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Discussion Starter · #1 ·
I posted this on the C30 forum and haven't gotten any hits. The car is a 2012 C30 but there are a lot more P1 Platform S40's and V50s so I am hoping somebody has some wisdom to share. Thanks in advance and apologies if cross posting is a problem. I didn't see it called out in the forum guidelines so here goes...

All, hoping for some help here.

I had a strange issue with the car. Will try to explain. About a month ago in the middle of the night the horn just started blaring. Car is parked in my driveway and had been sitting for hours. By the time I got to it after 10 minutes or so, the horn made a sick dying sound and went off. I got in the car anyway and fired it up. Car ran and drove fine, but no horn.

So, I just started digging into it. First thing to check was the fuse and sure enough it had popped. I put a new fuse in and the horn immediately started sounding and the fuse popped. Tried an even slightly higher rated fuse, same results, it popped after a few seconds. First conclusion is that one of the horns themselves is bad and pulling too much current, but that doesn’t explain the relay being on all the time.

I found an online wiring diagram that showed the relay for the horn. I did confirm that the relay is good and that it is being energized all the time. The wiring diagram shows it being controlled by the CEM under the glovebox. There are stories online of those being contaminated by water. I pulled this one out though and it is completely clean. No signs of ever being wet.

The CEM has an input that looks to go to the steering wheel module and then the horn contacts. I pulled the airbag out of the way and the voltage on the horn contacts is about 0.6V with the horn not pressed. I don’t know what the steering wheel module or clockspring module does with the horn signal but it it seems odd that the system would only put that low a voltage on the open horn signal which gets grounded when you press the horn. My suspicion is that whatever is between the horn contacts and the CEM is at fault.

My first question is whether any of the volvo documentation (VIDA) gives more in depth info on what is in the steering wheel module..

Here is what I found online:

https://www.manualslib.com/manual/53...age=219#manual

I’m an Electrical Engineer by day and do a lot of analog and digital hardware design. I should be able to tackle this with the right info. I am going to continue to explore how the CEM horn input gets connected to the actual horn contacts through the steering wheel module or clockspring. Just wondering if anyone else has had a problem like this. I searched a number of C30, and S40 forums assuming all P1 platform cars have the same basic setup.

Regards,
Dave
 

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The Horn circuits are very simple. The steering wheel push button switch activates a relay which provides power to the Horn.

You likely have a stuck steering wheel switch.

Disconnect the steering wheel horn switch. Does the horn stop? If so there is your problem.
The steering wheel switch switches the circuit to ground, so a permanent low means a stuck switch.

Be cautious working on the steering wheel as it contains an airbag. If you disconnect it you will get a warning lamp, which you can only reset with VIDA.


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Discussion Starter · #3 ·
Thanks for the reply!

I did disconnect the horn switch in the steering wheel and it still stayed active. The horn switch circuit goes through the CEM where I believe it gets combined with other inputs, such as the alarm so that it can be enabled by either path. What I would really love to see is a schematic for the CEM. That would help a lot. The Steering wheel module is also suspect since that is what connects the rotating steering wheel assembly to the fixed steering column harness. My next step is to disconnect the horn signal from the column harness and see if that turns off the horn.

Aware of the airbag dangers. Always disconnect the battery and then wait 10min or so before doing anything with those wires and never power up the car with it disconnected.

Dave
 
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