Yeah, you don’t need to say it! 
So, other than swearing, despair and quite a bit of self loathing what’s the likely outcome of this?
Here’s the story so far… Battery dead, went to get other car to jump it. Engine running on ‘good’ car (Golf) connected leads in (what I thought was) correct order, bit of a crackle with the final negative (so I thought) connection but didn’t think any of it. Engine running in Golf tried to start Mazda. Nothing. Not even an interior light.
Went to check jump leads, felt sudden dread (the battery leads aren’t coloured, in mitigation) and realised error. Connected up correct way, still totally dead.
What have I done then? I went to the main fusebox in the engine bay and the big black 80A ‘main’ fuse doesn’t look too clever and seems, no pun intended, fused to its terminals. I’m hoping that this is it and once I change that all will be good. That’s the optimistic view.
Anyone else done this? Or have any idea how much damage and to what I’ve done?
Now, if you’ll excuse myself I’m off to kick myself some more…
Thanks!
The 80 amp fuse is the place to start, it’s held in by a couple of bolts, so don’t try to force it. With a bit of luck that’s all that’s wrong, however we’ll see
Happened to me, last year. The main fues will be kaput, and if you have an immobilser, it will be kaput too - mine was (20amp fuse IIRC) Red top.
MX5 parts are selling main fuses at the moment for a lot less than I paid for one, but they didn’t have any when I needed it. Plus the 10% discount… - get ordering! For a horrible few minutes, I thought the PCM was shot, but the fuses do the job they are supposed to.
I think it will have shorted via the main fuse and alternator diodes, taking out the main fuse. So change any blown fuses and check all visible wiring. With luck the diodes may have survived. Recharge the battery the correct way round, it may be reversed polarised, refit and give it a go. If it runs, check the alternator is charging with a multimeter probes on the battery terminals looking for around 13V-14V at 2000 rpm…
Thanks chaps, fingers crossed for the fuse. Wish I’d checked here before attempting to remove but hey ho!
And Gerryn - when you say immobiliser kaput you mean just the mentioned fuse or actually kaput?
Slightly more hopeful than I was a few hours ago but I’ll be happier when I get a fuse and can try it out.
Sorry - I didn’t put that clear enough - it was the fuse, not the immobiliser. I think the alarm was on the same fuse, not sure now, but after I replaced the main fuse, the car still wouldn’t start, so I started checking fuses. This one is under the dash, I took the radio fuse out temporarily and used it, they are both the same amperage. I said 20amp, but might be a 10 amp - it’s a red top fuse, is all I can remember clearly! No other problems, the car works fine as usual.
Thanks all for the tips and responses on this; the happy ending to all this is that with the battery charged and the main fuse replaced (top tip on it being bolted in - had to remove the fusebox from its mountings to access one side though) the car fired first time and is restored to rude health. Alternator still working and plenty of charge going to the battery, immobiliser working as usual, all back to as it was before. Shins now healing from the day spent kicking myself after too!
Phew!
Good info here, with the help of a friend, I did the same thing on Monday this week. I’d left the side lights on in my NA and of course the battery was flat.
Fortunately, I had jump leads and while I was holding them on the battery in the boot, he was busy connecting the other ends to his battery when he uttered the fateful words " is it supposed to spark as much as this?"
Aaargh!
Well my little Mazda went home on the back of a recovery truck but after putting the Westco battery on charge overnight and spending £4.20 on a new 80amp main fuse, the little beauty started up with everything working just as it should!
Moral of this is always attach the jump leads yourself… Or always make sure your lights are switched off!!!