Mk1 Corroded PPF Earth Strap

A while ago, when I was driving the MX5, I had a intermittant starting issue that someone suggested was due to a loose PPF earth bolt (symptoms are, when hot, the engine barely turns over, as if the battery was flat). I then forgot about it, with everything else going on. Took the Mazda out for the first time in 5 months (and the last time was just a quick dash around). , got back, parked on drive. Then restarted to put in the garage. Nothing, just turned over really slowly, and the windows went haywire, which is a new one. Went away for a think. Checked batttery voltage; 12.6V, so nothing wrong there. Then eventually it started, but the idle was horribly low.

I finally made the effort to check that PPF strap, and the previous diagnosis seemed sort of correct. The bolt was tight, but the insulation was torn, the wires were corroded, broken and a bit crunchy.

So I need a new earth strap. I think this just bolts up near the battery negative terminal, and I need a 58" strap. However, I cannot find a replacement Mazda item from the usual suppliers, and my crimping tools/skills are useless, so is there such a thing as a supplier of ready crimped earth straps of different lengths (no ebay suppliers)? I think I might replaced batter to body and body to PPF straps with two new ones.

This guy has 30" straps

Perhaps bolt 2 together if you cant find a 58" one…

Ebay though, and I don’t do Ebay thanks to previously being burnt by it, but thanks for the tip. I’d prefer a single piece though. I think there are some ebay suppliers who do custom lengths, but they are on ebay…

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How about here

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Perfect, thanks

Worth noting that it is the battery negative cable. It’s one continuous length, from the battery terminal to the chassis at the side of the boot, then forward and down through the wheel arch to the PPF.

I had the luxury of being able to borrow a really big crimp tool so I was able to make one easily from 16mm² cable and crimp terminals. On reflection, since I cut it to length while I installed it, sadly I can’t give you the right lengths from PPF to chassis and chassis to battery in order to get one made up. That’s annoying.

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Pity about the length uncertainty. The 58" came from a posting of yours on another forum, but elsewhere, there are indications its 66"

So I can’t get an earth wire bolted to this point, and then to the PPF, it has to be one piece from the battery, with this connector (which I don’t know what its called) added in?

I could just run two cables from the battery earth terminal; one to the body, one to the PPF.

I did not understand your ( no ebay ) bit. I thought you meant you couldn’t find one on Ebay…durrrr !
I dont see why it would have to be one piece. A short one from the battery to the first bolting point then a second from the same bolting point to the PPF.

That connection there looks more like the cause of the problem than the one on the PPF. Are those wire strands broken?

Not the best of solutions to this bit of wiring, when mine broke on the battery side some years back I purchased a short lead form Halfords and it has been fine ever since.

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This is not my car. My car is blue, not red. Its an example of the bracke/clip I am referring to. The wire is fine, but what do I know? The PPF wire feels extremely fragile to touch, but you think this isn’t that much of an issue? Am I likely to experience issues replacing the stock battery-PPF lead with two new leads. As I understand it, Mazda will only supply this lead with the main loom.

I would be tempted to clean up the ring terminal and its seating area on the ppf and see if that improves things first.

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I recall some discussion about the length of the cable on miata net but it was so long ago…

Anyway, it might be worth saying that feeding the new cable through into the wheel well would be a bit more of a pain if it already had a terminal crimped on the end, so if that one at least can be crimped in place it’ll be easier. I got 2m of cable which was more than enough but I can’t say exactly how much I trimmed it. I used 16mm² cable and a 16mm² crimp eye for the PPF end, but I used a 35mm² crimp eye at the body connection in the boot, so that I could cut the cable there and feed both 16mm² ends into the same crimp. I can’t see any reason not to have two separate cable terminations and bolt both together to the body, one on top of the other.

I guess it’s easy to measure the body-to-battery length and if you subtract that from 2m then the remainder will definitely be enough to reach the PPF, even if it means leaving a small loop of extra cable in the tunnel forward of the spare wheel. The way to get the length exactly right would be to pull out the original and measure that.

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Fragile? Do the strands break? It’s all about the cross sectional area to be large enough to allow current to flow.

I can’t see that being a problem. I replaced the complete harness on Project Merlot Black with the one from the original Project Merlot, the issue on that was the strands at the negative battery terminal itself that were breaking.

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The problem presents itself after a long drive. Rather just be done with it.

Some of the strands at the PPF were broken. Wires “crunched” when I touched them. With the car on a steeply sloping drive, I wasn’t going to investigate more unless I had a solution at hand.

Perhaps I am reading too much into ground resistance.

A multimeter would help with that.

There’s also the braided earth strap next to the dipstick which does the same job of bonding the drivetrain to the chassis (though obviously it’s just a secondary connection, more local to the ECU I suppose) and it ought to be good enough to keep the car running normally if not for cranking (I’ve heard of it literally smoking when cranking if the main wire is broken). Worth checking the condition of that and maybe loosening the bolts and nipping them up again just to clean up the connections.

Replaced the PPF earth; I’ve been given to understsnd because the battery is in the boot, battery earth to PPF to Gearbox/engine is very important.

Original earth


I opted to not try and disturb the loom to which the old earth was strapped to, and routing through an existing hole. Grommet to be added. 6 foot wire used, which gave me enough slack to route the wire behind the electric aerial motor.

Grotty old earth wire