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So, with that backdrop, I cracked the key to ignition, put my hand on the horn relay, and reached my other hand inside to hit the horn button. (Note that on a modern car with radio volume, channel, and cruise control buttons on the steering wheel, the multiple contacts are handled via a “clock spring,” but on an old car with just the horn, it’s just a single ring contact and a single plunger.) In either case, it’s very common for the 50-year-old Bakelite or plastic housing that holds the sprung plunger to split, allowing the spring and contact to spill onto the carpet. On a 2002, the circular contact is at the top of the steering column and the plunger is on the back of the steering wheel, but on an E9 they’re reversed. If the manufacturer ran a wire directly from the horn button to the relay, it would eventually break from being twisted when the wheel is turned, so instead, there’s an arrangement with a circular horn contact and a little sprung plunger. The other side of the switch goes to relay terminal 85 through a Rube Goldberg-like path. One side of the switch is usually connected directly to the metal body of the steering wheel, which, since it’s bolted to the metal steering column, is well-grounded. There’s the horn button that you mash, which on modern cars is generally right in the center of the wheel, but on original 2002/E9 “bus” wheels is on the spokes.
The other thing you need to know is that, although I’ve simply labeled it above as “horn switch,” there are actually three separate pieces to it. The basic electrical underpinnings of the horn. With most relays, the on-off switch that triggers it is employed on the 12V side of the electromagnet, but on a horn, because the horn button is on the steering column and you wouldn’t want it to accidentally short to ground, it’s switched on the negative side. If 30 is fed 12V and 87 is connected to the positive terminal of your device, and if the negative terminal of the device is grounded, it turns on. Feed 12V to 86 and ground to 85 (or vice versa it doesn’t matter), and that energizes the electromagnet, which pulls the contacts together, connecting 30 to 87. 86 and 85 are the low-current terminals for the electromagnet. 87 and 30 are the high-current terminals for the thing you want to turn on. Nearly every relay used in a car for the last 40 years (and longer in German cars) has a set of standard “DIN” (it’s a German anagram) terminal numbers on it. I’ve written a lot over the years about relays, but in a nutshell, a relay is nothing more than a remote-controlled switch where the internal contacts of the switch are pulled closed by an electromagnet, letting you close a high-current switch with a low-current switch. If you don’t hear the relay click, then the problem is either in the horn button and its contacts, or the wiring between them and the relay.In this case, wiring the horn directly to the battery will tell you which of the two it is. If the horn doesn’t toot, but you hear the horn relay click when the button is pushed, then the relay, the button and its related contacts, and the wiring between them are working, thereby isolating the problem to the horn and the wiring between it and the relay.If the horn toots, obviously you have no problem.On a vintage car, the quick triage for the horn is to turn the key to the ignition position and hit the horn button. You can unplug these and substitute a pair of wires run to the battery, but it’s usually not necessary. However, because the horn is actuated by a relay, if you understand the basics of relay operation, you can often skip the wire-it-to-the-battery step. So whether it’s a horn or an electric fan or a bulb, my first instinct is to wire the thing straight to the battery to remove any question whatsoever about whether or not the component works. When any electrical component stops working, the first thing I want to know is if the component itself is at fault so I can get a replacement on order. Of course, it’s always possible that it’s the wiring, but that’s the last thing I’d check, not the first. But the problem can also be in the horn relay, or the button and its related contacts in the steering wheel. After all, it’s a vibrating metal diaphragm driven by a little electromagnet and it’s located up behind the grilles of the car, so after decades of weather, it’s not surprising that the diaphragm won’t move or the contacts are bad. Horn problems on vintage BMWs are fairly common. Last week I described patching the muffler on my ’73 3.0CSi, “Rene”, and preparing to drive it to the inspection station to get stickered, only to find that the horn didn’t work.
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