I am in the middle of building a Haynes Roadster and I am using a MK2.5 MX5 1.8 as a donor.

I absolutely hate car electrics and wiring generally and after a few hours yesterday wrestling with the original MX5 loom I am considering abandoning it and putting in a new loom which only has the circuits that I really need for my simple car.

The thing that scares me most about it is the immobiliser set-up and what connections are required to make the ECU work. I've read some horror stories about trying to fault find...

So I have come to a crossroads and have a decision to make, I would really welcome other builders thoughts and suggestions.

Options;
1). Persist with the MX5 loom, try and cut out as much as possible for unused circuits and hope for the best with the ECU = high likelihood of failing!
2). Try and force the MX5 loom into the car without cutting anything out, but this is bulky and messy = Easiest and cheapest, but not very pretty
3). Try and use only the engine loom and ECU and buy a basic loom from CBS or somewhere = Cheaper then option 4, but still a bit risky that the ECU won't work
4). and finally, abandon the Mazda wiring in its entirety and buy an aftermarket ECU, engine loom and a basic loom from CBS = Very expensive.

There may be other options that I have missed?

Please bear in mind that I am really bad with car electrics and have no pleasure or confidence!

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Originally posted by albertz
There may be other options that I have missed?

Please bear in mind that I am really bad with car electrics and have no pleasure or confidence!

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The problem with keeping the full loom is that it is so bulky and unwielding to try and neatly fit into my car. I think that the majority of the wires are just not needed, so I thought that installing a nice neat loom such as the one offered by CBS would be a good idea, but I am just scared about how to integrate that with the Mazda engine loom and make sure that the ECU is happy.

I was a bit hasty yesterday and cut out one of the door electrics plugs and stripped back all of those wires, but then I took the fear and abandoned it before doing anything really silly that I will regret.

When I built my MX5 based Locost I did this...

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3). Try and use only the engine loom and ECU and buy a basic loom from CBS or somewhere

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Thanks for the reply, that is what I was hoping to do for the car loom. When you used the Premier Wiring loom how easy was it to connect in with the engine loom and did you have any problems with the ECU, ignition barrel, immobiliser etc?

albertz - when I built my MK Indy (Ford Sierra donor car) I bought a Vicki Green wiring loom purpose made for this type of car. It only had the wires I needed for the simple car and was easy to use.

I left the original Ford Sierra wiring loom alone. It had too many wires and was the wrong shape for the MK Indy.

I think you have the right idea and will make it easier to complete the electrics of your Haynes Roadster.

I used the MX5 loom and was very glad that I did. I took out only the bits that I knew I would not need ie radio, electric door window systems, interior lighting, HRW etc and then laid the loom over the car. It didn't fir properly so rear lighting wires were extended a bit and then started the car up. You will need the key, the aerial syten around the steering lock or the ECU will not light up. Also the ECU controls the charge rate of the alternator and the wires for this take a very strange route through the car. I used al the MX5 instruments as well so eveerything just worked. The speedo was attacked with an angle grinder and the dash shaped to appear that it had individual instruments fitted. Then came a bit where I took out circuits that I was sure I didn't need but started the car and checked all was well before the next step. Finally, there was too much wire behind the dash so a bit of a pain but shotened each wire one at a time. The amount of wire I took out was amazing but it all worked and at IVA even the speedo read within 2 mph of correct throughut the speed range.

Ok it was a lot of work but very rewarding that it all worked as it should.

I have just wired up a frinds Tiger Avon using an MS221 ECU and Tiger wiring loom - that was much harder to get all working properly

Hi, Yes I agree totally with Ian's previous post.

Whilst building my Roadrunner Sr2 I used the MX5 loom, instruments etc also. Just take your time and logically remove wires/ circuits not needed. If you are not sure then leave the wires in....
By using the MX5 loom all connectors are a simple plug in. The only thing you will need to do is to mainly shorten wires. I suggest that once you have stripped the loom you lay it over the car and cable tie to the routes you need the wires to take. You will then be able to work out what goes where and how much needs taking out. Just make sure that when you remove the old loom that everything is clearly labelled as this makes identification much easier.
One point worth noting is the Immobiliser. I had the unfortunate problem of thinking just by removing the wiring all would still be ok, not thinking that the immobiliser is coded to the ECU.....wrong! The Immobiliser is intergral and can not be worked around. So if you do have an immobiliser fitted (think this was only done on Imports) then make sure you keep the Immobiliser, and aerial around the key lock as they are all coded.
Found this out the hard way and ended up having to source a second non immobilised Loom/ ECU.
However apart from my own error, this was much much easier and cheaper and rewarding than other alternatives...

Thanks for all the replies, help and advice, much appreciated. However, I am still a bit unsure of what to do, to be honest.

I've been talking to the guy at Premier Wiring and he has been very helpful and reassuring that their product can solve my problems. However he has advised that I should look to buy a MK2.5 Eunos ECU as apparently they don't have the immobiliser issue, can anyone verify that is the case or not, just before I start looking to spend some money on another ECU?

I do understand the satisfaction that others get from stripping, shortening and reconfiguring the Mazda loom, but I just won't get any pleasure from doing that, I far prefer to work with mechanical items and refurbish, make my own etc in the mechanical world. To me, electrics is a case of getting to the end in the easiest, neatest and most efficient way. Right now I am thinking that £240 for a Premier Wiring loom customised for my car is a good deal, especially if I can couple it up with a standard Eunos ECU and the existing engine wiring harness.

My biggest concern is that either;

1) I stick with the Mazda loom have no end of electrical-related problems and start to lose interest in the car, or...
2) that I buy the £240 loom, assume it will solve all my problems and then realise that I have misunderstood something and it doesn't work or costs much more.

Basically I'm still confused about which way to go for the best.

Mk 2 or 2.5 - can't see the difference really. believe me the aftermarket looms are no walk in the park simple from what I have seen, MX5 loom - ok don't shorten the excees wires and you have a bit extra loom to hide but Mazda designed it and it works in many MX5's!

KISS

Hi I did this previously on my Haynes and about to again on my next westy.
The mk2.5 ecu is very different to the mk2 one.
You have to use the mx5 mk2.5 instruments as a couple of the ecu wires run to the circuit board within the binacle
You also need to keep the immobiliser and the ring on the ignition switch (reads the key)
I have found an add for a “demob” kit but need to make enquires on that yet.
If you have a search online for 2003 wiring diagram from mellens it is laid out pretty well and is the one I use. That way you can identify each circuit on your loom. There’s heeps that can be removed and provides an ample source of coloured wiring.
As said earlier lay the whole lot out and label each connector, strip the wrapping off and that way you can identify each set of wires and shorten accordingly
As for the rear end I just identified the relevant wires and extended them to suit rather than using the cabin / boot harnesses

Good luck

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Originally posted by gremlin1234

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Originally posted by albertz
There may be other options that I have missed?

Please bear in mind that I am really bad with car electrics and have no pleasure or confidence!

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just keep the full loom, it should work.
you could probably take out a few complete sections, but that would be 'higher risk'

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