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Aftermarket horn help please
#1
Hi, ive changed the weedy standard horn on my 2017 enterprise van to a twin setup, much louder.
I've wired a relay in, however when I press the horn the lights go out??
I'm stumped! I've attached the diagram as wired in (there's a fuse to battery not shown), and I've since earthed pin 85 as I read another person had a problem and that sorted it but not this,
Any suggestions welcome
Thanks 

   
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#2
Where is the relay earthed ?
So where does this bit go then ?
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#3
OK, start off simple.

TEST 1
Does the new horn work if you apply 12v to the +ve terminal?
(yes = horn works, earth is ok)

TEST 2
Put the relay back in circuit.
Disconnect the original wiring.
Earth 85
+12v to 86
Does the horn work?
(yes = horn 12v feed OK, relay OK)

TEST 3
Meter the original wiring.
Are either of the original wires +12v all the time the ignition is on?
(yes = it's a switched ground setup)
(no = switched live)

TEST 4, only do if Test 3 = Yes
Meter the original wiring.
Is the other original wire earthed when the horn is pressed, and the ignition is on?
(yes = the BSI is switching ground OK)

TEST 5
Disconnect the original wiring from the relay.
Reattch the original horn.
Does the horn sound? Do the lights stay on?
(yes = original config OK)

At test 5, I'd be wondering about the mysterious, overintrusive BSI current monitoring. It *may* be detecting that the correct amount of current isn't being drawn, interpreting that as a short, and swiching off the supply to the horn (and other things)

I'd also take a look at the original and new horns - are they similar enough in draw that you can avoid the relay-switched setup completely?
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#4
Many thanks for the replies ffrenchie and shtu, answers as follows....

The relay isn't earthed apart from pin 85 which is the horn negative trigger
The horn is earthed at the inner wing earth point just behind the passenger headlight

Test1
yes = horn works, earth is ok

Test 2
yes = horn 12v feed OK, relay OK

Test 3
yes = it's a switched ground setup

Test 4 how do I test this?

Test 5 not done yet

I've noticed something changed slightly, in that the lights only dip off when the horn button is released, I'm sure the lights used to dip off on pressing the horn
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#5
TEST 4 detail

Disconnect the original wiring from the relay.
Find the wire that ISN'T the +12v.
Set meter to Ohms. (continuity setting is fine, maybe you have a sounder for open\closed circuits?)
Connect the meter to that wire, and also a known-good earth.

At this point the meter should show open circuit, infinifty, whatever. Same as it does without the leads connected.

Sound the horn.

The meter should read 0 Ohms, go BEEP, or whatever it does when a short is detected.
(Yes = The BSI is switching the ground as it should)

And finally,

TEST 6
Are the lights OK with the original horn only?

TEST 7 (re-edited to correct my stupid mistake)
Wire the original horn and relay in parallel, so that the original horn sounds, and the relay is energised.

Test. And when your hearing recovers, report back.
(Lights fixed = the BSI is doing weird things detecting current across the horn circuit.)


Beyond this, I'd need to look at a wiring diagram and have a think, maybe look for earthing issues, but that's pure speculation just now.
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