NC Immobiliser issues

Technical Area Electrics & ICE
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  1. My model of MX-5 is: NC1 PRHT
  2. I’m based near: Daventry, Northants
  3. I’m looking for technical help or recommendations on: Immobiliser issues

The factory immobiliser has started playing up on my car. Apparently at random, the immobiliser locks out the starter (with the red key-in-car warning light blinking rapidly). Sometimes it will start first time, sometimes it takes a couple of tries, or sometimes it will take a dozen or more attempts. Same behaviour if I use my other key (both are the OE ones that came with the car).

When it is in this “locked out” state my OBD scanner shows a “U0100 - Network” fault code. Once it has decided to start, the car runs and behaves perfectly normally and the fault code can be cleared and doesn’t reappear (until the next unsuccessful starting attempt).

I have checked the connections to the PCM under the airbox, and also the fuse box and junction box behind the nearside kick panels and they’re all clean as a whistle. I’ve also conducted a “watering can test” on the scuttle grommets as these did require attention when I first got the car 6 years ago. Zero signs of leakage there and the passenger side carpet is dry (which was not the case when I got the car - hence attention to grommets). I’ve also been round all the earthing connections I can find - all clean and tight.

Anywhere else I should be looking for “known issues”?

Does anyone know if the ring antenna is known to fail? I have a BMW motorbike and I know some it’s a potential issue on those. Can the antenna be bought separately (it’s not listed on the mx5parts website)? Is it coded to the car?

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I’ve literally posted something similar a few weeks ago.

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I don't belieeeeve it!

A year or two back I had issues with a non-functioning blower. Went down various exploratory paths and ended up having the Power MOSFET changed without actually fixing the problem. Then I stopped and thought about it a bit more and cautiously tried swapping out the blower relay (a 30 second job). Job done, Blower working again.

So I tried the same approach with this problem. Swapped the “Main” relay (R1 in the Haynes manual) for known good one (the blower relay mentioned above). Voila! 100% starting.

There’s a pattern developing here (the identical front fog lamp relay also failed last year) so I’m off to buy a couple of new ones ready to drop in when something else fails…

1 Like
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I’ve been watching this thread, and not commented as I’ve nothing of any relevence to add.
Its good to see that you found a solution, hopefully a permanent one.

Are these the same relay, and if so, do you know the relay number ??

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The ones on the car are marked “Denso AJ51”. The same relay is used for various functions on the NC. Mx5parts will sell you one for £15.66, and they are available used from eBay for around £6.

I bought generic RLY1307 from my local auto-electrics place brand new for £3.94+VAT…

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Just tried to cross reference that with my old place of work and it seems that they don’t do it.
Well they do in a fashion, RLY1307 is sealed, the one they do isnt sealed.

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I don’t think it needs to be sealed (I’ve opened up the casing on one of the failed ones and it certainly didn’t appear to be sealed).