Horn circuit fault

MikeT · Post by **MikeT** » 22 Jun 2007, 20:58

Does anyone know if there are any known weaknesses in the Xantia's horn circuit? Both horns don't work yet my basic meter testing shows 12volts at either horn connectors when the stalk button is pressed. I then hooked up a 12volt interior light (too weak?) bulb to test the power and it glowed solidly and brightly on either connectors.
Assuming both horns had failed, I put 12volts direct from battery to each horn in turn and neither made a noise. Proven!
I sourced a working horn (loud bark when connected direct to battery) yet it barely squeals when fitted to it's connection under the wing - on either connector! I spent a long time repeatedly trying to clean both connectors and it made no noticeable difference. I'm going to study the Haynes BOL wiring diagram later but I've just about reached the limit of my electrical skills. Any hints or tips please? Thanks, Mike

AndersDK · Post by **AndersDK** » 22 Jun 2007, 21:21

Horns usually consumes some 8-10 Amps. Your testlamp maybe 0.2Amps.

The testlamp may be bright and clear when wiring is loaded by the 0.2Amps. But when wiring is loaded with 8Amps the voltage loss may be that big the horns no longer works on the remaining voltage left.

A very common problem in car electric circuits.

My bet its the ground connection that is weak.

MikeT · Post by **MikeT** » 22 Jun 2007, 21:52

Hi AndersDK, I was ready to suspect the earth as all other cars I've ever worked on use the component mounting-to-car-body as their earths.. but these Citroen horns have both +ve and -ve supplied through a plug. I assume this means the suspect-earth-to-be-tested is elsewhere on the car? I'll read the Haynes manual on circuit testing as I've reached the limit of my DIY car electric skills, cheers

MikeT · Post by **MikeT** » 23 Jun 2007, 10:13

According to the wiring diagram in the Haynes manual, the earthing point for the horns is a brown wire going to a connector block (containing 13 channels) via connector socket number A5 into an earth equipotential post from which it exits through socket number A7 and on to each horn.

Does anyone know where I might find the connector block or the earth post and what they look like? Are the socket numbers marked?

When I do find the right earth, what's the most reliable method of testing with a multimeter? I presume I'm looking for zero resistance between the post and the horn connectors or should there be a small amount of resistance due to wire size and length? How much resistance indicates a poor earth?

jeremy · Post by **jeremy** » 23 Jun 2007, 11:10

Simple answer - connect the meter set to 20 VOLTS across the earth circuit - ie from horn negative (or earth) to the battery negative and operate the horn. There should be a minimal reading - ideally about 0.1 volts - max 0.25 volts.

If everything else is working - simply make a new earth - drill a hole, clean the paint from the surrounding area, put a ring terminal on one end of a wire and screw it to the bodywork with a self tapping screw then to complete the job paint over it to prevent future corrosion. The other end of the wire should be connected to the horn negative terminal.

DO NOT try and measure the current in horn circuit. many meters do NOT have overload protection for the 10 amp setting and you may wreck the meter.

MikeT · Post by **MikeT** » 23 Jun 2007, 18:56

Hi jeremy, I'll follow your instructions next chance I get. I presume I need the horn connected to complete the circuit which requires a very thin wired probe to avoid jamming the connector - will this distort the reading significantly?

I'm also interested to know what a higher than 0.25V indicates exactly. Can the same test method be used for the positive side of the wiring? i.e +ve batt to +ve connector pin set at 20VDC

Ideally, I was hoping it may just need the current earth cleaning up hence my request to find it though your method is, as you say, a simple solution that works well.

I do have a meter that will accept 10A+ inputs though it is, as you say, via a special unfused circuit.

Thanks very much for the info.

MikeT · Post by **MikeT** » 27 Jun 2007, 15:44

From negative feed of socket to negative pole of battery, when the horn is activated, I get a reading of 0.02 - 0.08V using the 20DCV setting.

I guess that's not near enough 0.1V? Does that indicate a bad earth?

jeremy · Post by **jeremy** » 27 Jun 2007, 17:17

0.02 volts is 1/5th of 0.1 volt - so the resistance is less and the earth would seem to be in order.

When testing by metering in parallel the higher the subject circuit resistance - the higher the voltage reading. In crude terms you are bypassing the subject circuit - and the greater its resistance the more current will take the bypass route through your meter.

jeremy · Post by **jeremy** » 27 Jun 2007, 17:24

The value of 0.1 - 0.25 volts when measuring a voltage across a circuit was taken from a Range Rover manual - for testing the alternator circuit. Personally I'd be looking for less of a reading but took the view that those who wrote the manual knew more than I do and so quote their figures rather than sending people to look for perfection.

MikeT · Post by **MikeT** » 27 Jun 2007, 17:25

Well that's good news then

You advised not to measure the high amps. My meter can accept 10A max, is that too low?

jeremy · Post by **jeremy** » 27 Jun 2007, 17:37

2 horns in parallel could easily take 15 - 20 amps.

You can test the horns by feeding directly from the battery.

You can test the supply by measuring its voltage under load. Connect the meter (set to 20 volts) to the horn feed and to earth. Operate the horn - if the supply reads 11.5 volts or more the supply is sound. If it drotps to 5 volts or less - then there is a resistance in the supply.

Horns can fail - especially if mounted wrongly - so that water gets inside (correct with trumpet facing down or in a position where they don't get wet). Sometimes they get lazy and a tap with something hard vibrates the points against each other and temporarily revives them.

2 fail together? - unlikely.

Look on this board - horn unit failure is not common.

MikeT · Post by **MikeT** » 27 Jun 2007, 20:47

At the moment, I only have one replacement horn fitted and that barks nicely when directly connected to a battery.

Ok, I'll try and read the voltage under load.

MikeT · Post by **MikeT** » 02 Jul 2007, 17:57

I haven't tested for voltage drop on the horn circuit yet but I did make one surprising discovery - one of the original horns began working today when tested directly to the car battery. It didn't work on first connection but on second try, after an initial constipated sound it gave a constant low note.

When I first removed these suspect horns the other week, I tested them individually with a 12V 7Ah fully-charged battery pack and two bits of wire. With hindsight, there is obviously a need for power - was this battery a poor one to use for this test? Additionally, I made no proper wiring connections - strands wound round the battery posts and me pressing the other ends to the horn terminals.

Using an original horn loom connector, as I said I got the first horn working this time using the car's battery. The second horn stayed silent though.

So you were spot on jeremy when you said - "2 fail together? - unlikely."