Build Your Own Harness: A Step-By-Step Guide

[NickyV]

Greetings, Fellow Comanche Enthusiasts!

 

I’ve spent the past few months diving into projects on my 1987 2.5L, 2WD, Short Bed, and this forum has been an invaluable resource. I’ve really enjoyed the time I get to spend just working with my hands and not thinking about emails I have to send.

 

But it occurs to me that I’ve spent a lot of time asking you all questions. I want to give back with a tutorial of my own.

 

I’m much newer to this than many of you, so I welcome any notes on things I could have done better or things I’ve done flat-out wrong.

 

I’m going to write a step-by-step guide on how I built a Headlight Harness Upgrade using @gogmorgo’s diagram. And I’m going to add some diagrams of my own. I hope this ends up helping someone else down the line.

 

NOTE: This is a LONG write up. If you know what you're doing, I suggest taking a look at the diagram below and then the step by step slide show I have at the end.

 

 

Step 1: Assemble the Materials

 

The first thing I did was add up the total length of 10 AWG and 14 AWG wire that the diagram calls for.

 

The following list turned out to be a little more than I needed, so I think it’s the perfect amount.

 

WIRE:

 

10 AWG: 30 Feet or 360 Inches 
14 AWG: 16 + 48 = 5 Feet or 60 Inches

 

RELAYS:

 

Two 30-amp headlight relays with the following connections: 30, 85, 86, and 87. Here's an example. This one is 40 amps, but you get my drift.

 

 

H4 HEADLAMP SOCKETS:

 

I scratched my head on this one for a while, because I was unfamiliar with it. Also, all the auto parts websites have my vehicle information by now and they were telling me certain headlights didn’t fit my MJ. I turned off the vehicle-specific search function when I was searching and that helped open up the results.

 

I ultimately went with this one, because it was available locally and the connector was ceramic, which I’ve heard is better than plastic. Makes sense when high temperatures are in play, right?

 

I also wanted to use the male end of one of them to make the connector for the existing OEM harness (see the second picture).

 

 

 

OTHER MATERIALS:

 

I had most everything else I needed already, but you’re going to need things like wire strippers/crimpers, big box of wire terminal attachments, a ton of zip ties, and electrical tape. Some other nice-to-haves are a soldering iron, heat shrink wrap, a heat gun, and other electrical components like that.
 

 

Step 2: Plan Your Build

 

I’ve charged headlong into way too many projects. And honestly, I could have taken more time with this one. But with a project like this, it really helps me to lay everything out in front of me.

 

 

I looked at @gogmorgo's plans over and over again and cut most of my wire to length before I did anything else. I also labeled my lengths of wire and cross referenced them with the drawings to see if it all made sense. One thing I wish I had done was also plan what kinds of terminal heads I was going to use at each juncture. Doing this would have saved me some time down the line.
 

 

Step 3: Slow and Steady (Learn From My Mistakes)

 

If you’re brand new, or relatively new (like me), don’t get flustered. Take your time.

 

Like Gogmorgo says, the diagram is not to scale, and it can be hard to visualize at first. Here are some things that took a little while to click for me:

 

1. Your relays are going to be right next to one another and very close to the battery. Once the new harness is built it will form the shape of a capital “L” — running from the battery on the passenger side of the engine bay, down to the passenger side headlight, then to the driver's side headlight. The short length of the “L” runs from the battery to the passenger side headlight. The long length of the “L” runs from the passenger headlight to the driver’s side headlight.
    
2. All of the 24-inch lengths of wire  correspond to connections at the passenger side headlight. All of the 80-inch lengths of wire correspond to connections as the driver’s side headlight.
    
3. If you do it like I did, you will have to do at least two splices. These will go on the positive lines. There were two ways I considered doing this.
    
   1. One way would be to connect the positive lines directly to the shovel terminal that attaches to the 87 terminal prongs on each of the two relays (high and low beam). Doing it this way will require the full 24-inches of wire. You may also need to get creative with how you connect the two ends of 10AWG wire to the shovel.
   2. Another way is to splice the shorter length of wire roughly at the right angle of the “L” shape that this harness will form when it is installed. Doing it this way will allow you to save a little bit of wire. This second way is the way that I did it.
       
4. You do not need to splice any of the ground wiring.
    
5. You do not need to splice the battery-to-relay wiring. Instead, you can just make two 8-inch lengths of wire. On one end attach them to the battery. In the middle, place your in-line fuses. On the other end place a shovel terminal connector to attach to the 30 prong on each relay.
    
