Key/FOB requires pressing the buttons for several minutes ONLY after the car has sat more than 4hrs. The last two times, it required me to use the metal key to unlock the doors. Most of the time, if I just continually touch the buttons, the car will eventually unlock.
If I let the car sit for just a couple of hours, the key works fine, everytime.
You can check the signal by looking at the tip of the key via digital camera.
Those days everybody has camera in his phone and digital camera will see infra-red signal that bare eye will not.
Change your keyfob batteries as suggested, and check the status of your CAR battery(s).
The second remote fob can be repaired depending on the damage. Someone needs to remove the printed circuit board and evaluate the damage. Does it have a triangular red panic button or a round one ? Maybe you could take a picture and post it here for the key experts.
that was the first thing i thought of as well when i read of the symptoms but
it was just a guess. to support this scenario, the remote locking would be
operating at a higher voltage threshold vs car starting and that seemed a little
odd to me since, usually, it's the reverse (electro-mechanical vs electronics
load demands).
so if this were true (low car battery), allowing it to sit for a spell meant that
the battery's health is relatively low and does not support locking system.
however, after the car is run a bit, the alternator replenishes the battery just
enough to allow the locking system to function relatively well.
the cyclical scenario restarts, i.e. the car sits and the battery restarts the slow
discharge again.
to determine the validity of this theory, one would have to log metrics of how
the battery is doing at rest, given starting load, at shutdown, etc etc and
perform comparisons.
A car can be started on as little as 10.5 volts. The problem is when the battery is below an accept able "at-rest" state- approximately 12.5 or better volts- the body control modules on not just Mercedes, but other manufacturers as well are very sensitive to those drops. Controls are lost, memories can be scrambled. It isn't an amperage thing, it really is a voltage thing. Think of electricity as a water supply: Amperage is volume, but if there is no pressure (voltage), volume does no good.
Remember that the battery is a storage device. It's your alternator that supplies the system once the car is started. Any battery has only so much capacity.Allowing the car to sit for a period with the draws a normal car will take is a drain on the battery. This draw process is harder on a worn battery with lowered capacity.
Don't need to determine a theory. Just check the battery after it sits for a period- say, overnight. Disconnect the battery, then you can either load test it, or you can check voltage across the terminals. Low voltage- approximately 12.4 or less- will make itself apparent.
Most cars will start with a lowish battery, when you unlock the car has been standing and if no drains the standing voltage should not be too low. Take a 230, one of the worst cars for low main battery voltage, the battery will turn off at 11.75 volt and still unlock with no problems, if the battery is too low to unlock the car, the the car will not start anyway. There is one rule with Mercedes that if the cranking voltage falls below 10.5 volt, the gearbox ECU may not set correctly, but that is something else.
Really if a car has enough charge in the battery to start, then it will always unlock, some keyless go cars might be more sensitive on this, but I have never known it.
A Mercedes cannot start with 10.5 volts. I know because I just experienced this a few weeks ago with my e55. It had exactly 10.5 volts and I measured it, directly from the battery. I didn't use the climate control for the measurement. I agree with the rest of what you said though.
The OP has a 2005 e320 which I believe has a dual battery configuration so it is a bit more complicated. If the system ( rear) battery is getting old and / or discharging (maybe due to some current leak / active modules) doesn't the battery control module detect and charge the system battery with the already charged aux. battery ? The OP indicates that after 4 hours, and many attempts to unlock the doors finally unlock which may be the time when the system battery voltage reaches and exceeds the threshold to operate the locks.
As I indicated, the first step should be to change the remote batteries and at the same time tensioning the battery terminals to ensure sufficient pressure on the batteries for good contact (there is a TSB on this), and if this does not work, monitor the system (rear battery) voltage 4 hours after the ignition is turned off with a digital voltmeter connected to the battery terminals with long leads outside the car. Or simply take it to a workshop or MB dealer which has the right equipment ( Midtronics MCR 717 tester or equivalent) to test the conductance and the capacity of AGM type batteries.
As others have said, the car can crank with a bad battery and yet not have enough power to unlock/lock the car.
I see this often enough in R230/W221....starter battery is good enough to start the car, but the aux battery in trunk is dead, so it won't unlock the car.
Though W211 does not use the aux battery under the air filter to start the car, the principle is the same here as well.
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