E89 DIY installing Hella Supertones on Z4


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Staff member
Aug 11, 2017
E89 Z4 35i, F10 535d
DIY: Installing Hella Supertones
The first time I hit the horn on my Z4, I was nearly embarrassed to be the guy behind the wheel of a car that made a tricycle like squawk horn sound. A horn fix was on my radar. When I had to debug my ambient temperature sender, the horns were close by and it was time to do an upgrade. I thought I'd post some photos and information, as this is an inexpensive upgrade and I'm pleased with the results.

First off let's cover what to purchase - about $50 shipped gets everything you need:
Hella Supertones. Got these of Amazon Prime for $33 shipped.
iJDMToy Wiring Harness. One can fabricate a harness, but $13 on Amazon Prime shipped, there was no point.

Photos are at the end of the post.

So first off, there are a pair of horns mounted in front of the driver front tire. To access this area, you pull the front driver fender liner and undertray (the fender liner is front-half). There is a small access plate to change corner headlight bulbs, at the top of the inner fender liner. A black plastic twist plate pops it off with your hands. The rest is simply 8mm black screws. When you've removed them all the liner falls right out. Note the temperature sender is attached to the liner - simply pop that off.

You'll see the two factory horns, mounted on aluminum tabs perpendicular to one another. Simply unplug them and unbolt them. The plugs disconnect by simply pressing the tab at the rear of the connector and pulling.

The factory horns are wimpy and pull very little amperage. They are wired straight to the horn switch (the steering wheel horn button). The Hellas are another matter in the altogether. They use a relay to supply significantly greater amperage. You will use one of the two horn connectors to switch the relay. The other factory horn connection is just left unplugged (and no longer needed).

To mount the horns, you'll need to ream the two factory mount points slightly - I used reamers (see below photo), but you can use progressively larger drill bits. Be careful to slightly enlarge, bit by bit, as you don't want to twist up the thin aluminum mounts. Once large enough, you may wish to bend the forward horn every-so-slightly forward so it does not touch the black housing to its rear.

Using the iJDMToy wiring harness, here's how I made the connections:
Relay: Mounted on the metal inner fender panel. I used a 3M wall hanger with double stick 3M tape.
Power Wire (thick red): I extended this wire 6", and placed a smaller loop connector at the end. I then ran this back to the charging post on the firewall. You'll notice there is a small hole at the bottom of the charging post. You can use a M4 nut and bolt, or a body clip and screw.
Ground wire (thick black): I ran this back to the shock tower, where a harness mounting screw gave a convenient body ground (note photo below where I point to the location).
Wiring pairs for Hella horns: These simply plug into the horns. Note the iJDMToy harness is made to mount a WRX setup with two horns across the nose of the car. I did not shorten the long harness - I just simply bunched the spare wire up and tied it off with a cable tie.
Horn switch connector pair: Ok this is slightly tricky. The remaining wiring harness line has a connector pair has two round tabs. You will notice they are slightly different (a male and a female). These need to plug in to one of the factory horn harnesses, but you'll note those connections don't match! I don't believe in cutting factory wiring harnesses so here's the solution. Take one of the stock horns and look at the connector - you'll notice the black connector body is clipped on. Lift the clips gently with a screwdriver, and slowly pry the connector body apart. It will come apart with a little work. Once off, you'll note the back of the black connector will take the male round connector pairs from the iJDMToy harness just perfectly. Since the second iJDMToy connector is a female (slightly wider), it won't fit. I took some needle nose pliers and folded the round female connector inward, making its outer diameter small enough that it could be a male fitting to the connector. Though it likely doesn't matter, brown on BMW is almost always ground. I ran ground to the black iJDMToy wire. For added security, I flowed a bit of solder into the two male connections. I then had some shrink wrap covering up the connection. At this point, you can plug this into either of the factory horn connectors. Leave the other horn connector unplugged.

This is a pretty straightforward installation. I've included a quick video where you can hear the horns (plenty of Youtube examples, as well).


Location of factory horns:

Mount relay on inner fender liner:

3M mounting tab for relay:

Power wire, extended:

Horn connector removed, and to be used to hook up the iJDMToy harness to one of the factory BMW horn connectors:

Ream horn mounting holes to fit Hella horns:

I used a mounting tab for sheet metal screw. Alternately one can use an M4 machine nut and bolt:

Power wire attached to positive charging post:

Charging post cover still fits nicely after horn power wire installation.

Ground wire mounted to shock tower wiring loom mount:

Horns mounted:

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Apr 5, 2017
07 335i 6MT e90
Nice write up! I am about to get a set of supertones as one of the horns on my car doesn't work anymore and with only one frequency, the horn sounds pretty lame. I'm debating mounting them behind the kidney grills since that would provide more open sound transmission, but also visibility. I could always paint them black to tone down the look, it's not a subaru after all. Thanks for the link to the harness - can't beat that price.
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