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2012 MP4-12C
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9,447 Posts
Discussion Starter · #1 ·
So... I didnt drive the car for around 11 weeks and the battery went dead. I went through the process of using the physical key to get into the car (bring some needle nose pliers to turn the physical key from inside the inlet cubby on the driver side as no human hands could have turned mine), to pop the hood and put on the battery tender.

The tender tells you have a bad battery. It lies. So you have to keep hitting the reset button between 30-50 times, but eventually instead of getting the red light from roxanne, it will move up one notch to the orange charging light. Let that go for an hour or so and you can turn over the car.

Now comes the problem. The car throws an ecu limpmode error. You cannot clear it with an OBD, you cannot even move to other screens on the dash to see how much battery power you have. Other things that dont work in this extreme limp mode, that also throws every other error the dash can show (air bag, brakes, tire sensors, everything is christmas on the dash)... some of those things are just bad error codes and the things actually work. However, some things critcially are disabled. Including turn signals. Brake lights. Hazard lights.

When youre in the state, driving and turning the car off for an hour and restarting, it wont clear. The limp mode code is causing it. So the only way to clear it is to take out the frunk, get to the battery, and disconnect it for a couple of minutes. Bonus, it sets off the alarm, which I measured as being over 101decibles, so ear bleeds come with the territory unless you bring some earplugs.

Once you reset, you might have to use your key to unlock/lock the car a few times and start restart to clear the alarm.

I still have an airbag warning on my screen, but im hoping a few drives will clear it out. When the ECU restarts your car might run rough idle and shift gears a bit wonky for a bit as it re-maps relearns everything.

Couple of shots with the frunk out. The lithium ion battery is actually way bigger than I thought it would be.

196341

196342
 

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2012 MP4-12C
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9,447 Posts
Discussion Starter · #3 ·
My doors and seats seem fine. the doors have soft close option, and not sure I noticed anything different about the window settings.

There is clearly some other backup batteries because the siren goes off even when the battery is disconnected, and the seat memory stayed in tact.
 

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These things can be frustrating. Dead battery after setting a while unplugged. Got the battery to charge after many resets of the charger. Many faults. All have cleared. Now windows won't work on drivers side correctly.. Right window control on drivers door operates drivers window. Left window control does nothing. Going to try the lock overnight procedure. I will let everyone know how it works. I wish the mother ship would give us some guidance or at least turn us on to some reasonably priced diagnostic tools to fix such things.
Whomever designed this system must have been a jokester. With the battery disconnected, the alarm stills works. Has to be another battery somewhere. (reset this one also?) Why do the door controls have to go to the central computer? That pull strap works fine. Car is way to complicated for what could have been simply designed features. Rube Goldberg comes to mind.
Oh well, going to bed and dream of an ALL analog car with the Mac's performance.
 

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Got back from the dealer today. Apparently the windows needed to be recalibrated. Took about 45 minutes. By the way, very few Macs available there. They had a Senna, a GT, and also a Chiron available.
 

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McLaren 12c 2012
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918 Posts
I suspect things like seat positions will be stored in NVRAM hence why they aren’t affected by a dead battery.
This has happened to me, including the seat issue, wouldnt move foward, after a while it sorted itself out luckily. When the battery drains the canbus goes and the engine wont start unless you take the negative off of the battery and yeah, have your ears blown in.

Its odd that when the battery goes the coding for these things goes as well, a first in a car I have owned.
 

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129 Posts
@ZombiePhysicist I'm in the same boat now on my 12C Spyder. Battery died. Pressed reset about 20 times to get trickle charger to charge up the battery. Car starts but dash lit up and limp mode error shows up. Before disconnecting the battery, is there any new knowledge/technique that has emerged over the years?
 

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@ZombiePhysicist I'm in the same boat now on my 12C Spyder. Battery died. Pressed reset about 20 times to get trickle charger to charge up the battery. Car starts but dash lit up and limp mode error shows up. Before disconnecting the battery, is there any new knowledge/technique that has emerged over the years?
This happened to me a couple months back. I did the battery disconnect for 10 mins, and it reset all but the airbag fault. Brought to the dealer and they reset for $50. I was told it will not go away by its self.
 

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2012 MP4-12C
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9,447 Posts
Discussion Starter · #15 ·
@ZombiePhysicist I'm in the same boat now on my 12C Spyder. Battery died. Pressed reset about 20 times to get trickle charger to charge up the battery. Car starts but dash lit up and limp mode error shows up. Before disconnecting the battery, is there any new knowledge/technique that has emerged over the years?
Bring earplugs. You just want to take one lead off the battery and leave it off for 5 minutes for the ECU to reset. The alarm will BLARE and your ears will bleed if you dont have protection. After 5 minutes put the lead back on. Most errors will clear (except for the airbag). You can ignore the airbag until you get it back to service and they will clear it. Good luck!
 

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Worked well. I don't have an airbag fault. All errors cleared PLUS my passenger side soft close motor that the car said was busted started working again. I didn't expect that.

If anyone else has a issue with the frunk latch not closing after resetting the ECU, press the green emergency trunk unlock button inside the frunk to "wake up" the electronics. The frunk unlock button inside the car might work too, but I didn't try that.

Thanks for the forum support! Adds an extra smile to the ownership experience.
 
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