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Marty, ASE Certified Technician
Category: Mercedes
Satisfied Customers: 54832
Experience:  Bosch Certified Master Technician with over 35 years experience in the automotive industry.
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I have 2008 Mercedes ml320. Locked it yesterday and left it

Customer Question

I have 2008 Mercedes ml320. Locked it yesterday and left it in the drive overnight and when I went to open it is morning with the key fob it wouldnt open. I took the key blade out of the key fob and it wouldn't open it either.
I spoke to a mechanic friend who told me to turn the key blade to the right whilst pulling the door handle then release and do it again. This opened the door locks alright but now I can't lock it at all.
The doors open and lock with the main switch inside the vehicle and also lock automatically when I start driving so I don't think its a door lock problem. I also only had two door lock units replaced 2 months ago with a Mercedes dealer (who charged me ‚ā¨1600 for the privilege so I dont want to go back there unless absolutely necessary ūüėĪ).
The batteries in my key fob are fine as I am getting the red light when I press the buttons. I have also tried my spare key fob and the vehicle is still not locking.
Its like the vehicle just stopped recognising the key fobs!!!
Has anyone any ideas?
Submitted: 1 year ago.
Category: Mercedes
Expert:  Marty replied 1 year ago.

Hi Michelle, my name is***** sounds like there may be a fault with the antenna for the remote signal. Try pointing the key fob directly at the driver's door handle from 6 inches away and see if the doors will lock and unlock with the fob. If they do it indicates a fault with the antenna. If they do not unlock the most likely fault would be the electronic ignition switch. The switch may need to be recoded at the dealer.

Expert:  Marty replied 1 year ago.

Hi, I'm just following up with you on this question. You never rated my service therefore I never got paid for answering your question. If my answer was helpful I would appreciate it if you would rate my service so I can receive credit from the website.

Customer: replied 1 year ago.
Hi, just had it at the dealers, they had it plugged in and said it didn't show up any definite fault so it is a process of elimination. They are suggesting firstly a new key (but up until the other day both my key and my spare key worked so I can't understand how both keys stopped working) they want €265 plus vat for this.
If that doesn't work they next want to replace the rear SAM unit at a cost of €545 plus vat.
Finally if that doesn't work they want to replace the rear infared transmitter at a cost of €204 plus vat!
I don't think it is the key as both are not working. I can't understand how they don't know what it is! What do you think, am I being ripped off?
Expert:  Marty replied 1 year ago.

Hi Michelle, they always want to rule out the key but if you are sure that both keys were working I would bypass the first step. If you are able to lock and unlock the car by pointing the key directly at the driver's door handle I would suspect the rear antenna module or the rear SAM is the problem.

Customer: replied 1 year ago.
Hi Marty, Neither key is working at the moment but both were working prior to this problem starting last Monday. As both keys were working I can't see how it would be a key problem as that would mean that both went wonky at the same time. Neither key work by pointing at the drivers door but the key did work again on Friday when it locked and unlocked the doors twice then decided to stop working again! I think I will bypass paying €245 for a new key and go straight to replacing the SAM unit, what do you think?Thanks,
Expert:  Marty replied 1 year ago.

Hi Michelle, if the rear SAM was bad you should still be able to lock and unlock the car by pointing the remote at one of the front door handles from 6" away. Are you sure they won't work that way?

Customer: replied 1 year ago.
Hi Marty, just checked with both keys directly pointed at drivers door handle and neither working. Do you think I should have the keys recoded or go straight to getting a new rear antenna module? Does the whole rear windscreen need to be replaced to sort this? Or is it a separate unit? Of the three options the dealer suggested which do you think I should try first? Thanks for your help. Michelle
Expert:  Marty replied 1 year ago.

The keys send out two signals. One is a long range radio frequency signal that is picked up by the antenna and the other is a short range infrared signal that is picked up by the receivers in the door handles. The signals are processed in different manners but have the electronic ignition switch in common. Since they don't work by either signal I would suggest replacing the batteries inside the key. Even though the red light is blinking the batteries might still be weak (although unlikely). Next I would suspect a fault with the coding of the electronic ignition switch. This sort of rules out the rear SAM and the antenna since the infrared signal is not processed through the SAM and the antenna. Try pointing the key at the passenger door handle just in case there is a problem with the driver's handle.

Customer: replied 1 year ago.
I replaced the batteries in the main key last week and I have just pointed both keys at passenger door and no joy. The dealer never mentioned recoding the ignition switch to me but do you think it is worth my while going back to them and asking to look at that first? They didn't seem to know what it is as nothing showed up on the diagnostic computer and gave me the name of a LockDoctor to check if the key was sending out signals and then to come back to them!! Sorry for being a pain but I don't want to throw money at it just fit the sake of it. I've already had 2 door locks replaced this year at a cost of €1600 and just paid the annual road tax of €1494! Think it might be time for it to go ��
Expert:  Marty replied 1 year ago.

I think I would go back to the dealer and inquire about the coding. Let them know what I said about the radio frequency signal and the infrared signal. The infrared signal does not pass through the rear SAM or the rear antenna. I'd be looking for something like a bad key (2 bad keys) or a coding issue. You can check the infrared signal from your remote by pointing it at your smartphone camera and see if you see a blueish light coming from the remote in your camera window. Click here for the instructions.