6. If you end up using a left over male H4 connector, remember to connect your wires as a mirror image of the diagram above. I made a mistake here and didn't discover it until the harness was installed. I had to strip wires and make some new connections while crouching at headlight level. Not fun. If you notice, in the picture above, there is a short length of red wire that isn't connected to anything. But that SHOULD have been connected to the high beam relay. Which leads me to No. 7:
    
7. If you notice in the diagram above, both high beam connections are on the left hand side of the female connector and all the low beam connections are on the top of the female connector. Remember that!

 

Step 4: Splicing & Dicing

 

I was practically brand new to splicing when I started this project. I am also very bad at soldering, and I don’t have a great soldering iron.

Here’s how I ultimately spliced the positive cables together:

 

Cut out a very small segment of the 80-inch positive wire. You can use the wire cutters to make the first two cuts around the outer edge of the wire. Then you’ll need a razor blade to cut from one of those cuts to the other cut. Then you’ll peel the slim piece of wire covering off, as if it were a bandage wrapped around your finger.

 

The location of this splice is very close to the connection point for the passenger side headlight connection. The connectors I used came with 14 AWG wires attached to them. I thought about devising a way to maintain 10 AWG all the way to the headlight connection, but I abandoned that idea, and it was helpful to do so.

 

That’s because it was way easier to wrap 14AWG around the exposed splice point of the 10 AWG wire than it would be to wrap the very thick 10 AWG wire around another 10 AWG wire. I took this picture to help you visualize the process.

 

 

Picture the white electrical tape as the exposed bit on the 10 AWG wire. The yellow bit is 14 AWG wrapped around the exposed portion. Use a soldering iron or thin strips of electrical tape to help secure it in place.

As I said, you’ll need to do this on both lengths of 80-inch wire.
 

Step 5: Making Connections

 

Now it’s time to put it all together. For this bit, I made some diagrams of my own. Red is for positive connections. Black is for negative/ground. 

 

Connect all the shovel terminals to the wires going to the relays. Connect the new headlight sockets to the wires. Prepare your connection for the existing OEM plug (female plug on the passenger side).

 

I found it helpful to use zip ties to fasten the longer lengths of cable together as I went along. This helped prevent tangles. When I was done, I covered the whole thing in conduit, which I think was helpful when installing in the MJ.

 

MAKE YOUR CUTS
 

 

MAKE THE DRIVER'S SIDE CONNECTIONS

 

 

MAKE THE PASSENGER SIDE CONNECTIONS

 

 

MAKE THE CONNECTIONS FOR THE FACTORY PLUG

 

 

MAKE YOUR BATTERY CONNECTIONS

 

 

MAKE AN INLINE FUSE ON THE BATTERY CONNECTIONS

 

 

Step 6: Hook it Up!

 

I could probably write a whole tutorial on how to do this as well. It took me a long time and I hit some road blocks along the way. Thankfully there is a detailed installation video from K Suspension that you can watch here. The main difference with the harness in this video is that the grounds do not run all the way back to the battery. I opted to create a harness where the grounds return all the way to the battery, because I hoped that would lead to a healthier circuit.

 

By the way, if you don't want to take the time to build your own, or don't feel comfortable doing it, K Suspension and many other companies make this exact harness. Some of them are even cheaper than what it would cost you in money and your valuable time to do it DIY style.

 

But, at least for me, tinkering is part of the fun!

 

Step 6.1: Hook it Up! (Pt. II)

 

OK... Just thought I'd add a few more pics of the actual install. I didn't document this process perfectly. But here goes:

 

1. It helps to take the batter out completely. This is also good because you know you won't accidentally create a short while the harness is flopping around. This picture is of when I was thinking I'd snake the harness underneath the driver's side of the battery tray, but you'll actually want to snake it under the passenger/fender side. It's a tighter squeeze, but a more optimal flow.
   
   
    
2. Bring the passenger side connections into the passenger headlight bucket. In this image you see the new male connector, the new female connector, and the old factory female connector (clockwise from top).
   
   
    
3. Bring the driver's side connection over to the driver's side bucket, snaking it along behind the grille. I don't have AC (yet). There was a lot of space back there for me.
   
