How to tell which factory wiring harness I have?

[Gas Pig]

I’m in the process of building a Bronco for are family. So over the past few years I have collecting parts.

In short I have a ‘74 302 mildly built and a ‘74 body. I thought I had a ‘74 dash wiring harness but I’m thinking it’s later then ‘74. What should I look for to determine what harness I actually have?

Thanks Jeff

 

[Viperwolf1]

Not much difference. Duraspark wiring was different under the hood and not all '74s had it. Everything under the dash should be the same.

 

[Speedrdr]

There’s 3 different Duraspark models and they each have a different colored plug(?) thingy where the wiring goes into/out of the box. Unfortunately, the only one I’m familiar with is the 76-77 model years and that connector is blue. I’m not sure how much that helps, but maybe it helps determine what year your harness is.


Randy

 

[DirtDonk]

Look for an engineering number identifier on the fuse panel. Even though they were all visually the same, there were minor variations which may have included changes in the engineering number.
Worth a shot anyway.

 

[DirtDonk]

Rear main gas tank sending unit connector maybe?
In that case only the 77 might be different, so it wouldn’t tell you the difference between a 73 or 74, or ‘76.

Wiring for dual tone horns perhaps?

 

[Ovalis]

The 74, 75, & 76-77 harnesses are different because the plug end connections have a different shape to them so they plug into the appropriate ignition module. Just use the correct ignition module for the harness you have.

 

[Ovalis]

If you have the horseshoe connection for the ignition coil, then it's a 75 and later. If it doesn't have the horseshoe, but it has the duraspark connectors, then it's a 74.

 

[Gas Pig]

I was always under the impression once ‘74 came the 75-77’s were all same. But that is not exactly true?

Here is a picture of ignition module plugs, distributor plug, motor harness plugs.

But what is odd the ignition plug end just misses fitting through the firewall hole. So maybe this body isn’t a ‘74 but a ‘73? Not that it matters much but just wondering?

Sorry for my ignorance on this later model questions! I have spent my of my Bronco building with ‘66 and ‘67. My first bronco was a ‘72 and never owned anything later then that! lol

 

[Viperwolf1]

'76-'77 harness.

 

[Gas Pig]

'76-'77 harness.

Now will it be compatible with my 74 distributor?

Thanks again for responding, Jeff

 

[Viperwolf1]

Now will it be compatible with my 74 distributor?

Thanks again for responding, Jeff

A '74 electronic distributor, yes.

 

[Steve83]

At worst, you'd have to re-pin a connector. Study this, the PREVious, & NEXT few pages:

(click this text)

 

[Gas Pig]

A '74 electronic distributor, yes.

Yes it the factory duraspark but just a regular coil

 

[Gas Pig]

My buddy has a good ignition module out of a ‘75… would that work with my 76 harness and 74 distributor?

Honestly I’m pretty green on 73-77 broncos. I rarely have had a chance to mess with them let alone really dig into one as much as I’m doing now with mine.

Thanks again, Jeff

 

[DirtDonk]

Check the color of the grommet and verify the orientation of the wires in the plugs.
If you don’t have it with you, have him send you a good picture of it.

 

[Gas Pig]

Check the color of the grommet and verify the orientation of the wires in the plugs.
If you don’t have it with you, have him send you a good picture of it.

Ok I’ll get some good pictures of it.

 

[Ovalis]

The 75 module is the green grommet, 74 is black & 76 & later is blue. The plugs on the 75 module are different than the plugs on the 74 harness. The ignition module & ignition coil are a one year only on the 74's. That being said, it's no big deal. I have the correct units for my 74, but I don't currently run them. I will after I restore it, but for right now I run the easy to get blue grommet ignition module & standard coil. I got the connector pigtails and cut off the exterior tabs on the connector, followed the wiring diagrams between the 74 & 77 and made a wire harness to plug in between my stock harness & ignition coil. When I'm ready to go back to my original 74 parts, I'll just unplug the harness I made and plug in the original parts. No reason to modify the original harness at all.

 

[Steve83]

My buddy has a good ignition module out of a ‘75… would that work with my 76 harness and 74 distributor?

As Ovalis explained, they work the same way - it's just a matter of different connectors. But it's probably cheaper & easier to just get the right parts than to piece together a wiring adapter to use the wrong one.

 

[Gas Pig]

The 75 module is the green grommet, 74 is black & 76 & later is blue. The plugs on the 75 module are different than the plugs on the 74 harness. The ignition module & ignition coil are a one year only on the 74's. That being said, it's no big deal. I have the correct units for my 74, but I don't currently run them. I will after I restore it, but for right now I run the easy to get blue grommet ignition module & standard coil. I got the connector pigtails and cut off the exterior tabs on the connector, followed the wiring diagrams between the 74 & 77 and made a wire harness to plug in between my stock harness & ignition coil. When I'm ready to go back to my original 74 parts, I'll just unplug the harness I made and plug in the original parts. No reason to modify the original harness at all.

Here is my buddy’s ignition module plugged into my harness. It’s a Wells which I’m not sure if this was OEM or not. When you said green grommet are you talking about the green grommet were the wires are coming out of the module itself?

 

[Ovalis]

Yes, where the wires comes out the color of the grommet determines which module it is. The 74 black grommets were 7 wire modules & the 76 & later blue ones were 6 wire modules. I can't remember off the top of my head if the greens were 6 or 7 wire, but by the looks of yours it looks like 6 wire. I assume the harness side of the connectors is 6 wire as well. Maybe your harness is from a 75 unless the protrusions on the connectors were cut off. They added protrusions to the connectors so you couldn't accidentally plug the wrong module into a different harness. I have some documentation that I'll try to find that shows what the connectors look like for the different years so you can see the differences. Another tid bit of information that I don't think you need to worry about, but just for informative purposes is the ignition wire which is a resistor wire to cut down the voltage feeding to the ignition coil had a different resistance for the different years based on what coils they were using.