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Looking at the many write-ups on how to do this, and while there is some good thinking there, most seem way over-thought in terms of the wiring. Why wouldn't it work to:
Run some red 1/0 from the battery positive terminal (in the passenger side bin) to the original positive battery lug in the engine compartment
Run some black 1/0 from the battery negative (in the passenger side bin) to the original negative battery lug in the engine compartment
Alternately, you could attach the existing negative lug to the chassis in the engine compartment, and ground the battery negative to the chassis near the bin (using the chassis to complete the circuit. I like the home-run idea better tho, maybe also add another one from the battery to the chassis near the bin for belt and suspenders.
I didn't do voltage drop calculations but 1/0 should do the trick given that's like an 8' run, probably less.
I have seen a lot of people add extra runs such as from battery positive to the starter, alternator, etc. But to me, if I get all the voltage to the original lugs it seems like it should work perfectly fine.
I grounded my negative battery terminal to one of the irons on the motor (which in turn is grounded to the chassis), and to the chassis in the rear where its mounted.
Might be hard to run 0/1 awg from the bin to your fuse box with the cable that thick, you might have to do some trimming. I run 4awg from battery to the fuse box, and ~8" of 0awg ground cable from the battery to the chassis, been this way for 10+ years without any issue.
You don’t need to run the positive cable from the battery back to the fuse box. You can land it on the starter stud and call it a day; the original starter cable now functions to backfeed the fuse box, resulting in a cleaner and simpler install. I used 1awg welding cable because it’s what I had; it is more than adequate.
Ground your battery neg right to the chassis under the seatbelt bolt or similar with a short cable.
There is no need or reason to do home runs as it does not buy you anything.
As long as the engine to chassis ground is in place, you are good to go.
Looking at the many write-ups on how to do this, and while there is some good thinking there, most seem way over-thought in terms of the wiring. Why wouldn't it work to:
Run some red 1/0 from the battery positive terminal (in the passenger side bin) to the original positive battery lug in the engine compartment
Run some black 1/0 from the battery negative (in the passenger side bin) to the original negative battery lug in the engine compartment
Alternately, you could attach the existing negative lug to the chassis in the engine compartment, and ground the battery negative to the chassis near the bin (using the chassis to complete the circuit. I like the home-run idea better tho, maybe also add another one from the battery to the chassis near the bin for belt and suspenders.
I didn't do voltage drop calculations but 1/0 should do the trick given that's like an 8' run, probably less.
I have seen a lot of people add extra runs such as from battery positive to the starter, alternator, etc. But to me, if I get all the voltage to the original lugs it seems like it should work perfectly fine.
There's no reason to not just ground the battery straight to the chassis in the luggage bin / rear seat well. Weld a bung if you have to.
I attached the positive wire straight to the fusebox, not the positive terminal, which would be much less clean and weigh more.
There's no reason to not just ground the battery straight to the chassis in the luggage bin / rear seat well. Weld a bung if you have to.
I attached the positive wire straight to the fusebox, not the positive terminal, which would be much less clean and weigh more.
There's a HUGE reason not to do so.
Car bodies are made of panels bonded together. If you ground the battery to a panel in the rear you will have a significant amount of resistance to the front of the car.
Many of the problems people have had over the years with battery relocation - slow cranking, odd electrical gremlins, etc. - can be attributed to just grounding the battery to a threaded hole in the back of the car.
You can either run a whole home run of cable to the original ground points to basically make a new starting/charging harness or connect the battery into the original harness.
Personally I refuse to relocate the battery. It solves one problem (battery in the way) and creates a ton of new ones. I'd rather run a smaller battery up front. I've been running a Miata battery for some time now and it works great, only down side is it will drain/go flat in about a month without a battery tender. But cranking, starting, car running, it's all good.
A couple things to note. I ground the negative cable to the engine block, and show you how to run the cable from the bin, through the fender. I also installed a 250amp circuit breaker. The JP3 positive battery post was used in my engine bay. This was done about 4 months ago, and while I haven’t been driving the car in winter, it always starts right up.
If it’s helpful I could also post a writeup on the forum.
Has anyone run the cables directly into the engine bay rather than out through the wheel well and then into the engine compartment? Wondering if they could go through the firewall next to the clutch MC?
This is my setup, almost done:
1/0 positive from bin to engine Bay
Short 1/0 ground to chassis using rear seatbelt bolt with paint removed.
4 ga ground from there to engine bay and trunk.
Using 200 A breaker, Battery Brain low voltage disconnect and Bluetooth battery health monitor.
Added aux fuse box.
Added extra wire to alternator to original positive.
Will be adding extra wire to starter positive.
Wiring Specialties grounds (alt case, block, chassis) and new exhaust ground straps.
