Alfa 156 Horn Fault Diagnostics
Just been over my '04 facelift 156's horn wiring as it had stopped working also. The horn initially was OK, then it would work intermittently (irrespective of steering wheel position), then gave out with a brief sort of intermittent feeble noise.
The 156 has 2 horns in the left of the engine bay as you stand in front of the car, looking at your lovely engine. They're just in front of the washer bottle, pretty inaccessibly located in the wing. You should be able to get your hand in if you've got skinny wrists. There'll be a black and a purple wire to each. Black is neg. Try putting a juicy current feed on the pos of the horn to check they actually work (mind the bodywork!). If they don't, check the grounding, and if still no, the horn is faulty. Rare for both to fail at the same time though. If they work, the next vaguely accessible test point is the horn relay. Alfa have kindly located this on the REAR of the fusebox next to your right knee when you sit in the drivers' seat (RHD). There are 2 bolts that hold the fusebox in - you'll need to disconnect the battery and the thick cables that go to the fusebox (both sides). After you've skinned your hands, it's a thin oblong relay, not the sqare ones. Take it out and test it with a 12V supply and a multimeter.
If that's OK then you're down to the cable that goes to the steering wheel boss. The way the horn works is that the relay does all the high power stuff, and the relay switching coil is activated by your horn pressing. The coil of the relay is connected to a pos feed, and when you press the horn, the other pin is grounded, energizing the relay. No good checking for 12V therefore. You will need to have the battery disconnected for 15 mins before proceeding.
There are (on my multi-function steering wheel) 3 allen bolts on the back of the spokes of the wheel. Unscrew. A washer keeps the bolts captive. The central boss should now be loose, including the airbag. Lift it away - it'll be on a yellow wire. There is a green locking tab on the back of the airbag connector. Gently prise it up. The connector will then lift stright up, away from the airbag, permitting you to store it (gently, away from the work area). You'll see a chunky red and black wire on the right, and a multi-connector on the left, both connected to a U-shaped black thing - there will be ribbon cables from the multifunction buttons that connect here too. Unplug both ribbons and the 2 connectors. Now it's worth checking that when one of the three 'buttons' (ie where the steering wheel boss screws are) is pressed, the circuit is made - check it at the pins where you pulled the chunky 2 wire connector off. If all is well, you have traced the fault to either the black U shaped thing, or the clock spring (see later).
The black u-shaped thing just gently prises out of the steering wheel, and comes into two halves by prying with a screwdriver. There's a small PCB inside, with some surface-mount components on the back. You'll need good eyesight, but trace the path of the black chunky wire and red chunky wire through to the pins of the multiconnector at the other end. On my car, there is a tiny surface mount 'bridge' that looks like a diode, labelled 0000 that had burned out (on the chunky red wire 'track'). It isn't a diode, as I've checked similar components on the same PCB and they're 0 Ohms in both directions. If yours has done the same, you'll see 2 conveniently located 'pads' on that PCB track either side of that bridge. Just solder an insulated wire between the two pads and check your work with the multimeter. It's fiddly if you're no good at soldering though.
If your black U shaped thing is OK, then you'll need to remove the steering wheel to access the clock spring. Put the wheels in the straight-ahead position. Prise off the plasticky nut protector. The steering wheel nut has a sort of outer metal castellated bit round it - you just ignore that, and use a 24mm socket over the whole lot. Take the wheel off, feeding the airbag cable and multicable through the hole.
Now you're looking at the clock spring. CAUTION! Don't rotate the centre collar while you're fiddling! There is a spring-loaded brake that should prevent this while the wheel is off. If you want to remove the clock spring there are 3 screws, but to get at them, you need to remove the steering column cowls first. 3 screws underneath, then 2 screws at 2 o clock and 10 o clock that hold the top cowl on (just under the lip). There is a connector on the clock spring at the 12 o clock position. You can then go inside the house with your clock spring, and check that there is continuity between the 8 pins at the steering wheel side, and the multiway (I think 12-way, but this includes airbag control) on the clockspring itself. I found a dressmaking pin useful to put sequentially in each of the 'holes' of the 8 pin connector to allow a connection to be made.
Hopefully you have located the source of the open circuit by now, and at least know which part is faulty! If you don't have a multimeter, get one from Maplins or similar. One that has an audible beepy continuity setting saves a lot of time, they're only about 15 quid these days, and as you've got an Alfa, you'll likely need it again!!
Best of luck!
Last edited by fletch106; 26-09-10 at 20:24.