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Need some help from the wiring guru's

crawln68

Bronco Guru
Joined
Oct 8, 2004
Messages
3,045
I need you to educate me on some wiring. I have a 76 Bronco, 302, auto trans. It has the original wiring from the factory. Coming from the Ignition switch there is a heavy gauge red wire with green stripe that is in a in its own black plastic covering. It looks to be some sort of resistance wire maybe, but I'm just guessing. Where does this wire go and what is it's true purpose? The reason that I ask is because with the key in the "On" position or with the Bronco running, this wire is getting very hot! Almost too hot to hold in your hand. This can't be normal. So what's really going on here?
 

ransil

Bronco Guru
Joined
Sep 6, 2003
Messages
8,125
its normal for the most part, you find a lot of harnesses with a blob of insulation melted together.

It goes to the coil, comes out of the fire wall near the throttle linkage
 

jckkys

Bronco Guru
Joined
Mar 15, 2012
Messages
5,212
If you look at several inches of this wire you will see "resistance wire" printed on the side. If it's getting very hot, the problem could be the coil. What brand/model coil are you using and is it getting hot? It should be a Motorcraft DG-314 or a quality replacement. Even if it's the correct coil, they go bad eventually. Stay away from speed shop crap.
 
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crawln68

crawln68

Bronco Guru
Joined
Oct 8, 2004
Messages
3,045
If you look at several inches of this wire you will see "resistance wire" printed on the side. If it's getting very hot, the problem could be the coil. What brand/model coil are you using and is it getting hot? It should be a Motorcraft DG-314 or a quality replacement. Even if it's the correct coil, they go bad eventually. Stay away from speed shop crap.

This actually crossed my mind because sometimes (not all the time) the coil does get hot. The PO put some kind of aftermarket "fancy" coil on there. I don't recall the name of it right now. I'll look at it when I get home from work. Maybe the coil is starting to fail.
 

DirtDonk

Contributor
Bronco Guru
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Nov 3, 2003
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48,743
It only gets hot sometimes? I've never seen an ignition coil that did not get too hot to touch during use. Of course, the more air from the fan that flows over it, the cooler it will remain, but they've always seemed too hot to me.
But that's what they do...

As does the resistance wire. It's separate and not part of the rest of the harness because it can get so hot it would damage adjacent wires, and it's expected to fail at some point too, so this makes it easier to replace.
Wit some models, it was literally replaceable due to having a separate connector between the switch and the firewall that allowed you to replace just that section should it ever fail.

Are you having a problem with the electrical system? Or is it just that you noticed it getting hot and wanted to make sure it's not going to become an issue?

Paul
 

House

Contributor
Minutia Militia
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Aug 12, 2004
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2,409
IIRC it should have 1-1.2 ohms of resistance.
 
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crawln68

crawln68

Bronco Guru
Joined
Oct 8, 2004
Messages
3,045
It only gets hot sometimes? I've never seen an ignition coil that did not get too hot to touch during use. Of course, the more air from the fan that flows over it, the cooler it will remain, but they've always seemed too hot to me.
But that's what they do...

As does the resistance wire. It's separate and not part of the rest of the harness because it can get so hot it would damage adjacent wires, and it's expected to fail at some point too, so this makes it easier to replace.
Wit some models, it was literally replaceable due to having a separate connector between the switch and the firewall that allowed you to replace just that section should it ever fail.

Are you having a problem with the electrical system? Or is it just that you noticed it getting hot and wanted to make sure it's not going to become an issue?

Paul


Yes and yes. I'm having some electrical issues so I was tracing wires and taking apart wiring looms and that is when I noticed that wire getting very hot. I could even smell it. So I had no idea if that is normal or not. The PO had it wrapped up along with the main wiring loom under the dash and then had it put in the plastic wiring loom. It sounds like it should be by itself and not all wrapped into the other main loom?
 

jckkys

Bronco Guru
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Mar 15, 2012
Messages
5,212
The resistance wire on '76s is 1.30 ohms. The thread here has all the Ford ignition specs and part numbers for 1956-79 ignitions;https://pantera.infopop.cc/topic/duraspark-vs-msd-distributor. I don't know when hot is too hot, but my OE '77 ignition never melted any insulation. One coil leaked the oil out in 50 yrs. and many Fords. I never had a reason to buy any after market coil.
 

