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Brunos 03 Benz Sl500

442 Views 8 Replies 4 Participants Last post by  Tom Manning
My 03 Sl500 sat in the driveway too long and the front battery is dead. The car is locked and I m unable to unlock it even with the spare key. Anyone has an idea how I can unlock the car?. Thanks
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2011 SL550 AMG Sport Package
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134 Posts
Can you not use the little skeleton key that comes out of the end of your key FOB to unlock the trunk (never heard of that failing) and then put a low-amp (2amp or less) trickle charger or battery maintainer on the rear battery?

It will take awhile but that will re-charge both the rear and then the front batteries, the same way it maintains them both if you hook a battery maintainer to the rear one. That works on dual-battery Jaguars anyway and I believe it should work fine on one of these as you can use a maintainer on the rear battery to keep both batteries up and the trunk always opens with the little skeleton key that pulls out of the end of the key FOB since that's totally mechanical function and requires no power when opening the trunk with that method.

It's gonna be slow (like 8-24 hours if both batteries are totally dead) but should work. As I have came to understand it the front battery is also just a starting battery anyway, your rear battery is what makes your door locks work so it sounds like your rear battery is also dead.

Someone please correct me if I'm wrong but I believe I am correct.



My 03 Sl500 sat in the driveway too long and the front battery is dead. The car is locked and I m unable to unlock it even with the spare key. Anyone has an idea how I can unlock the car?. Thanks
 

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2005 SL 500 R230
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443 Posts
Charging the rear battery with an outboard charger will not charge the front battery.

One of the functions of the battery control module is to charge the front battery; they're not wired in parallel unless relay K57 has kicked in (which is another function of the BCM). Without the car on, there's no path to charge the front battery.

The front battery probably has plenty of oomph (technical term for CCA - cold cranking amps) left in it - they can sit for months and months and not discharge. Charge up the rear battery (which is the one likely to be flat) and see how you go.
 

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2011 SL550 AMG Sport Package
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Then how does battery maintainers connected to the rear battery also maintain the front one?

I assumed it was like a Jaguar where if both batteries are low connecting a maintainer to the rear one first charges it until it gets to a certain voltage, then the BCM (which is powered by the rear battery) senses the correct voltage in the rear battery which triggers it to then open a circuit and the front one also begins to charge.

I mean we know maintaining the rear one also maintains the front one so I'd think it must work something like that.



Charging the rear battery with an outboard charger will not charge the front battery.

One of the functions of the battery control module is to charge the front battery; they're not wired in parallel unless relay K57 has kicked in (which is another function of the BCM). Without the car on, there's no path to charge the front battery.

The front battery probably has plenty of oomph (technical term for CCA - cold cranking amps) left in it - they can sit for months and months and not discharge. Charge up the rear battery (which is the one likely to be flat) and see how you go.
 

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2005 SL 500 R230
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443 Posts
Then how does a battery maintainer connected to the rear battery also maintain the front one?

It won't; you need a separate charger for each, as the circuit indicates, and as tested. Try it. A charger connected to the rear battery will not charge the starting battery.

The only possible way would be a fault in the system causing K57 to lock on, wiring both batteries in parallel. I've never tried to charge two batteries at once from one charger; I don't think it would work with modern chargers due to the current and voltage sensing circuits inside.

The manual warns against charging the front battery with the rear battery connected, by the way.

As I have posted before, forever eliminate your battery discharging issues by fitting a quick isolator in the negative line on the rear battery. It seems that all R230s that don't get driven much suffer battery drain sooner or later.
 

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'03 SL500's Tanzanite Blue, Aegean Blue, Mars Red, Alabaster White '11 S550 Flint Gray
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My first suggestion would be to get a locksmith to pick and decode your lock and make you an emergency key. You need to be able to access the trunk in case of PSE or latch failure.

Another option would be to charge the front battery through the starter cable. The power cable to the starter comes directly from the front battery. Assuming that your dual battery system is working, the front battery will then take over and allow you to start the car.
 

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2005 SL 500 R230
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443 Posts
For some odd reason the key blades in both my fobs didn't fit the lock. I bought new blades from an MB dealer already cut to the car, after I passed proof of ownership. They weren't expensive - about $75.00 IIRRC. Has OP tried the key blade yet?
 

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'03 SL500's Tanzanite Blue, Aegean Blue, Mars Red, Alabaster White '11 S550 Flint Gray
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I read 'spare' key as the blade, but re-reading, it doesn't sound like he tried the blade. If my blade keys didn't fit, I would suspect that the locks were swapped. Obviously not the case for your car.
 

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I just looked at my fobs. Both are original - pretty sure of that, but the odd thing is I had actually ordered only one new blade, not two. I looked at the invoice. So one of the blades in one fob didn't fit the locks initially, but the other one was good.

The replacement blade is identical to the good one, so what I reckon may have happened is when the car went in for service before I bought it, the monkeys at the dealer swapped the blade with another car's. Could they really be so stoopid? Hope not.

New blade cost $AUD71.00, probably less than getting a locksmith to make a new one.
 
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