Wiring Harness : where to begin?

[jakester888]

Time to start the wiring on the MK4. I am using an un-trimmed wiring harness from a 93 Mustang Convertible. I do not plan to use the wiring diet, just going with wiring whole (if possible) and trimming after engine install & fire-up. The FFR manual is not too useful to suggest where to route the wiring, attach points, etc.

Can someone post pics or links showing their wiring harness install?

Thanks, Jake.

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[Mustang Man]

I guess the Mk 4 manual doesn't do/say much about the donor wiring. I presume they think everyone buys a complete kit these days? You might want to look into finding an older manual (from a Mk 2 or Mk 3) for sale, or even buy the digital version, as there's much more in those books on the donor wiring and how to route the harnesses and even use the factory Mustang switches (ignition, headlight, etc.) if you're thinking of that as well.

I must say, installing the harness undieted is going to leave you with a lot of bulk/connectors that you'll never use (power windows/locks, power top, seat lumbar, etc.). I understand you might be on a budget or trying to save money using the donor wiring, but you can find basic "street rod" wiring kits for under $200 that are fully labeled and will save you a lot of time and frustration, plus the wiring is new, and not two decades old.

If you do stil wish to use the donor wiring I would invest in a wiring manual (EVTM) from eBay for your model year and at the least trim some of the non-essential stuff away and rewrap the harness.

HTH...
Mark

[jakester888]

Mark.

I'm planning to prune/trim after install and engine running. Any suggestions on where to route/attach on the frame? Which connectors fit through the hole in the footbox?



Thanks, Jake.

[jakester888]

Which connectors fit through this hole in the footbox?

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[lbperry]

Jake;
x2 on what Mark said. I had an EVTM and a complete Ford wiring diagram when I used the harness from my '88 donor. And I still spent hours/days tracing wires and trying to figure out what to use and where everything went. 20 year old connectors are pretty brittle and easily broken. If you're going to stay EFI with the ECM, (I did), I guess there's some justification for going that way, but it's going to be a big job. Several guys said that they laid the harness on top of the car and figured out where everything went that way. That might be a good approach if you decide to do it yourself.
Good Luck,

[jakester888]

I own the EVTM. Still just looking for a few good pics showing frame mount locations and which cables go through footbox. :)

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[DARKPT]

I have the Coupe rather than the Roadster, and also started with a '93 Mustang convertible harness. Since the cars are very similar you should be able to use this as a starting point:

I have the fuse box above the driver's left knee, and the ECU to the right of the passenger's right foot. Wiring from the ECU comes out the top of the passenger footbox on the Coupe (a hole in the footbox top matches an oval rubber grommet from the Mustang harness), then runs to the back of the engine for fuel injectors etc.

I put the coil on the driver's side between the brake master cylinder and the left front upper control arm. The front light wires all exit the footbox next to the master cylinder (there's a round rubber grommet on the Mustang that fits a hole in the footbox front wall). I ran the rear harness inside the trans tunnel on the driver's side (near the top of the tunnel). In the rear, I ran the harness down the driver's side for the rear lights, then across to the fuel pump.

Hope that helps!

[jakester888]

Darkpt : Yes! Thanks.

This helps too...

http://farm5.staticflickr.com/4085/5081695795_f128408b3c.jpg

[CapeCoralCobra]

Jake,

DARKPT gave you a good summary. Its been a while, but if I remember correctly, regarding the hole at the top inner corner of the footbox you posted, after running the harness up the tranny tunnel, I ran all wires related to the the fuse box, clutch safety switch, front harness (headlights, etc., which then go out the hole next to the MC) and the gauges through that hole. I made it a little larger to avoid pinching anything. I don't need a heater down here, so I put the ECU behind the glove box and ran the engine harness though the firewall right behind the engine. I cut a new oval hole to match the grommet and sealed up the one above the pass. footbox. Makes for less/shorter wires running around the engine bay. I mounted the starter solenoid on the pass. side motor mount bracket.

I laid out my harnesses & dieted them before the install, but I followed the advice of previous builders and didn't remove any ground connectors from the harness other than where I deleted the whole section containing them.

[DARKPT]

There's a few weird things to watch for when using the donor harness. I can't say I resolved them, but maybe you'll do something different:

-The "exciter" wire for the distributor is in the same harness that feeds the lights. For me, that wire comes out the front of the driver's footbox, goes all the way to the front x-brace, then across the car and back towards the engine on the passenger side. I stuck it under the mount for the air filter and ran it up to the distributor. If I had it to do again I'd probably have that wire just come up with the injector harnesses and save 10 feet of wire. :)

-Kind of a similar story for the radiator fan. I have a relay sitting on the passenger side of the x-brace. Almost all of the wiring for that relay originates on the driver's side (control circuit is part of the front harness with all of your lights, relay power comes from the main junction that I mounted near the coil). However, the ground wire from the coolant temp sender runs from the relay up the passenger side of the car to the back of the passenger side head where I have my sender mounted. It all works fine, but it seems a little funny to have such a large loop just to control the fan. If I had it to do over again, I'd probably run the fan control wire out the passenger side footbox, pair it up with the temp sender wire to the relay, and just have the relay power coming from the driver's side.

