Mercedes Horn Troubleshooting
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Mercedes Horn Troubleshooting
I can't tell you how many times I use my horn in Atlanta traffic, its bound to happen. However when I drove the ole '81 300d, the horn did not work. The horn is not something you think about until you need it and can be a mystery to how to fix. Fear not! Follow along with me as I troubleshoot the horn and get it working!
Obviously you should check the fuse the very 1st thing and make sure you are checking the correct fuse. I did and the fuse looked fine. Mistake number 1. Never trust a fuse, trust your meter! You will see why later.
Make sure to keep in mind the horn circuit is only powered when the ignition is in on or run position. Not in the off position.
My fuse cover legend says the horn is fuse #6. I check it and it was fine. Come to find out someone had in the past put the wrong paper legend on the fuse cover. The MB manual shows the horn fuse as #14. So I was not even looking at the right fuse. I make these errors so you don't have too.
The horns are located under the front bumper, one on each side.
When you press the horn button your volt meter should read voltage. Mine did not.
Since I had assumed the fuse was fine you can see how to troubleshoot the system if your fuse is really ok. We will need to remove the horn cover and check the contacts for power and proper operation. To remove the horn cover just gently pull on the rubber cover around the edges and it should come off.
This is what it looks like after its off. Its really easy to do.
The horn rocker plate is held in by 4 screws with springs below them so the plate will spring back after you depress it. Remove the 4 screws.
On the back of the plate is the ground connection. Disconnect it.
Here is the plate removed. See the 4 springs, don't loose them.
Here is what it looks like without the horn plate. Two power contacts One of mine was broken on the right, so I used silicon to repair it.
Use a 10mm alan socket to remove the center steering wheel bolt. I had to use an air impact wrench to get it off.
After the steering wheel is off, test the contacts. You should get battery voltage with the ignition on or running. If you don't then lets keep checking....
We need to check the power coming in at the combination switch next so remove the rubber cover.
Just slide it over the combo switch and the cruise control stalk.
Check these 2 screws for power. If not then lets move on.
Remove the knee panel to expose the wiring.
Underside of the knee panel.
Pin 4 should have battery voltage. Mine did not. So whets up? I was thinking I had a wiring problem between the harness and the fuse block. Not a fun job to trace down. So I went to my trusty MB factory manual and looked at the wiring diagram. Oh no! The power for the horn was at fuse 14 NOT fuse 6!
Fuse 14 looked fine, but I removed it anyway and found corrosion on either side of the fuse. I cleaned this up with a small wire brush.
Back to the bench to test the fuse. On my meter the wires and probes themselves have .5 ohms of resistance. I always keep this figure in my mind when I test for resistance. A fuse should have almost no resistance.
Oh my look at this! This fuse looks to be fine to the naked eye. The metal is not burned, or broken and I thought it was fine, but meters don't lie. It is showing infinite resistance, just like if it were burned in two.
Old and new fuse.
Test new fuse and it shows continuity with almost no resistance.
I installed the new fuse and tested it to ground. Ahh battery voltage.
If you check the fuse like this you will read 0 volts whether the ignition is on or not. Remember a fuse just passes power like a wire and will read no voltage. Voltage is read across a difference in potential or a load. A fuse is not a load or power consumer so no voltage is registered.
I then tested the horn and beep beep it works fine now. Now just reinstall everything in the reverse order you removed it. The moral of this story is that I should have tested the fuse with my meter FIRST not last. Could have saved me some work, but I also would not have been able to show you the horn system and how to test it. So I guess it was all worth while.
When I am working on ground doing automotive work, my knees really take a beating. I got some of these knee pads from Sears and I am all smiles now. I wish I had gotten a pair of these years ago. No more knee soreness. These are the best knee pads Sears has and worth every penny! Get some.
Now get out there and drive around a beep your horn!