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The near-new battery seems to have been randomly going or not flat while the car has been unattended. Recently this was accompanied by the speedo intermittently packing up, slower than normal cranking on morning starts and the airbag light coming on. Charging the battery overnight appeared to solve the speedo and the starting. Disconnecting (but not charging) the battery last night seems to have reset the airbag light.

The battery dropped from 12.97v to 12.85v while disconnected overnight last night. Is this normal or a sign of a failing battery?

How do I check if the car is consuming any power when otherwise switched off?

What voltage are you getting with the car running? Maybe the alternator could be going bad. 12.8v is a fully charged battery, the voltage will drop slightly after just being charged over the next few hours.
The alternator should be able to run everything unless there is a massive load IE (completely stuffed battery or a short) so the speedo being dodgy I would be checking that first.


Try checking the amp draw with a multimeter maybe there is a parasitic draw from something left on.
http://www.wikihow.com/Find-a-Parasitic-Battery-Drain

Started ok at work. Drove 3kms home with the airbag light flashing again. 13.03v on the battery. It's charging at just over 14v.

Airbag light flashes 6 times then stops, then flashes 6 times etc. What's it telling me?

Link to blink codes: http://wiki.miata.net/tiki-index.php?page=Airbag%20Blink%20Codes

Dropping to 12.8v is pretty normal, just the battery stabilising.

The driver's side air bag clock spring can sometimes fail, causing that code.

Edit: or maybe a loose plug/connection on the clock spring? I had that code in my NB8A and found the PO had cut the plug off the clock spring to fit an after market steering wheel.

6 blinks is clock spring failure or disconnect.

Try pressing your horn and seeing if that works.

Thanks for the thought but horn makes no difference.

Could this be another symptom of the ECU issues ... ?

With this car, anything could be anything.

davekmoore wrote:Thanks for the thought but horn makes no difference.

Because the clock spring connection was broken I was left with no horn. The connector for the horn and the airbag are common albeit with separate wires.

Mine has the OEM horns and a Nautilus. I know the Nautilus is working but don't know about the others because they're drowned out by the volume of the Nautilus. Will disconnect the Nautilus and check if OEM horns working.

it might be irrelevant but I cannot see any value in having the standard horns connected as well as an air horn.

Connected the multimeter as recommended. This was after everything had fully cooled down so no chance of fans starting up, and after disconnecting the boot/rego plate globes and taping up the plunger on the driver's door room light just in case I had to open the door to check any fuses.

The reading was fluctuating between 0.59 and 0.8 amps (59-80 milliamps?) on the 10 amp setting. Eric the Car Guy on the interweb suggests 50 milliamps is too high.

After a further 20 minutes it settled down to flickering between 53 and 54 milliamps.

After another hour it was at exactly 50 milliamps and the battery was at 12.47 volts.

Battery is on a preventative charge tonight and it looks like tomorrow night will be spent checking fuses etc to find what the parasitic draw is.

Of course, neither the recent installation of the Haltech nor the work-around done by the tuner's auto electrician to deal with Haltech supplying the wrong loom (without alternator control), are ruled out yet as potential ongoing electrical problems.

The 6 flashes of the airbag light are indeed a warning of a fault in the driver's airbag or circuit so that'll be a separate investigation.

That would be 590 and 800 milliamps.