Okay, so I got sick of not having a horn and decided to do something about it on my day off.

I have a 1990 740 GL that I bought in 1999. It now has over 300,000 miles on it and still drives pretty darned well. I could use some suspension bushings and such and springs, but that is a story for another day.

I have read here several times about the problems with these horn buttons, but I have not ever read a really clear description. But everyone's ideas for removal and such were all good. So I decided to look for a temporary fix to see if the horn actually still works. In 1999 the right button did not work at all and the left one required nearly painful squeezing to get anything. I always dreaded my inspection sticker experiences, having to convince the mechanic that I could indeed honk my horn even if he had some trouble with it.

So today I pulled on the left one and it popped right out. I disconnected the wires. NOTE that I stupidly did not think to disconnect the negative battery terminal first. Also note that my horn does not seem to get any juice while the car is off - none at all. So this was not a problem, thankfully. However, had I touched those connectors with bare hands with the car running… well, you can see how lucky I was. On my car you can touch the wires together barehanded so long as the engine is OFF and the key removed.

The buttons are held in place with an upper and lower snap-in retainer that can be easily pried out of the wheel with care. I found that an up and down (front to rear) motion works a lot better than a side-to-side one because of the orientation of the clips.

The button pulls away from the wheel easily, held down only by two wires with blade type connectors that slide free with a little work but no tools of any sort. (No detents, just a smooth blade for the connector to slide on and off of.) One wire comes off the side at an angle and one comes straight out of the back of the button and there is enough slack in the wires to do this without fear of pulling anything loose.

I got the left button inside and took it apart. It has four clasp points, two of which are like little ledges that the other parts nests beneath and two that are obviously snap-in locks. I opened up the button with a pen knife to the retainers on the snap-in side and the cover came off with zero problems.

It is essentially three parts that you need to be concerned with. first is the button itself, then the bottom half and then there is the copper plate. The copper plate is easy to remove. But the two retainer bits on the plastic base need to be figured out so that you can remove it without damage. I used a small round file in one of the two holes in this plate. It is curved and has a small contact point on the reverse that meets up with a larger contact point on the plastic base. The curvature makes the thin plate act as a spring while conducting current at the same time. And that spring that carries juice gets hot.

The button cover has a plastic protrusion that contacts the copper plate/spring and they eventually melt from this heat and slowly squish down until it only makes contact when you have the button depressed. When you use your horn the plastic heats up and deforms a tiny bit, eventually leaving a gap that allows your button to become loose and wobbly feeling. If your buttons are no longer working and they are showing this jigglyness then this is probably your problem. You need something inside this setup that takes up the slack and makes contact between the melted button nub and the copper plate/spring.

I chose something that will melt really fast, but it works perfectly right now. I need to replace it with something that will act as a very large heat sink that can dissipate the heat of the copper plate while not getting too hot itself. I have decided to fill the button cover with a brass piece that will fill the entire void of the button cover. Brass is able to absorb a lot of heat and such a part will be very inexpensive for me to create as I am a brass musical instrument repair technician. Filling the void of the button cover would spread the heat over the entire surface of the horn button rather than putting all on the tiny contact point that bears the entire force of our pressing. So a thin brass shim will probably do the job of filling the gap temporarily, and a thicker one will be the permanent answer.

And right now? Uh… I have a small section of a dress shirt "collar stay" that fits perfectly. But it is made of a rather soft plastic that will melt faster then the Volvo plastic. So the needed pair of brass pieces will get made in the next week or so, and the larger parts will probably just be a small stack of shims cut to fit into the plastic button cover. I will use .5mm sheet brass for this and it ought to take me about two hours of work to make everything.

By the way, when I discovered the problem and applied my temporary fix I was pleased to start up my 18 year old 740 and have not one, but two perfectly functioning horn buttons and a nice, loud, buzzy Euro horn!

Now to fix that pesky passenger front seat window button…