Golf 7 No Horn after BCM upgrade for LED headlight retrofit

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sammie M

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2016 Golf GTI,
I recently managed to complete a Golf 7 to 7.5 LED facelift retrofit including LED headlights. Have everything recorded and working perfectly....except the Horn...it has just stopped working since the BCM change.
Any ideas where in the coding I could look or what it could be?
Old BCM 5q0937084an
New BCM 5q0939084cf

Your help would be greatly appreciated.

Thanks
Sam
 
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darkchina811

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have you checked the fuse and/or the horn relay? have you checked if the horn works with the ignition on? there is an adaptation that can shut down the horn without ignition.

anyway, post an auto scan.

besides that, did you replace bumper and fenders to be able to install mk7.5 headlights? what other parts?
 
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sammie M

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Horn works fine when I reconnect the old BCM.
Fenders etc replaced, headlights all working fine.
Will post an autoscan soon, but where else in the coding can I look? It's weird!
 
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myounus

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Your new bcm will have many different adaptation channels. Once you post a scan it'll be easier to work out.
 
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DV52

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^^^ hmm......... I hope that it is a simple matter of changing a software switch in the code-string, or altering a setting in the adaptation channel database in the BCM

However, according to the WD, the horn initiation signal happens on the steering column control module (J527) @ address hex16. The horn output signal (the actual volts/wave-form that sounds the horn) however is generated by the BCM (i.e. an entirely different control module).

This suggests that the end-to-end process for the horn function on a MQB car is predicated on sending a message over the CAN network - I reckon. This also suggests that there needs to be compatibility between the firmware versions in both j527 and J519 to send/receive the horn initiation CAN message.

Could be something to bear-in-mind when dealing with this issue.

Don
 
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sammie M

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Thank Don,
Can you please confirm the BCM pin connection that drives the horn?

Thanks
Sam
 
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DV52

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Sam: sure, but I think that I answered this question in the forum in another place


Here is my cut-down version of the WD for the Horn-relay on a mk7 Golf:

KFwuAgJ.png


So, the horn relay (J413) is connected to T46b /24 on the BCM.

As you can see and if I'm reading the wiring diagram correctly, the +ve side of the relay coil for J413 is connected to fuse #15 on the B fuse box (the one in the engine bay) and this coil is energized when the BCM (J519) receives the correct CAN message from J527 (as per my previous post) by earthing the other side of the coil (via pin #24, socket B inside the BCM).

I guess that you could confirm the operation of this pin on your BCM by connecting a multi-meter (switched to volt mode) to pin#24 and earth. Without pressing the horn plate, pin #24 should be somewhere near the car's rail voltage. Press the horn pate on the steering wheel and pin # 24, socket B should be shorted to earth (i.e. zero volts) if the CAN message has been received correctly from J527

As for your assertion (on another forum) that the horn relay is connected to T46b/14 and that this pin is missing on your new BCM, my information suggests that this pin is dedicated to F387 Bonnet contact switch. The second thing that I would mention about this assertion is my surprise that you don't have any error message from the BCM if the connector pin for J413 is missing on your unit.

Intuitively, I would have expected the BCM to monitor the presence of some Volts on the horn-plate solenoid pin in case the fuse blows, or the coil itself is faulty. With the horn plate not pressed, the horn plate pin (be it pin #24, or another pin) should see an open-voltage (~13.8V) on the other side of the solenoid wire. However, If the connector pin for the horn-plate is not not connected on the BCM, the solenoid pin sees no voltage at all - hence I would expect a resulting error message in this instance. This is a very long way of saying that if you do not have an "open-circuit" error message on the BCM - it's more than likely that the solenoid is connected to the correct pin (I think)

Don
 
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