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Interfacing with OEM Wiring Harness

Practical Motorsport Wiring - Club Level

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Hi everyone!

Just completed the wiring courses and thought they were excellent. I am in the process of building a Spec MX-5 (Spec MX-5) race car. These cars are the newest iteration of the Mazda Spec Miata based on the NC chassis (2006-2015).

Mazda Motorsports is advising not to make a custom harness as the series requires use of the OEM ECU and a Mazda Motorsports supplied tune. The tune requires retention of the keyless-entry and anti-theft modules.

So my question is - any recommendations on interfacing (eg. radio, data systems, cool suit, etc.) into the OEM wiring harness? My initial thought is to identify existing branches to repurpose (eg. use the OEM radio power to power my Motorola radio).

The interior is completely stripped as was needed for the roll cage installation. My plan of approach is to:

- Reinstall OEM wiring harness

- Reinstall gauge cluster, anti-theft module, keyless entry module, ECU

- Start the car up and begin unplugging connectors to determine what is and isn't needed for start-up

- Remove from loom or repurpose unneeded connectors

Thoughts or advice?


Hi Steve,

In this circumstance I would recommend building a separate harness to handle the extra functionality that you require in the vehicle with minimal connections (data communications, ignition power), rather than trying to re-purpose the factory loom. Building a separate loom will usually be quicker and neater than trying to re-purpose an existing loom, and will be easier to fault find. You also don't need to worry about causing a fault in the OEM loom from adding extra devices into it.

Thanks Stephen, appreciate the advice. I think I will go that route and will be a great opportunity to build my first (accessory) harness. For the OEM loom what is the best approach to terminating unused connectors / ends?

If they are connectors that have just been unplugged, I would leave them as is, unless they are in a location that will be exposed to damage (temperature, moisture, impact) in which case I would either cut the connectors off and shrink adhesive heatshrink to each wire end, crimping the open end of the heat shrink closed with some pliers whilst it is cooling to ensure that it is fully sealed, or I would cover the whole connector with heatshrink. Other exposed wires would be heatshrunk with the adhesive heatshrink.

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