   
    
4. Bring the new driver's side connection into the driver's side headlight bucket. FYI, getting these connections into the buckets was one of the hardest parts of this — at least physically. It's a real contortion act. Be patient. Wear gloves or be prepared to slice up your hands. I also ended up taking out the turn signal on this side. Not sure I needed to, but I got stuck for a while on this step and I wanted a better view.
   
   
    
5. Figure out where you will place your relays. In this picture, you can see where I chose to put my relay and where I decided to put my negative/ground terminal connection. You can use sheet metal screws, but I used zip ties. I might change that in the future. There were many holes already drilled on the interior side of the passenger fender on my '87, which made this pretty easy.
   
   
    
6. Put it all back together. Don't judge me! I need to clean up my battery and replace cables and all that. It's on my list. Anyway, here is the only picture I have in this entire thread of my inline fuses. The fuse block at the top of the pic has a transparent plastic cover, which is good. I will eventually find a better, cleaner way to mount all this stuff, but this is working for now.
   
   
    

All right! I think that is it on this now. Se you all in the comments!

[89 MJ]

Holy cow, this is a very thorough write up. Looks like you did good work. I didn’t see it mentioned (but it’s possible and likely that I missed it), but I just wanted to add that you can get heat shrink versions of these:

7 hours ago, NickyV said:

 

I recommend them because it makes your life a heck of a lot easier than using these connectors and then needing to put heat shrink over them. 
 

And then as an alternative to solder, the solder stick type connectors seem to work pretty well. They can’t be used in all cases where solder is needed, like a 2 into 1 type situation, but they do work for a 1 to 1 type scenario. 

[NickyV]

Huh... Thanks for letting me know about these, @89 MJ. I literally didn't know they existed. I mean... some of the butt connectors I have are probably actually just this. Wish I'd realized that. Will keep in mind for the future!

[89 MJ]

6 minutes ago, NickyV said:

Huh... Thanks for letting me know about these, @89 MJ. I literally didn't know they existed. I mean... some of the butt connectors I have are probably actually just this. Wish I'd realized that. Will keep in mind for the future!

They are pretty slick! You do have to be careful though. There is a fine line between burning the plastic and melting the solder nicely that you have to try to balance between. But if you can build a harness, you’re more than capable of not wrecking the connectors haha. 

[gogmorgo]

Dang. Hard to believe I put this together ten years ago. I always intended to put together a write-up but got sidetracked trying to make a harness that wouldn’t interfere with the factory daytime running lights. Good on you. 
This is still IMO an excellent electrical project to tackle as a beginner. It’s a simple circuit that involves a lot of connections but it’s easily broken down into small steps. People are often apprehensive about taking on wiring projects, and I think this one really helps to build confidence.

I think in the original threads where this diagram shows up I would have mentioned the 10awg wire is massively overkill for the stock headlights. I had decided at the time I was going to do something with high-powered H4 lights so I overbuilt my harness in anticipation, but after having the relays installed I haven’t ever felt I needed to upgrade beyond  sealed beams. You could very happily make the entire harness out of 14 gauge to run sealed beams. Electrically I’m sure you wouldn’t even need 14 but for mechanical strength and corrosion resistance I don’t ever really use anything smaller. 

I hadn’t really figured out soldering irons either when I made that diagram. I crimped absolutely everything, and any soldering I did at the time was done with a torch. As I’ve since found out, the trick to soldering irons is cheating. Melt a blob of solder onto the iron, then use that blob to conduct heat into the wires. It works way better than just trying to heat up the wires with the iron. Keep the iron sitting in the blob on the wire until the solder pulls itself into the wire. Using a torch will heat up the wires more quickly but the flame is liable to melt and burn your insulation.

It’s also going to be difficult to solder wires with corrosion on them, even using flux or resin core solder if the corrosion is heavy enough. Ideally you’d replace the wires entirely but that’s not always practical, so I try my best to scrape the wire back to shiny metal before twisting and soldering.

 

I’ve had issues with my spade connectors coming off the relays, or bending and contacting other relay pins, or just getting loose over time. You can get relay pigtails that hold all the spade connectors together. Really seems to help keep everything connected. 

