- My model of MX-5 is: NC (2008)
- I’m based near: Reading
- I’m looking for technical help or recommendations on:
Can anyone point me towards a step-by-step procedure for replacing the main fuse box, please.
Is this something that can be done by a competent amateur? - Thank you.
Hello and welcome from Aberdeenshire 
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I have never done it but i think I would attempt it as I enjoy a challenge. There would be a LOT of wires under there and it would need to be well planned and take it slow and methodical. You’d need to make a list of fuse numbers, size and use and label the wires to each fuse as you’d need to remove the complete box before installing the new one. Also take plenty of photos as you go. You would possibly need a special tool to remove the wires also. As I say, I’ve never done one but that’s how I would look at going about it. Good luck! 
One has to ask why? It’s not a normal maintenance process and I’d suspect not straightforward either.
Remove battery.
Unscrew, unbolt, unplug, unplug, unclip, unplug, etc; fiddly but possibly OK.
Reverse the procedure to refit.
BUT! Be warned.
Collateral damage (mostly randomly placed brittle fractures) in an ageing loom etc is extremely high in the possibilities. I was nervous just moving the fusebox an inch in my 25AE (only ten years old wiring) to gain access to the dip beam bulb.
Now, if the connectors and wires require repairing! Big job.
Ouch!
Consider new looms going to it as well as simply the fusebox.
If the ends in the connectors are cooked, you will find most wires will be too short for enough spare to make a reliable repair.
And you will need exactly the right crimp tools and correct contacts.
As an electronics engineer I’ve been doing this sort of thing for ages, and I would only disturb it to that extent if I had no other choice but scrap the car.
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Many thanks to everyone for your quick and helpful responses. Why? is a very good question. I put the car into the local dealership to investgate a parasistic current leak (which goes away when you pull Fuse #5 (ROOM)). Last time it was the bonnet catch. The door switches are fine. This time, after three days and with little supporting evidence, they said that the engine wiring loom and fuse box needed to be replaced due to ‘water damage’. Their quote was north of £1k. My post was an attempt to see what other options I might have before committing so much money on a ‘repair’ that might ultimately not solve the problem. I had hoped that I might be able to unplug something to confirm that the fuse box was faulty but your helepful responses are enough to confirm that this task is almost certainly beyond my limited skill set. Many thanks again,
I would be surprised if replacing the fuse box and engine loom cures a battery drain… If you do decide to pay the dealer to replace the parts, makes sure that you agree the next steps in advance if it doesn’t fix it.
Thank you for your good advice. I was not convinced either. Given that the leakage drops to zero if I pull Fuse 5 (ROOM), this implies that the fault is likely to be downstream of the fuse (i.e. not in the engine loom).
There seem to be many switches, sensors, etc (like the bonnet switch) that are protected by Fuse 5 - one of which may not be going to sleep when everything is turned off/closed. I’ve been looking for a circuit diagram that I could use to identify what to check.
Thanks again for taking the time to reply.
Do you know how many amps are being drawn when the car is asleep and how long is the car being left undisturbed for when the reading is taken? Are you able to measure it yourself?
Is the below of any help?
Just what I was looking for Thank you. That will be a big help.
Yes, I have a proper ammeter and can measure the drain myself. I was getting a repeatable leakage of 210 mA (should be about 40-50 mA) after leaving it to settle once everything is turmed off (say 20-30 mins). The dealer reported a higher value (1 amp).
BTW I have fitted a temporary switch to the battery earth terminal so I can still use the car.
Are you clicking the bonnet latch across, closing all doors and windows and locking the car? Try waiting an hour with the car in this state and noting the reading, I assume that you are connecting the ammeter and leaving it connected during the waiting period. From memory, the workshop manual states up to 40 minutes. Have you got any aftermarket accessories fitted?
The last one that I tested dropped in 3 stages, starting at 230mA, then to 80mA, then 50mA then 20mA.
Yes, forcing the bonnet latch closed and turning everything off , locking, etc. Yes to leaving the meter in circuit during the waiting period for 40 mins (per manual). No aftermarket accessories. It is a tin-top with conventional entry/ignition key.
There is defininitely a parasitic leak. The newish battery will go flat after 4-5 days it I don’t turn my temporary switch off after running the car.
Thank you for spending you time on this - much appreciated.
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I had the same issue on a motorbike a while ago. I tested everything. Turned out the new battery was faulty.
Thanks. I’m pretty sure the battery is fine. It keeps its charge over time as you would expect.
Just one other thing thing that has come to mind, the dealer didn’t mean the in car fuse block did they? These can be subject to water ingress through a plastic screw insert in the body at the bottom of the windscreen that holds the scuttle panel trim on.
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But you said it goes flat after 4-5 days? Have you tried disconnecting it, fully charging it and checking to see if it holds the charge when disconnected? Batteries can be funny things at this time of year. Sorry, I’m not an expert but I just go on past experience.
That is another real possibility that I haven’t explored get. I had to replace the grommet a few weeks ago after passenger foot well carpet became wet. I don’t know whether the in-car fuse box is downstream of Fuse 5 (ROOM). I’m still finding my way round your schematic.
I also had to clean out the rear drain tubes recently (lots of water behind passenger seat).
Thanks again. Yes, the battery holds its charge fine when disconnected.
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