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GBS Engine Link Plug - help


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Hi Guys I'm hoping someone can help me make some progress with my gbs wiring, I'm a bit stuck on the engine plug which spurs off from the chassis harness to my MX5 ME221 ECU.

It's mainly the top 3 pins I need to figure out where they go, the others should be ok. 

1. Which pin do I use for main 12v ignition to the ECU? (On the OEM diagram it comes from the main relay to most of the sensors except coils and to ECU Pin 4B), I'm a little confused because Pin 2 sounds like the one but I'm measuring no voltage here. I've put a link to the schematic of the relay.

2. Pin 3 main relay control (coil, injection, lambda) I'm planning on connecting the ignition coils here but the notes state "emerald switches to earth" but I'm using an ME221 will this also switch to earth? I don't want to blow anything inside the ECU.

If there are any mazda builders that can confirm which pins they used for what that would be a great help. 



Not sure why it won't let me add a photo from imgur I'm sure I used this method last time. 

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This is from memory, and that is fading due to age.

Please note that the main loom was designed to be used with an engine/ECU sub loom, and Emerald ECU.

As you mention the MX5 ECU I assume you are using that, and making your own engine loom.

I do not know if the MX5 ECU switches the same way, you will have to investigate.

Loom plug

Pin 1, ignition switch live, 12V+ when ignition is turned on.

Pin 2, is 12V + from the main ignition relay pin 87, but only when Pin 3 is grounded

Pin 3, the ECU grounds this pin, which switches on the main relay pin 86

So, switch on ignition and Pin 1 gives 12V +, this wakes the ECU, which grounds pin three, which in turn, switches on the main fuse board relay, which makes pin 2 go 12V + for engine components.

I hope that makes sense.

You can easily check with a test light, and a bit of wire to ground Pin 3, you can also check Pins 4 cooling fans and 5 fuel pump/pumps relay by grounding them.

Note when pin 4 is to ground Pin 12 goes 12v+ to feed the high pressure fuel pump

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Thanks that explanation has helped me to understand it better. I've connected pin 1 to my ECU and confirmed all sensors are receiving correct voltage, the sensors left are O2 sensor and coil pack which on the Mazda are powered from a different wire compared to other sensors. 

I'm still struggling with pin 3, do I connect this to my coil pack and O2 sensor 12v supply? Basically these are my last 2 sensors I need to supply 12v to. But when I connect my coilpack wire I am reading 6.8v at both coilpack pins which is confusing me.


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no, defiantly not.

consider pin three as the ECU's way of switching everything, related to itself, ON.

Switch on the ignition and pin 1 goes live, this feeds everything NOT associated with the ECU system, that needs a live feed, Alternator field terminal for example, and ECU power.

On getting a live feed, the Emerald ECU switches pin 3 to ground.

I have no idea which way around your Mazda ECU switches it, normally ECU's use ground switching, but I do know that one of the ECU's on the MX5, live switched the coil packs, you have to mess with the Emerald settings to get these to work, so it's possible it could live switch other things as well.

One of the reasons an aftermarket ECU, Emerald is one of many, is preferred over the original.

With Pin 3 to ground, ignition on, Pin 2 goes live, feed from the ECU relay, see fuse board diagram.

Pin 2 is the power for the ECU stuff such as Lambda and Coil packs.

Check your donas wiring diagrams, as I say some coil packs are live switched, cant remember which age range of MX5.

I hope this makes sense.



fuse board.pdf

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Thanks Simon, so I think my main problem is that the MX5 ME221 ECU works differently to the Emerald that GBS use (guess who's wishing he bought an emerald now!), it does not have power supply pins which can be earthed etc. I'm guessing because it's a plug&play ECU. 

Looking at the mx5 diagram it has 3 power supply wires:

Main Relay - Wht/Red - goes to ECU & every engine sensor apart from the 2 below

Engine Fuse - Blk/Wht - 12v supply to coil pack (ECU activates the coils with 2 separate wires) 

Meter Fuse - Blk/Yel - 02 sensor heater supply & check engine light 

Let me know what you think but the only way I can think about making it work is grounding pin 3 to chassis or battery to activate Pin 2. I was then thinking of adding another relay into the mix because I wasn't sure running everything from pin 2 was the right thing to do. Something like sketch in the link. 


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I have decided to go with the extra relay as per the photo in my previous reply, everything seems to be getting voltage as it should so happy with that. Now to wire my gauges & column switches up! 

Reg - R53 FGC

mk1 1997 1.8

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