on more "modern" 12v systems, they avoided the overheating issue with the resistors.

This was even more important prior to the mid-seventies because the points ignition triggers were just as, if not more sensitive to high voltage than the coils were. A set of points and a condenser run on 12v for very long didn't live very long.

Now that we have the option, I always prefer to eliminate one more potential failure point when doing new wiring and leave out the resistors. This way it's just normal wire running at the full 12v of the system, but making sure to use compatible components in the ignition. Coils and triggers in other words.
Ignitions like the Pertronix Ignitors, HEI distributors and MSD ignition control modules with capacitive-discharge designs all love more voltage. So keeping them fed with 12v or better (like the 14.5 that we get when the alternator is working) makes them happy and puts out as much spark energy as they're capable of.
All good things.

Paul

 

[Steve83]

Early 12V coils used low-quality copper (high resistance = high heat, high chances of impurities causing corrosion & failure, etc.) and low-quality insulation (wax paper, lacquer, etc. = sensitivity to heat & high chance of internal shorts). So the full 12V was only used during cranking, when a strong spark was critical. Then, once the engine was running, the coil voltage dropped back (via the resistor wire) to reduce its temperature & prolong its life.

Modern coils are designed better, and use much-better materials, so they can tolerate the full 12V without generating as much heat, and they tolerate higher temperatures better. So they don't acutally have an internal resistor; they have higher resistance (they use less current) because they're inherently more-efficient & -durable.