When I bought the car, the horn didn't work. I wasn't to worried about it as I only use the horn a couple of times a year. Well after almost hitting three different cars during the first week I decided I'd better fix it fast. It seems other cars don't see a light blue 928, or assume I won't risk a collision with their Camry, Toyota pick up, or Altima.
I started with the fuse and relay. Referencing the fuse and relay chart I checked fuse 15 first. I used a ohm meter and found that it was ok. Next I switched relay X (horn) with relay XXII (defroster). The defroster and horn relay have the same part number (928.615.117.00). To remove the relay, I just pulled hard and wiggled a little. Luckily relay VIII was not used and XXII is on the end, so I had more room to work. I tried the horn, but it still didn't work. I then switched them back (first mistake).
Next I checked the horns. They are located behind a cover in the passenger side front wheel well. I jacked up the front passenger side of the car and removed the wheel. I then remove the 8 mm bolts that hold on the cover and removed the cover to expose the horns. I disconnected the wires to the horns and used a 12 volt power source (old battery charger) to test each horn. Both worked. I cleaned the contacts and replaced the wires, cover and wheel.
Step three was the check the horn button. To remove the horn button, grab both ends firmly, and pull away from the steering wheel. (Don't allow the horn to pull away to far, as there is a wire attached to the back of the horn button.) I found the wire disconnected and covered with electrical tape. The terminal on the back of the horn was broken off at the base, and a prior owner had just taped it up instead of fixing it. I untaped the wire, and tested it with the ohm meter. It read 12 volts. I then touched it against one of the terminals on the steering wheel, but no sound, Arg!!! (other words used, but not for print).
After thinking about it, I decided to go back to the relays. I remembered I didn't test the defroster when I had switched the relays to confirm the horn relay was good. I switched the relays again, and touched the horn wire to the steering wheel terminal and I got sound.
A quick trip to European Automotive Salvage Yard - Porsche (EASY) in Emeryville with relay and horn button in hand, and I was able to get replacements. The horn button was from a 944 and said Porsche instead of 928, but I could live with it (hybrid?). I installed both while there to verify that they worked, which they did.
Since then I've only had to use the horn once, but at least, they knew I was there. The entire process took several days, but actual work time was about an hour, and the cost was $25.
With cover off
With Light up
Copyright 1999, George A Suennen