Resolved - Help! Horn only works half the time - Mercedes Benz SLK Forum

SLK R170 General Discussion SLK 200, SLK 230K, SLK 320, SLK32 AMG

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#1 Old 05-12-2019
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Question Resolved - Help! Horn only works half the time

Days to go before my 1997 R-170 has its umpteenth MOT test, I have a horn problem.
I've spent ages on this, and thought I'd fixed it twice, but it keeps failing, and then working, and then failing with no pattern.
So, the buttons on the steering wheel do work, they do send to the K40 unit.
The K40 looks fine, no bad solder joints. Following instructions from YouTube, yesterday I bypassed the K40 relay with a standard car relay, which clicks when horn pressed, and sends current to horn.
The horn works (though the second one is dead and now unattached). I replaced horn with similar and the same things happens, so not the horn.
So... what happens is, it's working... then I press it and it sort of dies mid blast, then it doesn't work for a while, and then I'll try it again and it's fine. Then it goes again. Arghh. The fact that it makes this funny squonk noise leads me to think it isn't the relay. Or the switches.
One more thing... if I disconnect the horn switch connectors from the K40, all sorts of warning lights come on... which is odd - so even with the new relay connected the K40 is still in the picture... could it be pulling down the horn voltage some how?
Any ideas gratefully received!

Last edited by Avel Du; 05-16-2019 at 09:20 AM.
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#2 Old 05-12-2019
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Originally Posted by PaulXLE View Post
Days to go before my 1997 R-170 has its umpteenth MOT test, I have a horn problem.
The K40 looks fine, no bad solder joints.
Any ideas gratefully received!
Some ideas,
Sounds like you have inspected the K40. But Faults in a old solder joints can be invisible to the naked eye.
Quick test is to swap it with one you know is working.
Alternatively, reflow every solder joint in it.
Also very carefully check all the tracks on the circuit board for cracks with magnifying glass.
Check continuity wherever possible.
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#3 Old 05-16-2019
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Thumbs up Maybe it's fixed!

Well, I checked out the K40 again. I checked out the horn again. I checked out the wiring I did to bypass the K40. I messed up one of the connections! Duh.

So I fixed it, and now the horn (oh please) seems to be working. The fault I had was intermittent, so you never know. But it seems solidly to be working now.

Let's hope it stays that way, in which case can I commend to anyone with horn problems (no sniggering at the back) the following short video on YouTube :
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#4 Old 05-16-2019
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Cheaper than fixing the brakes.
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#5 Old 05-17-2019
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The restraint shown by 'members' to the 'I have a horn problem' is commendable....!
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