For making Y-splices, if you will, or a 2-into-1, instead of trying to splice into the middle of a wire I’ll snip it off so I effectively have three wires. This makes it much easier to slide on heat shrink tubing. I’ll pick two of the three wires to lay parallel next to each other and one pointing opposite. I’ll strip back one of the pair of wires further than the other two wires. This lets me twist it with the paired wire and leaves a longer untwisted tail that I can then twist into the opposite wire. Then I’ll solder the whole thing together, making sure the paired wires lay parallel and as close together as I can get them. If you’re splicing this way, it’s imperative to use dual-wall heat shrink tube, hold the parallel wires together, and make sure the adhesive lining melts. It may seem a little sketch but it will hold up under the business end of the trucks spreading the salt on the highways, and if it works there I’m pretty confident it’ll work anywhere. 
 

But it is possible to eliminate the need for a mid wire Y-splice entirely in this harness, and run separate wires for each headlight back to the relay. Two ways to go about it. The first is just to use a female spade terminal for a larger wire diameter at the relay and just push both wires into it.

The second involves a fancier relay. You’ll notice the standard relay has both an 87 and 87a terminal. The 87 pin as you know gets power when the relay is energized, and the 87a gets power when the relay is not energized. There’s lot of functions you can use that for. For example if you had fog lights that you wanted to power only when the high beams aren’t operating (most jurisdictions require this) you could run the power for the fog lights off the 87a terminal of your high beam relay, so when you switch on the high beams the fog lights lose power. 
But you can also get relays that are externally identical to the standard relay but with a second 87 terminal in place of the 87a. This will allow you to run each headlight to separate 87 pins on the relays. You’d be running the extra length of wire up to the relay, but you’d have less complicated connections as a result. 
 

As far as crimp-and-seal, or the solder-and-seal connectors go, I’m a little skeptical.
It’s very easy to compromise the heat shrink coating while crimping, even if you’re using the correct crimpers for insulated connections, and anywhere the crimp doesn’t get made securely leaves you a gap for any moisture that makes it way in to sit and corrode your wire. 

I’m also not convinced the low-temp solder used is enough to ensure a good connection, or firm enough to provide mechanical hold. 
For butt splicing wires I prefer to twist them together tightly for good mechanical hold, solder them for good electrical conduction and to fill all the gaps inside the connection to prevent moisture intrusion, and dual-wall heat shrink tube to seal them and provide strain relief to the connection. I try my best to twist both wires around each other, not just wrapping one around the other, for the tightest hold I can get. If I’m twisting together two wires of dissimilar sizes, I try to twist the larger one around the smaller, which usually forces the smaller one to bend as well, otherwise you generally just end up wrapping the small wire loosely around the larger. 
When I’m attaching terminals to the ends of wires, I prefer to start with an uninsulated terminal (or pull the insulation off it), crimp it on, fill the barrel of the crimp with solder, and then heat shrink tube it to seal it. This again results in a firm mechanical hold from the crimp, a good electrical connection and filled voids in the connection via the solder, and seal and strain relief from the heat shrink tube. 
If I remember when it comes up on something I’m working on, I’ll try to take pictures of what all I’m talking about to better illustrate. 
 

But that all said, there’s lots of ways to do everything. The method that is easy, efficient, and affordable for you and accomplishes what you need it to do is the correct method. Not everyone is dunking their wiring in salt water for half the year, or shaking the crap out of it, or covering it in lumps of mud or ice to dangle and put stress on it. As long as whatever you do works for you, it’s good. Don’t let perfect get in the way of good.

[NickyV]

Thanks for the response and your thoughts on this project, @gogmorgo. This forum and YouTube make projects like these way more approachable than they would have been before, but one thing I find challenging is figuring out the right search terms. Just knowing the term "relay pigtail" would have been helpful for me to find the part that I wished I had — the one in your picture. I live in the Bay Area/Northern California and I don't have to worry about salt on the roads and ice. Hopefully my shovel connections will hold up for some time. But I will make a note of this and I may eventually upgrade. It would be nice to have everything sealed up real tight.

 

I'm also glad you brought up the relays with the 87a terminal. I actually used one of those, because while I apparently have the patience to build a harness myself, I don't have the patience to order parts on the internet when I'm rearing to go on a project. As it happens, one of the relays I bought is a standard relay (30, 85, 86, 87) and the other is one with an 87a (because that's what they had at the auto store when I went to get my relays). I just left the 87a empty. But I think I used it for the high beams. At some point in the future, I may figure out how to make it so my fog lights go off when high beams are on. Then again, I've never actually used them while on the road. This is not a daily driver and I keep the covers on them most of the year... occasionally pulling them off just to make sure they are still working.

 

I kinda feel like the only time I'd actually use them would be on a forest service road, far away from the prying eyes of the CHP.