A dead horn on an R107 is usually caused by one of four things: the fuse, the horn relay, the contact in the steering wheel or the horns themselves. Here is how to find which one without replacing parts at random.
How the horn circuit works
The R107 horns are behind the front grille or bumper area. They are powered through a fuse and a horn relay. When you press the horn pad or ring, the circuit to the relay is grounded through a contact in the steering wheel and steering column. The relay then sends full power to the horns.
Step 1: Fuse
Check the horn fuse in the engine bay fuse box, including the contact surfaces. Clean or replace it. See electrical troubleshooting.
Step 2: Relay
Have a helper press the horn while you listen or feel the relay. If it clicks but the horns are silent, the problem is after the relay: the wiring or the horns. If it does not click, the problem is the relay or the steering wheel side. Swapping in a known-good relay of the same type is a fast test. Relay location varies by year; check the diagram.
Step 3: Steering wheel contact
The horn contact works through a sliding contact or contact ring behind the steering wheel. Dirt, wear or a broken spring prevents the ground connection. Removing the horn pad and the steering wheel gives access. On cars with an airbag (later US cars), follow the airbag safety procedure in the factory manual, including disconnecting the battery and waiting before working on the wheel. If you are not comfortable with airbag systems, leave this step to a shop.
Step 4: Test the horns
- Disconnect the horn wires.
- Connect the horn directly to the battery with fused jumper wires.
- If the horn sounds, the horn is good. If it does not, it is faulty or corroded.
Horns can sometimes be revived by adjusting the small screw on the back, but corroded horns usually need replacement.
Replacement
Original-style horns are available from Mercedes and specialists. Fit a matched pair, since the R107 uses two horns with different tones. Clean the ground connections at the horn mounts when fitting them.
Quick answers
- Why does my horn sound weak? One of the pair has failed or a ground is poor.
- Why does my horn go off by itself? The steering wheel contact may be shorting to ground. Disconnect the horn until it is fixed.
- Can I fit louder aftermarket horns? Yes, using the existing relay.
- Is the horn the same on all R107s? Mounts and connectors vary by year.