Has anyone run the cables directly into the engine bay rather than out through the wheel well and then into the engine compartment? Wondering if they could go through the firewall next to the clutch MC?
the 1990-2005 miata has a battery in the trunk, and Mazda ran a short ground to the body right by the battery, and the positive cable goes to the power plant frame and then the starter post. then they have a bigger ground between the engine and body.
its really simple
the 1990-2005 miata has a battery in the trunk, and Mazda ran a short ground to the body right by the battery, and the positive cable goes to the power plant frame and then the starter post. then they have a bigger ground between the engine and body.
its really simple
This.
If spot welded unibodies had such bad conductivity, they wouldn't use them to ground things.
The FD was originally built as a four-seater, believe it or not, so there is sufficient metal in that general area to achieve a good ground. One of the rear seatbelt holes will work fine.
It's not going to be any worse than the engine to chassis ground.
At least for me, it has worked perfectly. I did use very thick cables, though.
the 1990-2005 miata has a battery in the trunk, and Mazda ran a short ground to the body right by the battery, and the positive cable goes to the power plant frame and then the starter post. Then they have a bigger ground between the engine and the body.
its really simple
My setup has ~0.2 volts drop at idle (+/- 14.6v) between the battery and the stock frontmost (i.e., behind the front bumper) common grounding point. I confirmed this while troubleshooting an annoying headlight issue that wound up being a loose c/b circuit screw (also in the picture)... sucks to be stupid!
Last edited by Carlos Iglesias; Mar 2, 2023 at 04:57 PM.
Reason: English talk gooder me.
My ground is right where your fuse panel is, I forgot the voltage drop measurement, been like 10 years since I put the battery there. I do recall the ohm test was about 0.1ohm from the battery post to the engine bay ground, and I had zero issue ever.
I relocated. Used 4AWG welding cable for both lead and ground. Ran the ground a home run to the block and also a local ground. Ran the positive to the starter lug and with stops to a breaker and battery disconnect switch. The JP3 positive post is a must to prevent the floppy fuse block.
As mentioned before, the main unibody panels are seam glued, and grounding wasn't great when these cars were new.
I relocated. Used 4AWG welding cable for both lead and ground. Ran the ground a home run to the block and also a local ground. Ran the positive to the starter lug and with stops to a breaker and battery disconnect switch. The JP3 positive post is a must to prevent the floppy fuse block.
As mentioned before, the main unibody panels are seam glued, and grounding wasn't great when these cars were new.
No idea what you are talking about. Unibodies are spot welded together and then glue is used to seal the seams.
OTOH, the North American market cars are all early-model FDs, and my is a mid-model (96), so maybe they made improvements after 95.
I haven't had issues with fuel pressure on the stock wiring, either.
No idea what you are talking about. Unibodies are spot welded together and then glue is used to seal the seams.
OTOH, the North American market cars are all early-model FDs, and my is a mid-model (96), so maybe they made improvements after 95.
I haven't had issues with fuel pressure on the stock wiring, either.
It's easy to see when all of the interior is removed. Unibody sections with some pinch welds here and there, but much seam glue is used. I cannot speak to any model years beyond '93, but it is quite evident for those with these years.
A home run ground is necessary for a relocated battery for these reasons. Anyone messing with these cars long enough (and they are still around) will tell you.
I also ran a 10AWG "stitch" cables between the unibody sections using some of the interior stand-off studs.
/thread jack
Anyone know what this thick wire on the front bumper pass side common ground is for? It got hot enough to melt through the plastic.
I was able to pull the wire and ground connectors and clean them well and also added extra ground wiring and sanded off the paint.
/thread jack
Anyone know what this thick wire on the front bumper pass side common ground is for? It got hot enough to melt through the plastic.
I was able to pull the wire and ground connectors and clean them well and also added extra ground wiring and sanded off the paint.
That looks like the JC-01 or JC-02 ground points on the front harness. Both are up front under the bumper, and it appears from the FSM section Z wiring diagrams that those heavy gauge black wires are the grounds for the cooling fans. If I'm not mistaken, one fan grounds via JC01 and the other fan via JC02.
That looks like the JC-01 or JC-02 ground points on the front harness. Both are up front under the bumper, and it appears from the FSM section Z wiring diagrams that those heavy gauge black wires are the grounds for the cooling fans. If I'm not mistaken, one fan grounds via JC01 and the other fan via JC02.
AWESOME! THANKS I can feel at ease now.
My fans died not long ago and I smelled the electrical burn smell, I thought it came from the fans themselves. They were replaced with RX-8 units.
Now they have extra healthy ground paths as I cleaned the common grounding things, sanded the paint off where they are bolted to and added extra wiring to the chassis batt- on the driver side and to the chassis below the fan relays on the pass side.
