Can I run any tests on my ECU ?

So, the garage says you have no compression on cyl 4. What readings did they get?
You did the compression test with all the plugs out, coil packs disconnected, so the engine spins freely on the starter? What readings did you get?

So someone is incompetent. If it’s the garage, then I’d suggest you need to check their work, or pay someone you trust to look at it. Was the belt installed correctly? Could it have snapped? Have you actually checked, or are you going by the word of the same person who told you “no compression”?

It seems unlikely to me to be an ECU fault unless a) your pax footwell has been filled with water due to blocked roof drain, or b) someone has been meddling with the wiring and shorted something out. Even then, there is a fair chance the ECU fuse would save it. (have you checked the fuses?)

Sorry, but the path to madness is expecting total strangers on the internet to magically solve all your problems…

Yes, Keat, I checked that the belt had not snapped or slipped.

I took the oil filler cap off and cranked the engine which showed that there had been movement on one or both of the camshafts. I then went through the process of removing and re-installing, correctly, the timing belt. 

I have checked every connector I can find. They all appear to be connected correctly.








As I recall your experiences with a brand new 30th ND whose gearbox was trying to shove itself into the transmission tunnel also drove you to distraction…ending up in a replacement car.

Sometimes I think folks just need to reach out…as you did… for a bit of support…from relevant forums… on the internet when all else fails.

Be fair.



The notes I took on the readings are in Spain, so I am working from my memory, which is that the garage told me that I had between 9 and 10 bar on 1,2 and 3, and something like 3 bar on cylinder 4.

With the coil packs and plugs removed, and using my own, brand new compression tester, I saw 4 more or less identical readings of 10 + bar across all 4 cylinders.

Am I a competent engineer ? Far from it, but I can read a pressure gauge, and I have learnt how to read a multimeter to check for AC and DC voltage, resistance and continuity.

I see no evidence of any significant footwell water, ( just a minor bit of rust on the ecu retaining frame, probably due to wet shoes), and I have checked all of the fuses.

I have to disagree with you on one point. The internet has allowed me to discuss my MX5 problems with others who have a similar interest in this particular  car.

From everything I have learnt since joining this site, I see this as a path to enlightenment.

A frustrating path, most definitely, but a true learning path.

Let’s not forget that nobody is obliged to read my posts, and there is even less of an obligation to respond to my posts. However, I sincerely appreciate the responses I have received, even if my problem is not yet solved.





Hi Phil, Due to it pi**ing with rain here today and nothing much else to do I’ve had a trawl through the circuit diagrams.
I’m no expert but I can read a circuit diagram, also there are so many variations that what I have been looking at may not be exactly the same as your car therefor treat the below as my thoughts and no guarantee of accuracy.

It looks to me as you have lost the power to the ECU, which is why you can’t connect via the OBD port, and a lot of the sensors and other stuff, which is why you are getting no voltage to the crank angle sensor ect.
It looks to me as though the main 12 volt feed to the ECU ( pin 4AF on the ECU connector, red and white wire ) comes in via the 30A Fuel Injector fuse, this then goes to the Main Relay. Both of these are located in the Main Fuse Block in the engine bay. The Main Relay is normally open and is turned on when the ignition switch is in the Start or Run position. The feed to energise the Main Relay comes from the ignition switch, through the 15A Engine fuse, located in the fuse block under the dash. When the ignition is turned on this energises the Main Relay and this passes the 12 volt supply to the ECU, crankshaft position sensor, camshaft position sensor, mass airflow sensor, EGR valve, and a mess of other stuff. My gut feeling is you have lost this supply. Can you check if the Main Relay is energising ( hopefully you should hear it click when the ignition is turned on ) and that the 12 volt feed from the Injector Fuse is present? Have you checked this fuse? Also, if the relay does energise, that the 12 volts is being outputted to the white and red wire that then goes to all the parts mentioned above. If the relay does not energise you need to work back towards the ignition switch. It’s also possible the relay itself is faulty.

Sorry it’s been a bit of a tome but hopefully a few things for you to check.

Good luck and let us know how you get on.


fantastic diagnosis.

And more probable than the ECU failing in my experience.

Hmmm, so going back to your Haynes Manual “fault finding by numbers”, did you check for the two basics, fuel and sparks?
I.e. were the plugs wet if you check them immediately after cranking but failing to start?
If you pull a plug and ground it to the head do you see a nice fat spark?

If the answer to either of those is no, you at least have a place to start.

Good luck!

Hi Dave,

Thanks for taking the time to to offer your diagnosis. (Should I be grateful for the lousy weather you’re having ?).

Your analysis and explanation were very clear to me. 

To summarise, I need to carry out the following steps:

  1. Check the 30 amp Fuel Injector Fuse, and replace if blown.

  2. Check that the Main relay functions properly, and replace if it doesn’t.

  3. Check that I have 12 volts at the harness side of connector 4AF, and then take from there if I don’t.  







Sounds like a plan. Report back and pleased to be of some help


This is an area where mechanical instincts and electronic control units tend to be in conflict with each other.

The very first things I checked, 18 months ago, were for wet plugs and a spark from a grounded plug. Not recommended by Mazda, but I saw neither.

However, if my understanding is correct, the ecu relies on signals from the crank position and/or cam angle sensors to send signals to the injection and ignition systems, which are required to send the fuel through the injectors and the sparks to the plugs.

Since my ecu appears to be completely unresponsive, it is probably not sending the injection and ignition signals, which would imply that I am not going to see wet plugs or a spark from a grounded plug.

Is my understanding incorrect ?



Thanks to all who have responded on this thread so far.

However, as I am now in the north of France, and my car is in the south east of Spain, I am not going to be in a position to work on the car for a few months, so it might be a good idea to put this to bed for now.

When I get back to the car, following some of the advice I received on here, I will have a few things to check out, so I will report back when I have any news. 




Certainly sounds like an electrical problem. In addition to checking the fuses as Dave has suggested you also need to check the earthing points, both on the battery post and on the car body and engine.

I’m not sure where the earthing points are on a Mk2 but an easy way to check the earthing is to connect a jump start lead to the battery negative and then clip to the engine and to other earthing points in the circuit.

Also when checking for 12 volts at various positions in the electrical circuit it is a good idea to connect the multimeter negative back to the battery negative post rather than trusting possibly defective earthing points in the car.

Hope you can solve this, never easy tracing electrical faults.


Topic locked for now by request of OP.