I just bought an XK 4.2 yesterday. Both keys were in very poor condition and I couldn't start the car using Keyless Go, only by inserting the key. I then replaced the battery in the key and was at least able to open and close the car using the remote control (but still didn't have Keyless Go).

In the evening, I tried to unlock the car again, but only the turn signals gave a short signal and the car did not open. Today, I inserted a new key battery again and cleaned the microelectronics in the key. Now, however, the car is not responding at all. I can see that the alarm light is still flashing.



Questions:

• Is the car battery probably dead?
• If I open the car with the key blade, the alarm will probably go off. It's possible that the key won't be recognized because the battery is dead, is that right? So I would have to open the hood and connect a battery booster, correct?

Any help would be greatly appreciated. I'm very frustrated (and scared of setting off the alarm) 😅

Thank you,
Philipp

 

If you unlock the liftgate (boot, trunk, lid) using the key, you can access the main battery directly. Be ready with a 10mm to disconnect the battery if the alarm sounds, then charge it.
This is my first idea, just to get started, there are more qualified people here. Keyless-Go? You must be a Benz owner.

 

Thank you.

Will the alarm go off if I open the trunk with the key and there is still some power in the battery (the red alarm light is still flashing)?

If the battery runs out on its own, could the alarm also be triggered?

And yes, you're right, I've also driven Mercedes, and in German-speaking countries we usually use the term “Keyless Go” regardless of the brand.

 

And one more piece of information: After purchasing the vehicle yesterday, I also noticed that water is dripping into the passenger compartment via the AC condenser. According to the seller, he is unaware of this issue (of course). I therefore assume that the problem has existed for some time. Could the water leakage also be related to issues with control units and key recognition?

 

The water is unrelated. Search the forums for Duckbill and you’ll find a whole host of threads that cover this known issue. As well as some very creative solutions.

 

You can put a charge on the battery using the external positive post, located under the car, behind the left rear wheel, usually covered with a rubber cap. I connect the negative charger lead to the tail pipe, seems to be a good enough ground point on my car.
I don’t think the alarm will sound if opening the car door or trunk with a key.

 

Whenever there's an electrical glitch, the prime suspect is a low battery, regardless of age. If the voltage in the morning, after the car has sat overnight, is less than 12.6 V, you need to recharge it using a real charger - 10 Amps or more - not a puny battery maintainer or trickle charger.

Winter is coming or may already be where you are. Cold weather is a battery killer.

 

Solved: It was a problem with the keys. I transferred the electronics from one key to the other and it worked right away. Keyless now works again too. At least with one key.
Thank you very much for your help, and I'll tackle the water ingress problem next.

 

Did you have the car inspected before purchase?