Locating and fixing the horn :(

So I annoyingly failed my NCT (MOT) yesterday as my horn didn't work, so I've done some initial testing today but can't seem to easily locate the source of the problem :-(

1. Fuse box

First test was to continuity test the fuse in the internal fuse box for the horn and check that that was fine - it is (*and I tested all the other fuses just for good measures)

2. Air bag switch

Second test was to disconnect the battery, unbolt the air bag and test the switch on the air bag that it actually closed circuit when pressed - it did. Then I short circuited the pin that this is connected to on the ribbon cable so ground to see if the horn sounded - it didn't :(

So I guess at this point it's either that a. the horn really is dead, b. the relay for the horn is dead or c. the wiring is broken as I can't seem to think where else the problem might be?

Anyone got any good suggestions on how to locate and test the horn (I believe it is behind the fog lights from searching previous posts) or test the wiring from the steering wheel to the horn? I did try testing from the fuse to ground, but that just gives continuity regardless of whether the air bag switch is pressed or not pressed

Also - I wanted to swap the relay with the fog light relay to see if that was the problem, but can't seem to easily remove the relay to try this as even with the two side 'tabs' loosened it didn't easily wiggle out

Thoughts much appreciated and a few photos attached :)

Airbag and horn wiring (ground in top left)

Attachment 83172

Pins for airbag and horn - horn is pin 4, airbag pins 5 and 6

Attachment 83171

Testing airbag switch to ground

Attachment 83173

Testing horn wiring to ground

Attachment 83174

Horn relay - how to remove and test?

Attachment 83175

Did you test all the fuse(s) in the engine bay, too? I know none of them are labelled for Horn, but worth checking!

Do you feel the relay clicking when you press the horn switch?

Take the column cover off, and try checking the other side of the clock spring - it could then be that it's the clock spring at fault.

Quote:

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Originally Posted by Confused

Did you test all the fuse(s) in the engine bay, too? I know none of them are labelled for Horn, but worth checking!

Do you feel the relay clicking when you press the horn switch?

Take the column cover off, and try checking the other side of the clock spring - it could then be that it's the clock spring at fault.

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Good ideas there Garry - many thanks and I'll try them out and report back :thumbsup:

Another afternoon of investigating today into my horn problem, and I think the issue is that the Horn relay is not getting a 12V feed, so when the steering wheel horn is pressed, and the circuit completes to ground, there is no 12V source to throw the relay to connect and send 12V to the horn

I tested the relay by swapping it with the one below for the front fog lights (they are exactly the same model number) and the front fog headlights worked with both relays, so that leads me to believe the relays are good

When it metered the front fog light relay socket, it had the following voltages

[ --- ] Ground
[ ]
[ --- ] +12V (constant)
[ ]
[ | | ] Ground, +12V (but only when front fog headlight switch was turned on)

So this would mean that the bottom two pins trigger the solenoid to close the gate on the top two pins when the switch is pressed and 12V is fed to the line to the front fog lights if my thoughts are correct

For the horn relay there is no voltages on the top two pins, so no voltage to give the horn when the bottom two are closed

For the horn relay there is a voltage across the top two pins of 9V when the key in the ignition is on, but this circuit is the reverse, so 9V on the top and Ground on the bottom e.g.

[ --- ] +9V (constant)
[ ]
[ --- ] Ground
[ ]
[ | | ]

When I press the steering wheel horn button I can hear the solenoid clicking but there is no sound from the horn - so definitely the wiring to the relay is working as is the fuse wiring

Does that sound logically correct and if so, and I double checked the horn fuse, where would be the next place to go investigating does anyone think?

Did you check you had 12v both sides of the fuse?
:happy:

Quote:

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Originally Posted by TAR

Did you check you had 12v both sides of the fuse?
:happy:

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Good suggestion and just checked and yes there is +12V on both sides of the horn fuse

I did a bit more investigating this afternoon and with the ignition key turned on (I thought the horn worked without the ignition on but obviously not!) and I do get +9V across the top two pins of the horn relay and when pressing the horn button I can hear the relay clicking, so it sounds like power is getting to the relay

From metering the relay socket without the relay and the horn not pressed I get

[ --- ] +9V (voltage tested to chassis ground)
[ ]
[ --- ] Ground (connectivity tested to chassis ground)
[ ]
[ | | ] +10V (voltage tested to chassis ground), Floating (connectivity tested to chassis ground)

When the horn is pressed I get

[ --- ] +9V (voltage tested to chassis ground)
[ ]
[ --- ] Ground (connectivity tested to chassis ground)
[ ]
[ | | ] +10V, Ground (+10V flows across the pins)

So I have a cunning plan to rig up a temporary 12V horn as per attached photo and use this just to get through the NCT

My thoughts were that if I run a wire from the bottom right pin (Floating / Connected to +10V) that this would trigger the horn but that doesn't seem to work for some reason - maybe I need a clean +12V to the bottom left pin and then this will give a clean +12V to the bottom right pin when the horn is pressed?

Here's someone else's wiring diagram of the Denso relay and this seems to match my measuring too [Note: but I think my assumptions about the Low Current Circuit and the High Current Circuit and labelling are wrong]

BTW I know this isn't just a VR4 related problem but all the help very much appreciated!

Attachment 83176

Denso relay

Attachment 83177

Temporary 12V horn just needing a feed

Attachment 83178

[Edit] Actually looking at the other person's diagram perhaps I would need to wire the horn to the middle pin labelled "+ Power Out"??

That seems to match this 5 pin relay diagram and this 4 pin circuit diagram...

Attachment 83179

Attachment 83180

So just to update my own investigations into this

1. We have connectivity from the air bag switch to the relays as the voltage is tested as changing on the low voltage circuit when the horn is pressed
2. We have +battery on one of the pins on the high voltage circuit
3. I've just tested the relay using a 9V battery to trigger the low voltage circuit (the silver vertical pins) and this then connects the high voltage circuit pins together successfully (tested using multimeter on connectivity beep setting

So everything is correct so maybe the horns themselves have just died?

Plan tomorrow is to get some crimp terminals and breakout the relay from the fuse box so that I can connect my Harley horn to the high voltage circuit pins at one end, and a source +12V from the battery to the other hvc pin and extend the two silver low voltage circuit pins so that the horn still triggers this circuit - sounds like sound logic so far in my head!

Testing the relay with a +9V battery

Attachment 83181

Where are the horns behind the fog lights as removing the drivers side one I can't seem to see any horn there?

Attachment 83182

Attachment 83183

9v and 10v doesn't sound right. There should be 12v minimum on any positive connection. The horns may not be working due to a low voltage applied to them. :happy:

Quote:

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Originally Posted by TAR

9v and 10v doesn't sound right. There should be 12v minimum on any positive connection. The horns may not be working due to a low voltage applied to them. :happy:

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Thanks Tim (@TAR) - that totally makes sense and no idea why there would be a voltage drop on the HC circuit!

If I can break out the relay tomorrow with crimp and wire extensions then I can hopefully test this and both the replacement horn and the original ones and see if we get a working circuit with a clean +12V :thumbsup:

So just to close down my own thread, thanks for the tips everyone and I went a bit creative but success at the end of the day - rigged up the relay with some wires to a bolted on horn which passed the NCT :)