Horn Failure - Circuit testing

Quite suddenly, out of the blue, my horns stopped working when pressing either of the steering wheel buttons. The wiring circuit for both the horn operation and the triggering of the SRS driver's airbag pass through a rotary coupler fitted underneath the steering wheel on the column itself. When horn failure occurs, there are 2 other major concerns:-
  1. Does the security alarm system still work?
  2. Does the driver's SRS airbag still work?
POSSIBLE CAUSES:

Click to enlarge The electrical circuit shows that the relay that controls the horns can be triggered by either the steering wheel buttons or the alarm ECU.


  • Try pressing both buttons. If only one of the steering horn buttons is failing to sound the horns, then the fault lies within the steering wheel itself.
  • Otherwise check the horn/ECU fuse in the 'under-bonnet' fuse box. This is the right-most fuse when standing and facing the front of the car.
  • If the fuse is okay, test the Security Alarm function.


    SECURITY ALARM:

  • The Security Alarm function can be easily tested by opening the driver's window, getting out of the car and set the alarms. Leave it for a minute or two to ensure the volumetric sensor is set. Then stick your hand or head in through the window and see if the alarm gets tripped. If YES, then the circuit from the battery via the fuse to the relay is okay.
  • If the alarm does not sound the horn, then it is likely that the relay is failing.


    SRS AIRBAG:

    NOTE: Before carrying out any electrical examination of the horns/SRS/steering column, DISCONNECT THE BATTERY AND LEAVE FOR AT LEAST 10 MINUTES FOR THE SRS AIRBAG SYSTEM TO DISCHARGE.

    Remember too that disconnecting the battery will mean your radio will loose it's security code and will have to be re-entered.


  • Does the dashboard SRS warning light stay on?
  • If YES, then it is likely that the steering column rotary coupler has failed completely.
  • The coupler is relatively delicate and from your own point of safety it is recommended that you ask your Rover Dealer to test it and replace it if necessary as soon as possible.


    TESTING THE HORN BUTTONS:

  • Assuming that the horns sounded when the alarm was tripped, then a simple test will determine if the failure is either in the rotary coupler or the horn buttons themselves.

  • Remove the upper and lower steering column nacelles by undoing the three screws underneath the column. The two nacelle halves clip together and excessive force will easily damage the clips.

    Click to enlarge Click to enlarge Click to enlarge The clips seem to be designed to pull apart, but are fragile and can break.


    Click to enlarge Testing out a way to undo them with my old nacelles, it seems that if you can get your fingers into the square holes for the indicator and light stalks, then gently pull upwards and downwards to separate the two half at this point on either side. The rest should then come apart.

  • Gently ease the lower half over the ignition key barrel, and remove.
  • Two connectors are now exposed at the top rear of the rotary coupler. The right hand side connector is for the horns. The purple/black wire is the horn circuit +ve feed from the ECU relay. The black wire is the earth or ground lead.

    Click to enlarge Unclip this connector and temporarily connect the back of the pins using a short length of wire or something like a paper clip. Never probe directly into the front face of a connector as this can cause damage and subsequent failure.

  • Re-connect the battery.
    1. If the horns now sounds continually, then the problem lies in either the rotary coupler or the horn buttons themselves.
    2. If the horns remain silent, the problem lies between this connector and either the relay (purple/black wire) or the earth point (black wire).
    3. The purple/black wire is connected to the left hand connector of the Alarm ECU, which can be accessed by removing the side panel in the left hand footwell.
    4. The black earth wire is connector to an earthing point sited on the inner wing, just to the right of the fuse panel.
  • Disconnect the battery again and wait 10 minutes before you start to refit the connector and steering column nacelles.


    ROTARY COUPLER OR HORN BUTTONS ? :

  • To test this you will need to remove the steering wheel.

    NOTE: Before carrying out any electrical examination of the horns/SRS/steering column, DISCONNECT THE BATTERY AND LEAVE FOR AT LEAST 10 MINUTES FOR THE SRS AIRBAG SYSTEM TO DISCHARGE.

    Click to enlarge To remove the airbag, undo the two screws from the rear of the steering wheel. Note that these are set at an angle, pointing in towards the driver's position.

    Click to enlarge Lift out the airbag, and remove the connector from the underside of the air bag.
    Keeping the airbag with the front face uppermost, place to one side. Do NOT drop or place anything on top of it.

  • Both the horn buttons are commonly joined to the connector on the top face of the rotary coupler. Unclip this connector.
  • Using an electrician's meter, test for continuity across the wires to this connector when depressing the horn buttons.
  • If the horn circuit is okay, then the fault lies in the rotary coupler. If both the steering wheel and the nacelles have been removed, you can test for continuity across the rotary coupler via both connectors.


    For further details of the rotary coupler itself, go to Inside the Rotary Coupler