I had a similar problem recently and I checked all the earth connections I could find, took them off and cleaned them, and the part of the car they came from. The easiest one to check is the one fastened to the left side of the engine block near to the bulk head. Can you check the plug leads where they go into the Coil and even if they look ok, clean them too with emery cloth. Finding all the earth connections may be a problem as my car is a gear change car. Don’t forget the earth from and to the battery. There are earth connections underneath the car too
No, this has not been suggested before, so I’ll have a mess with the gear selector just in case 
Yes, the earths has been suggested before but no-one was able to give details on where all the points are. In the meantime I will have a look and see if I can identify the areas you have suggested and check them, thanks.
Ok, I don’t think its the gear selector, it appears in good shape, tight movement and if I put the gear selector in certain places other than Park or Neutral, the engine doesn’t turn over at all. Thanks for the tip though, all suggestions are welcome.
I couldn’t find the bulkhead earth you mentioned.
In the boot with some of the trim removed, I can see a thick black/yellow wire going from the battery to the body in the boot, then it appears to go on from there though the body down towards the bottom of the car.
I need to jack the car up so I can try and locate it and follow it from there but I need to go out now, so will get back to that tomorrow hopefully.
Thanks.
Very frustrating. In my experience (limited to an Peugeot 306 to be fair) an immobiliser fault means that the engine won’t even turn over.
The only automatic I’ve driven (Toyota) similarly won’t even turn over unless the selector is in ‘P’, but the Mazda system may be different.
On a motorcycle I’d be checking the kill switch and the side stand switch. It sounds as if something is refusing to trigger the coil packs.
Given the random start for the previous owner I still suspect a tiny break in the loom somewhere. You have traced the CAM and CAS sensor connections but it could be something else.
I once had an Alfa75 that would fail to start at random. A good wiggling of all the multipin connectors I could find under the bonnet finally tracked it down, but in that case it was the fuel pump not running.
I tracked the Black/Yellow wire from the Batt Neg through to the bottom of the car where it terminates onto the chassis, all seems good.
I also connected a Jump Lead to the Batt Neg and clamped it on the engine just to be sure but made no difference to the crank no start condition.
This reminds me of Ford Orion many many years ago.
3 garages looked at it and couldnt figure out what was wrong.
It took me about an hour to find it.
The roll pin in the bottom of the distributor had snapped, causing the gear to spin on the shaft.
One minute the ignition timing would be OK, the next miles out.
This got me wondering if something similar could be happening with yours.
Maybe the crank sensor is out of alignment or does it have a ring that it reads its postion from and could that ring be out of alignment.
Then I found this, which talks about crank sensor adjustment bing the thickness of a credit card.
https://forum.miata.net/vb/showthread.php?t=550485
Does the MK2 have a Cam Angle Sensor, from the images here, it would appear to look a little like a distributor.
https://www.miata.net/garage/cas_oring.html
Having connected a jump lead from the engine to the battery neg that would seem to rule out an engine earth fault.
I would go back to checking the coil packs. The next step would be to find someone with an oscilloscope and check the trigger voltage from the ECU to the coil pack, you should see a 12 volt trigger. A good auto electrician should be able to check this for you.
An intriguing fault if ever there was one. Sorry if I’ve missed this in an earlier post but where are you located? As per the above post by PR-Y I think it’s time to get an oscilloscope on the ECU to check the inputs and outputs. I have an oscilloscope and a reasonable knowledge of electronics but unless you are located in east Berkshire not a lot of use to you rearly.
As I say, post your location and maybe someone local can help.
Good luck.
D.
I recently built a Westfield with a 2001 1.8 MX5 engine and had a similar fault.
Firstly, is your car an import! I thought all MK2 UK cars had immobilisers, your black box could be a KPH to MPH converter??
From memory, the coils are fed with 12v and are triggered by being grounded.
I had cranking but no firing as I had the 2 ignition wires the wrong way round. There are 2 outputs from the ignition switch, both have power, but only one has power when the engine is cranking-could this be broken or the ignition switch faulty?
You could check the main relay to see if it’s getting power during cranking and the same with the fuel pump relay (although this won’t stop you getting a spark).
Ian
Thanks Combemartian, I’m in Worcester (Midlands).
I’m sure theres many an MX5 enthusiast who would be confident they could nail this sucker and if any of them see this thread and fancy a little holiday in the Midlands, perhaps coinciding with an event at the NEC / O2 in Birmingham, I’m only 20 odd miles away and have a couple of spare rooms, so I’m sure we could come to a mutual agreement
P.S.
I did have a mobile electrician out who checked voltages and signals from the ECU to the sensors and the previous owner did supply a spare ECU he’d bought previously thinking it might be the problem but both original and spare ECU don’t make a jot of difference.
Yes, it’s a Roadster 10th Anniversary model manufactured in 1998 but classed as a 1999 MK2. Because its a Roadster, its still pre ODBII and your right, the black box turned out to be the KPH to MPH converter.
Interesting info about the coils and the ignition, after all the other things tried, that sort of symptom certainly sounds more likely.
I know it’s easy to be dismissive of them particularly when you want the satisfaction of identifying and resolving the problem yourself but have considered taking it in to a Mazda garage or a local independent specialist and get them to look at it for you? They might well be able to identify the problem quickly. It might cost you a small fee but might also mean you get the car back on the road quicker. Appreciate that would mean getting someone to tow you or paying for it to be put on a trailer though. Just a thought…
It’t not going to be easy for me to transport it to a dealership or specialist, coupled with the burden of costs of getting it there , I fear the man-hour rate that these places charge would exceed the value of the car.
Where about in Worcs are you?
Just outside, Crossway Green, DY13
I’m in Shirley, Solihull, so not a million miles away, I’m just stuck for time at the moment!
If it isn’t running by Easter I will have more time.
Problem with the auto is the gearbox ECU, it has lots of connections to the PCM/ECU and I don’t know what they do, and how they affect the running of the car.
The car should run on a default mode if you have a sensor fault, unless you have more than one issue. All the sensors have 12v and ground, so it’s worth checking these with the ignition on. Faulty EGR shouldn’t create a problem. You need to make sure the main relay has 12v on it with the ignition on and whilst cranking.
That should keep you busy for a while!
Oh, wow, about 40 mins away at most
I’ll try and check the relay and report back.
While I have very little knowledge of car electrics, 42 years with BT taught me a lot about fault finding.
This is an intermittent fault that has now become a hard permanent fault. Almost impossible for this to be due to something incorrectly connected such as crossed wires. Unlikely to be due to a faulty component as they will usually fail completely. The far more likely cause will be in the wiring. Could be worn insulation causing two wires to short or a wire to be grounded. Most likely a disconnection in a wire which initially caused an intermittent break which has now become permanent.
Often worth standing back, thinking, then start again. The electrical system is just a system. In this case the system input is 12v from the battery. The system output is a spark at the plugs. You have the input, you have no output. You need to start at one end or the other and work your way through the system until you find the faulty link. As everything else electrical is working then the logical place to start is the output, i.e. missing spark. Coil pack and ECU are sub-systems. If they do not give the correct output do they have the correct inputs?
I have cleared many faults over the years which others had failed to identify by asking two questions. Is it plugged in? Is it turned on?
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