Alarm (?) beeper going off randomly

  1. My model of MX-5 is: NC 3.75 PRHT
  2. I’m based near: __Chorley
  3. I’m looking for technical help or recommendations on: Possible alarm fault

Hi all.
I am experiencing a “beeping” noise starting and stopping randomly when the car is sitting parked up outside my house. At first I thought it was the reversing sensor, as it sounded very similar, but have disconnected that and it’s still happening. It’s seems loudest in the boot, so presumably the sounder is located in there somewhere. I’m now wondering whether it’s the alarm, but not getting any flashing lights etc. Where it gets even stranger is that I disconnected the battery while it was going off and it didn’t stop! It gradually got weaker, then eventually stopped so presumably fed by a capacitor somewhere. Having managed to stop it yesterday (probably more by luck than judgement), it was OK until this afternoon when I unlocked the car and it immediately kicked off again. Nothing that I do will stop it - locking/unlocking, starting the engine and so on. It will randomly stop of its own accord, then start again after a few seconds.
It’s really got me baffled. Any ideas would be most welcome.

I find if I leave my phone in the car it sets the alarm off. No sure if it is just when it connected to the Head Unit or not.

Not sure if car alarms are like bike alarm - but they have an internal battery so the alarm is not disabled by a baddie disconnecting the battery.

If you deliberately set off the alarm, does it make the same noise?

As @McTrucky says, Thatcham 1 approved alarms (which I guess the NC3 should be) must include a back-up battery, so your description above does sound as if it might be alarm related.

Haven’t tried that yet. Battery disconnected at the moment, so will try it tomorrow if I can figure out how to create an alarm condition.

To create an alarm condition, arm the system with the remote fob, let it set for
a short while then unlock the door with the key.
You can stop the alarm by inserting the key in the ignition and turn on.

Have you tried disabling the alarm after locking the car? Double lock the doors as usual (press the lock button on the key twice) then within about ten seconds press the alarm disable button (bottom right small button on the key). The indicator lights should flash twice to acknowledge it worked. Now the alarm is disabled until you next unlock / lock the car.

Thanks. That’s one of the weird things - I can start the car and it still keeps beeping. No alarm indication on the dash either.

Thanks. I’ll give that a go tomorrow too.

That button will only disable the Intrusion Sensor - it doesn’t disable the whole alarm system. Also, it will work if you just single lock the car; you don’t have to double-lock it first.

I recently experienced the same with the alarm going off randomly…it turned out to be caused by a hands free device that I had left in the car still switched on that was affecting the alarm, switching off the hands free and removing it from the car solved the problem.

Any bluetooth device like a phone will start it off if left within range.

If it’s sounding without flashing the indicators it points to the alarm triggering via the battery back-up. It either loosing it’s 12v feed or ground from the car or the siren has an internal fault. I’d suspect the latter.

Thanks for the input.
Having reconnected the battery this morning, so far it’s OK, but will leave it for a while to see what happens when I next unlock it.
Tried triggering the alarm by locking using the fob (both one-press and two-press methods) then opening the door with the key and nothing happened.
If I had discharged the back-up battery by letting it beep itself out with the main battery disconnected, will it take some time to get its charge back and then act up again?
Any idea how to get at the sounder? Sounds like it’s in the boot somewhere.

Yes I think the sounder’s behind the plastic trim in the boot, the trim’s easy to remove.

Another way to set off the alarm is to leave a window open, lock the car, wait for a minute or two for it to arm & then stick your hand in through the open window & wave it about.

As another poster has said, the “disable” button on the fob just disables the interior sensor. As far as I’m aware it isn’t possible to lock the car without the alarm automatically setting. As the alarm’s a standard security item, if it’s disabled or removed that might affect the insurance?

Be aware that this only works if the roof is closed. When the roof is open, the interior sensor is deactivated. (Don’t ask me how I found this out, but to be fair it is mentioned in the manual).

That’s right :grinning:

It sounds like the battery backup is not holding charge and is beeping top tell you to replace it. I had one on another alarm do this a few years ago (not an MX5)

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Thanks again to all for your input.
It lasted until 2pm today, then started beeping again unprovoked and wouldn’t stop. Went on for quite a while after I had disconnected the battery, so the battery backup is presumably what keeps it going. I did wonder whether the delay between reconnecting the battery and the beeps starting (about 5 hours today) is the backup getting up to charge, which then somehow triggers the beeps, crazy as that seems.
Another aspect of it is that it isn’t disabling the car, as you would expect the alarm to do, as I can start the engine no problem, even while it is going off.
Seems like I will have to pull some trim off and take a look at the beeping thing.

Pulled the trim out of the boot and removed the sounder. I was worried that it might have some sort of feedback to prevent the car from starting if it was disconnected, but everything runs OK - just no beeps thank goodness.
Only 3 wires to it - measuring blue to black (ground) gives a little over 12 volts and blue/red to black 11V or so, measured at the connector. According to the wiring diagram blue is the main feed and blue/red the signal from the controller. Does anybody know whether the blue/red wire should be live in no-alarm condition or dead? Presumably the fail-safe would be to have it live to hold the alarm off. If that were true, then it must be the sounder that’s faulty.
I can buy a replacement sounder - plenty on ebay - but suspect that it might be a fault in the control module, as so far I haven’t managed to trigger an alarm, which should flash lights as well as the audible, so might change that too.

For the record, I replaced the sounder with a second-hand one and no more beeps, so hopefully solved the problem.

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