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When the car is turned off the horn and radio works, once I…

Customer Question
When the car is...

When the car is turned off the horn and radio works, once I turn the car on/ motor on the horn and radio stops working

Mechanic's Assistant: Is this an aftermarket stereo, or did it come with the car?

Came with car

Mechanic's Assistant: What is the model/year of your Mercedes?

2010 c 300

Mechanic's Assistant: Are you fixing your 300 yourself? What have you tried so far?

I haven't tried anything

Submitted: 8 months ago.Category: Mercedes
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Answered in 15 minutes by:
9/18/2017
Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 8 months ago
Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 5,882
Experience: 20 years Master Certified excellent fix rate and have access to factory wiring, and instructions.
Verified

Hello, my name is***** and i will do my best to assist you.I am going over your story now and will reply back soon with some info,thanks!

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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 8 months ago

I see that this may be the first time you are using JustAnswer, if there are any questions you have about the process and how it all works please ask as it can be confusing at times. Not all questions have a easy or quick answer especially in the car sections like this. There may be some back and forth until we can come to a conclusion so please bear with me if I ask questions, it is just for me to get a better understanding of what may be happening. I have no doubt that I cannot help you get this resolved or pointed in the right direction. Below will either be an answer to your question or a few questions to help get us on our way,Lou P.

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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 8 months ago

there is a relay that powers up what is called the circuit 15r power circuit that is key on and engine running power.

its relay K in the drivers side engine bay fuse box. we need to make sure the relay is getting power and ground across terminals 85 and 86 OR terminals 1 and 2 depending on the relay numbers near the terminals.

if you dont know how to test or check relays and circuits and plan on having a shop do it then the possible issues are the relay,

front same control module or the ignition switch .

we ideally need a scan tool to check the signals to see what is not allowing the circuit to stay on, my best idea is a failed relay but it all depends too on if the alternator is charging, there are quite a lot of things that go into the relays and circuits being powered up

so let me know what kind of information you want from me and I can help for sure,

Lou

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Customer reply replied 8 months ago
The car starts and runs, the lights work etc. It's the horn and the radio that stops working once I start the car. Are the relays like fuses and can I change them my self? If so which relays do I ask for that auto parts store and is there a diagram that you have of my relay / fuse location and names? Also I think the alternator is charging cause I have no issues starting the car.
Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 8 months ago
Ok so for the alternator , just being able to start the car does not tell you that the alternator is ok. Only an alternator test can. We NEVER assume anything is ok without the proper test that is a number one mistake with technicians these days . Yes there is a relay you can try and change I will go back over the diagram to make sure that only the radio and horn are on that circuit because if there are more items then we may need some further things to try . Either way I will get back with some more info soon
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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 8 months ago

we need to check fuses 10, 31a , 9, 21 , if not then you will need to check and replace relay K in the engine bay fuse box.

Lou

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