DirtDonk

Contributor
Bronco Guru
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Nov 3, 2003
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I could even smell it.

Probably not good and a good thing you're taking note of it.
To me that's a sign that it's at that too hot point. Though it might depend what it's wrapped in, or what it's next to that might be overheating.

So I had no idea if that is normal or not.

The interesting thing about resistor wire. It's a smaller conductor, made purposefully longer than needed. Both to create more resistance.
Then it's expected to heat up, as heat is a result of resistance, AND resistance is a result of heat in most cases at least with common wiring.

A good example of a resistor wire would be a pair of 100' long 16ga extension cords trying to power a refrigerator!;D

The PO had it wrapped up along with the main wiring loom under the dash and then had it put in the plastic wiring loom. It sounds like it should be by itself and not all wrapped into the other main loom?

Correct for the most part. All of the ones I've seen through the model years were separate and wrapped in a heat resistant protective sleeve, but then doubled over (it's longer than most other wires would need to be) and then taped to one of the main harness trunks.
So while it's usually at least partially visible and outside the main harness wrap tape, it's still wrapped up with the other wires with some tape too. Probably some heat resistant tape at that.

I've never seen one in plastic split loom that was not a re-wire job by a PO. Even though they have high-temp split loom, I'm not sure what it's rating is and if it's ok to use it. I always buy that tar-coated fiber tubular loom specifically for high-temp conditions. You see it near engines and such, like around starter cables that tend to get close to the exhaust system components.

Paul
 

House

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2,409
IIRC it should have 1-1.2 ohms of resistance.
The resistance wire on '76s is 1.30 ohms.

Odd that their generic chart shows 1.30 Ω, the 1976 Bronco wiring diagram shows 1.05-1.15 Ω, and the factory service manual says 1.30-1.40 Ω. It must not be that important of a value... LOL
 
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crawln68

crawln68

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Joined
Oct 8, 2004
Messages
3,045
Does anyone know where I can buy a new one of these resistance wires? I can't seem to find anything on the vendor websites. Mine definitely in need of replacement.
 

Jdgephar

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Joined
Sep 25, 2012
Messages
1,357
Does anyone know where I can buy a new one of these resistance wires? I can't seem to find anything on the vendor websites. Mine definitely in need of replacement.
Probably easier to run regular wire and one of those big ceramic resistors.

Sent from my SM-G950U using Tapatalk
 

DirtDonk

Contributor
Bronco Guru
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Nov 3, 2003
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48,743
I bought my first resistor wire from Ford, the next one from the local auto parts store. Back then, both had them in stock!
Both of those were "modular" though, meaning they were pre-terminated with the proper connectors for only certain applications.
The issue of which one then, depends on if you have connectors or not. Many were wired directly to the key switch and were part of the harness, so not easily replaceable.

On the flip side of that, you can purchase just a roll or length of resistor wire and figure out how to terminate it yourself. But I would imagine it's hard to crimp, solder or whatever, to add the proper termination for your particular harness. And just crimping inline would probably be difficult, so you'd want to solder if that's possible.

As mentioned, this is likely why most just opt for the external ceramic ballast resistor instead. Or eliminate the ignition components that need it and run regular wire.

Paul
 

Slednut10

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Guru? That's funny!
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Sep 26, 2013
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If you are trying to troubleshoot with the key on and engine not running your coil will heat up.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
48,743
I'm having some electrical issues so I was tracing wires and taking apart wiring looms and that is when I noticed that wire getting very hot. I could even smell it.

If you are trying to troubleshoot with the key on and engine not running your coil will heat up.

This crawlin. Never ever leave your key in the ON position without the engine running. Even while troubleshooting stuff.
If you can't use the ACC position (when everything else except for the ignition and voltage regulator) are powered up, then you should disconnect the ignition coil from the system.

Many an ignition coil, module and/or resistor wire have met their maker when the key was left ON too long.

Paul
 
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