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Old 02-03-2018, 04:26 PM   #36
John V
No more BMWs
 
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Join Date: Apr 2005
Carmudgeonly Ride: Ram, MS3, CX-5, RX-8
Location: Glenwood, MD
Posts: 14,753
Alright alright. I haven't posted in a while.

Wiring! That's what's on the menu lately. I bought this 2006 Miata to scavenge parts of the "front" wiring harness. The RX-8 and the MX-5 are pretty similar, but the wiring is quite different. Both cars have dedicated engine harnesses, and since I'm using the MX-5 engine (sorta), that doesn't have to be modified. On the MX-5, the engine harness plugs directly into the ECU, located in the engine bay. But both cars also interface the engine harness to what Mazda calls the "front" harness, which handles the lights, ABS, fans, air conditioning, CAN bus, and a bunch of other things. The front harness is also what connects to the engine fuse box. I wanted to make it so the MX-5 engine could plug into the RX-8 front harness and behave exactly like an MX-5, for future troubleshooting.

The first step was to thoroughly understand each car's wiring, and develop a cross-reference sheet. I figured this would be challenging, and it did not disappoint. Even though there are only about fifty unique signals that needed to be spliced in, Mazda locates them in different places on each car, they use different wiring codes, and... it's complicated.





Once I had all of that sorted out, I bit the bullet... I had to go cut the MX-5's wiring harness. Mostly I needed two connectors, each of which plugged into the engine harness and routed signals like the CAN bus, the taillight and reverse lights, the starter interlock, etc into the interior harnesses.



The white and gray connectors in this picture attached to the MX-5's engine fuse block. I figured I'd find a place to attach them to the RX-8 chassis, near its fuse block, which is in roughly the same location as the MX-5. I cut them with enough wire attached to allow me to easily splice them into the RX-8's wiring harness. The big gray connector with the lever lock is the second connection to the MX-5's engine computer. I had to cut that off as well.



Here they are all cut out and ready to start working into the RX-8.



Moving to the RX-8, I had to cut out the sheathing and tape wrapping its harness. I'm converting the RX-8 from electric power steering to hydraulic, so I figured I'd lighten the front harness load and simplify the wiring by removing all of that wiring, plus all of the emission control wiring (which is substantial in the RX-8 as compared to the MX-5).



To the right of that pic is the fuse box, which is where all of these wires come and go.



And on the passenger's fender, a bunch of wires that need to get eliminated from the RX-8 wiring. The fuel pump resistor, the air pump, the EPS computer wiring... most of this is going away.



The first splice I made was a bypass for the two-speed fuel pump relay that Mazda runs. The rotary requires so much fuel at high RPM that they run the fuel pump at low speed during idle and at a faster speed when necessary, to avoid heating of the fuel. I don't need this for my setup as I'm converting the fuel system to a true return-style system, so this will simplify things down the road. Basically I'm bypassing the low-speed relay and always running the pump wide open.

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