No ignition

Possibly a bad ignition switch, but you need some pro help. Our steel keys tend to be rough on ignition switches.

 

When you say the voltage meter registered 12.5V, is that measuring directly on the battery posts? What is the measured voltage after attempting to recharge the battery?
I had odd symptoms when my battery failed, including no dash lights. A friend with a 550 had the same symptoms as you and despite the battery giving a correct measured voltage it was worn out.
The key seeming stuck into the switch can be caused by the steering lock. It only takes turning the steering wheel left and right a little to ease the key out.

 

There is actually a fuse for the ignition switch on the 456M. It's found in the passenger footwell, but not on the regular fuseboard.

It's in a box like this:

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Fuse 10 supplies power to the ignition key. Would you be able to take a closeup photo of the stickers on the cover?

The holder and the cover are seen in this photo:

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P.S. Isn't there some logic on the 546M GTA which prevents you from removing the key with the gear lever not in PARK? And isn't there a manual override pushbutton somewhere? (or am I thinking of another car?)

 

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If the locking mechanism does require battery power, that may be a sign that there is something wrong with the battery, but you'd expect some sign of activity with 12 volts at the battery. But as Timmo says, strange things do happen.

Did you turn off the battery with the battery cutoff switch and forgot about it

 

Is it clear from the fuseholder cover decal which fuses are which? I'm just wondering if it's possible to get the fuseholder cover on back to front.

 

Is your battery good? Can you put it on a charger disconnected from the car? Measure your battery under load. If a ctek or other charger won't take it, hook a know good battery up to it and put the good one on a charger then it can trickle charge the bad one. Is your trunk closed or other dome lights all off so nothing is draining the battery

If it died like that after a drive, I'd also suspect your alternator too.

 

OK... My fuse suggestion was just a possible cheap fix, but in the absence of other data, I guess the next step is the battery. Battery cutout switches have also been known to fail. Maybe try cycling that on/off.

 

There's a 2 pin plug on the top of the panel...

See white arrow

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On your relay board, large dual lug, upper left in the pictures. Follow it back from there. On the 456GT it's mounted to the car, on the M it's on the door that flips down. Still not likely your issue
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If you have no power to the car at all, it's unlikely that the relay panel battery plug would stop power getting to everything. In the engine compartment, there is a maxi fuse holder:

1) Two fuses there separately feed the 2 pins on the main relay/fuse panel
2) Another fuse feeds the smaller fuseholder in the passenger footwell.

There are other maxi fuses for the HVAC and ABS/ASR system

You could get a voltmeter and check for power going to the maxi fuses in the engine compartment. Use the chassis as an earth. If you get no voltage at the maxi fuses, then perhaps it's :
1) a battery connection issue
2) a battery ground issue (including the battery cutoff switch).

 

I found this image...

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Fuse 10 appears as a 20 amp fuse. Is this different from the sticker on your fuseholder?

 

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Have it tested from a battery box or pull it. That's good that the Ctek shows the battery is fine. Moving on... like Ian says check engine compartment fuses and cycle the cutoff switch or assure the negative is grounded

 

What happened? Was this resolved?

 

Turn it clockwise, turn it anticlockwise. Sometimes it can be a little stiff.

I'm wondering if you can do a resistance check between your battery negative and the chassis (in both cutoff and not cutoff). See if anything changes. Note: I don't know if battery voltage will affect the ohmmeter readings.

 

Did you check the voltage on the fuses (both sides of the fuses with fuses inserted) with the black meter probe on the chassis?

 

So no dome lights, no windows droop when opening the door? Car is totally dead? Are you getting power at the fuse board? Are your battery terminals secure on the battery? It does sound like this will be a simple solution once you do identify it.