"Here's a few tips on modifying the Miata electrical harness.
Note: This was done on a 1994 Miata. There may be differences between the generations.
Note: Here's where I downloaded the wiring diagrams from. The Haynes manual WILL NOT cover everything. I very highly advise against modifying a harness using just the Haynes manual. They do not cover half of the important items, and often do not show multiple wires--there are a lot of splices inside the harness, and some connectors have more than one wire coming out of one particular point. However, I used the Haynes manual to back this one up-- you might want to go scan the Haynes diagrams and staple/paperclip them seperate from the Mitchell diagrams.
http://www.madracki.com/miata/wiring.html
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Before attacking the harness, it might be good to skim through the diagrams to associate yourself with items you think you will want to remove.
Here are some of the items I chose to delete:
-popup headlight system
-all external lighting
-all internal lighting
-power windows
-power steering
-power mirrors
-power antenna
-rear defogger
-interior heating and cooling
-radio
-cruise contol
-airbag system
There are others, but those were the main items.
The orange and blue connectors are airbag sensors. They are a real pain, as they go throughout the entire car, and they take up about maybe 1/8 of your harness. They also have their own module, the "diagnostic module" or "supplemental restraint module" as it is called in the diagrams. This is something you will notice in several areas--Haynes, Michell, and regular guys like us all have three different terms for some things, and you may spend some time scratching your head thinking "WTF?". Anything related with airbags goes, but still check where the lines go before snipping.
I found it best to spread the harness in the general shape it would have been in the car, and hammer a few nails in strategic points to keep it from getting all tangled when you are trailing wires. Definately strip all tape and loom before trying to remove anything, because it is completely impossible to track otherwise--especially to consider there are quite a few splices, too. I brought mine inside to keep things slightly cleaner, and I placed a 4x8 sheet of plywood on two of the sawhorses I used to hold up my build table.
My donor car had just about every option except the kitchen sink, and ABS. ABS has it's own module as well, so if you have ABS, you will have 4 modules--the ECU, the ABS, Airbag System, and Cruise Control. That makes a LOT of wires. At one point I counted over 100 wires thick--thankfully I just had the three modules.
Mitchell shows a "Main Relay" in the diagrams. We scratched our heads for a while, and finally, after me reading the wire colors to my dad, we figured out that is what is labeled as the Fuel Injector Relay on the main fuse box lid.
If you intend on removing the AC/heating, make sure you don't just hack the wires--the engine coolant fan relay is in the bundle. I ended up pulling all the wires on the driver's side harness forward from the firewall except the ground, the radiator fan, the MAF sensor, and the diagnostic port (not to be confused with the diagnostic module). When I finished the harness, I pulled the diagnostics port and the ground behind the firewall, so there are only two sets of wires on the drivers side.
The TNS relay, mentioned in Mitchell, is the running lights relay. I deleted it, as I will have headlights on/off and turn signals, no running-lights-only setting. This is not to be mistaken with the DRL setup (Canadian).
There are a few diagrams in the Mitchell package that mention DRL wiring. Just because we're not in Canada doesn't mean we don't have the wiring--I deleted a few junctions that simply looped. Also, some of the automatic transmission wiring found it's way into my harness as well, usually going to a connector, and then not coming out of the other end, always causing a flurry of double-checking and freaking out until I'd find what it was.
Which brings to mind: How do you intend on wiring your headlights? If you are not using the stock column assembly, i.e. the stalks for lights and wipers, you will probably be better off stripping the entire headlight/external light assembly out of the harness and make an external headlight harness. That is what I am doing. It'd be too much of a hassle trying to wire up switches and selectors for that, as well as the fact that you are no longer using factory lights--you will have different wiring, most likely. If you are using a pressure-switch brake light, then you will probably want to remove the entire rear light assembly. That's the entire bundle of wires that goes down the passenger door sill on back. It connects to the main harness, and then to the battery harness, but other than that, it is on it's own. I did not have to do any stripping or cutting there.
Floor mounted pedals not only delete the use of a brake light switch, but also the clutch switch that came with the Miata, so I deleted all the stuff that came from that, and the two large wires that go to the switch are power wires, and I simply spliced the one to another, as that was just a dead-switch, making the car only able to start with the pedal down. So I'll be able to start the car with the clutch up, but what moron is going to start in gear?
That covers everything I could think of while going through the diagrams again.
When splicing, unless you already have a crimping tool, they cost a bit. Usually 50-100 and up. You want the ratchet type--the cheap kind really suck, and you're not going to want to do 200+ crimps with them. Butt splice connectors tend to be pricey, depending on the type. I live about 15 minutes from a Fastenal, so I have quick access to them, although I heard Home Depot has them as well. However, the guy there stated over the phone that they are for larger bands, i.e. 6-12 gauge, rather than 10-12, 14-16, etc. You'll usually need about
100 of the red ones (22-18g, about 10 bucks),
5-10 of the blue ones, (16-24g) and 5-10 of the
yellow ones. (12-10g) I chose the nylon coated type, there are vinyl, which tend to be cheaper, but are harder to crimp, the heat-shrink coated, which cost about 10 bucks for 25, and the heatshrink-and-self-solder type, which are REALLY expensive, and definately not necessary.
If anyone needs those parts, I can get them and ship them, as well as I'm willing to lend out my crimping tool if the person in question will pay for the shipping, and return it in a decent amount of time. I figure, it'd be easier to pay 10 bucks than 100 bucks--and I always ship fast.
If there's any questions about the Miata harness, I'd be happy to help if I can. Also, if anyone is relatively local and wants a hand with it (it really works better with two, one to read off wire colors and the other to match them with the diagram), I'm willing to help. "