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Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 23298
Experience:  BEng hons Mech engineering, in auto industry 22 years
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2012 MX-5 Mk3.5 2.01Sport Tech Roadster. Could not unlock

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2012 MX-5 Mk3.5 2.01Sport Tech Roadster. Could not unlock car with fob this morning. Light works on fob, so presumed battery flat. However no obvious reason why this should be - all doors and boot properly shut. Unlocked driver's door with key, tried ignition - nada, as expected.This has happened to me before (albeit when a door has not been properly latched, leaving interior light on - so cause obvious), so I know the drill. Connected charger and switched on. Charging light comes on for 2-3 secs then goes off with a loud click from the charger (thermal trip?). Car alarm then goes off (not going to make me popular with neighbours at 0730 on Sun morning!). Charger continues to cycle on 2-3 secs, then trips (fully charged light flashes on). As soon as battery has enough power for alarm to sound, alarm flattens it again.Checked doors again. Found that passenger door, though fully shut, will not open - even from the inside (there's no external lock on the passenger side). Wondering now if interaction between failed door lock on pax side and alarm system (perhaps thinking it has detected unlawful interference with lock?) has caused battery to flatten with alarm remaining permanently triggered.Car last driven on Friday evening with passenger but with no unusual indications...


this sounds like the battery may be damaged

I've known this to happen if they become very discharged / completely flat and the current pulled through them when you try to start then buckles the plates

and batteries only tend to last 5 years anyway so I'd say it sounds like time to replace yours

as to why it was flattened off they yes its quite possible the partially shut door is to blame and once you've replaced the battery it would be wise to check for a current drain

I'd first check the easy things like is the boot light or any other interior light staying on then check that the main battery connections are tight and in good condition also check the earth strap to the engine /gearbox.
Next I'd check the current drain aspect, the best way to find what circuit is causing the excessive currant drain is to fit a current sensing multimeter in series with the positive battery lead on the car and the battery, or use a current clamp meter on the battery positive cable.

A typical current drain with everything off and un-alarmed should be around 0.05Amp and if its higher than this remove a fuse and re-measure, if measurement doesn't change then replace the fuse and remove the next.

Once you've isolated the circuit with the excessive drain check the wiring connections and plugs to components and the wiring harness that runs by the engine bay fuse box - the wiring harness chaffs / rubs through and gets damaged - but its not always easy to see so be careful when checking..

Also check that the alternator is actually charging the battery in the car by taking a battery voltage reading with the engine off then another with the engine running. The 2nd reading should be higher than the 1st and climbing steadily. And also check that the alternator is secure to the engine as this provides the ground for the charging circuit and make sure that the engine / gearbox earth strap is secure and the contacts are clean and bright.

if the alternator appears not to be working - take it to a electrical specialist and ask them to test it as sometimes you can replace just the diode pack and this will repair the alternator without requiring replacement

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