-The fuel pump cutoff/reset is wired so that it ends up behind the driver. In the Coupe, I just mounted it below the floor back there with a hole in the floor that I can reach through to reset it. You might end up with it somewhere in the trunk. I've seen some builders say to put it where you can reach it while belted in. I haven't had it trip yet except when testing it before I installed it, so it is fine where it is for me. Something to consider if you're concerned about having to reset it. You can't use it to kill power to the fuel pump, so it's not like having it nearby would help in an emergency...

Finally, I ended up documenting the changes I made to my harness (especially the front and rear harnesses) to help me troubleshoot down the road. I also repurposed some of the wires in those harnesses to do what I needed and needed a way to document the new use of those circuits. I was able to ditch 2 of the 3 harnesses going to the rear of the car by moving things around. I used a free product called ExpressSCH (I have version 7.0.2) to make diagrams of the changes. I didn't document every circuit on the car, just the ones that I modified or created. If you want my diagrams, you can grab them here: Coupe Wiring Diagram (http://www.carrollclan.com/FFR/BAMA COUPE WIRING DIAGRAM.sch)

Good luck, and take your time. I spent about 10 weeks start to finish on the wiring. You can't hide much of the front harness on a Coupe. :p

[skullandbones]

The above post by DARKPT brings up a good point. If you are working with a new unbundled set of wires you can lay them out and arrange them according to best (shortest distance) from A to B. In the donor wrapped harness this involves a lot of unbundling and rerouting. So I laid mine out as I mentioned all over the chassis. I also had a new engine harness EFI from Summit. And some of those wires went to areas that were not the best route. So I spent some time rerouting them the hard way. But after that when everything was where I thought it should go, I wire tied everything to it's nearest frame member or brace and routed the ends to their respective device such as the radiator fans, horn, MAF, fuel pump and fuel leve sensor, etc. When it was time to permanently mount the wires in their wire looms and insulated holddowns, it made the process a lot easier. There were a few instances where I saw a reroute was necessary even after I thought I had thought it out completely. The sacrificed wire ties even though temporary really simplified the process in the long run. I would start at the fuse box left of the driver's side. All of my wiring to the rear when through the trans tunnel and that was the battery wires too. My only regret is not creating an access door in the trunk to get to the cutoff switch without going underneath the car. I'm not sure if there is a perfect way to run all the wires without having some issue somewhere in the project. Good luck, WEK.

[jakester888]

All great points.

Pics pics pics... please.

[lilnuke]

I have the fuse box above the driver's left knee, and the ECU to the right of the passenger's right foot. Wiring from the ECU comes out the top of the passenger footbox on the Coupe (a hole in the footbox top matches an oval rubber grommet from the Mustang harness), then runs to the back of the engine for fuel injectors etc.

I put the coil on the driver's side between the brake master cylinder and the left front upper control arm. The front light wires all exit the footbox next to the master cylinder (there's a round rubber grommet on the Mustang that fits a hole in the footbox front wall). I ran the rear harness inside the trans tunnel on the driver's side (near the top of the tunnel). In the rear, I ran the harness down the driver's side for the rear lights, then across to the fuel pump.

I did nearly the same thing with my MKII as far as routing.

The extra I will add: I mounted my starter soleniod on the 1" tubing above the trans, behind the motor. I also re-routed all of the wiring from the fusebox to come out one of three places: 1) the front of the footbox (lights, horn, coil, and rad fan) 2) the side access (alternator, trans harness, rear harness), or 3) dash area (dash harness & computer power). Computer came out and ran along the 2x2 behind the motor along with wires from footbox.

I recommend that you use your EVTM to strip and diet the harness. It is a pain and takes a while but you will get rid of several pounds of useless wiring. Several of the harnesses are to long and would benefit from shortening unless you want the wiring all curled up. One way to do it is lay the harness on the car and verify that everything works, then diet one circuit at a time, testing in between. When you are finished, your harness will be much lighter and ready to secure in place once you wrap it up again.

You will also be able to reroute the wiring to have more of it hidden, it really adds a professional touch. I have run across a few (been a few years now) builds that did not take the time to diet & route the wire harness efficiently. Let's just say it really detracted from the car when the hood was opened and all those wires were everywhere.

Use the clamps that came with the kit or get some of these insulated clamps and rivet to the frame to hold your harness.
http://www.elecdirect.com/images/pageheaders/RubberInsulatedCableClamps.jpg

[skullandbones]

X2 on the above post. I forgot to mention that if you don't diet the wire harness if it is a donor one, you will run the risk of not being able to route the bulk of wiring thru that hole in the right side of the driver's foot box. I don't know if I had more critical wiring at that area or just the design of my body harness but it was tight. Any extra wires you have in your harness will just make that bundle more difficult to route, secure, and hide for areas where it counts. WEK.