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Registered 1997 SLK 230
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118 Posts
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!
 

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Premium Member 2001 SLK320
Joined
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1,032 Posts
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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Registered 1997 SLK 230
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118 Posts
Discussion Starter #3
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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Registered 2016 SLC300
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249 Posts
The restraint shown by 'members' to the 'I have a horn problem' is commendable....! >:D
 
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