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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 54039
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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I've tried synchronizing my key/fob weeks with no luck 1999

Customer Question

I've tried synchronizing my key/fob for several weeks with no luck for my 1999 SLK230 Seems like I've looked and tried every combination posted on forums. For the last couple weeks what has worked is turning the key to the ON position until the pre-start "hum" ends, then I can start it. This "hum" originally lasted a couple minutes, now it lasts 20+ minutes. I bought the car last spring. Replaced the battery in the fall. Garaged it for six months during the winter. This began happening a couple months after I put it on the road this spring. Even replaced the starter. Any ideas?
Submitted: 2 years ago.
Category: Mercedes
Expert:  Marty replied 2 years ago.
Hi, welcome to Just Answer. My name is***** is nothing to synchronizing the key. Once the key is programmed to the car there is never a need to synchronize it in order to start the car. It sounds to me as though you are describing the symptoms of a faulty K40 relay. The K40 relay powers the control units in the car. When the relay malfunctions you are unable to crank the motor. I'd suggest replacing the K40 relay for this problem. The relay is located in the module box next to the battery. The part number for the relay is 210 540 00 72.
Let me know if you have any questions.