The horn is quite a complex electrical/mechanical system. It seems quite simple but when you start tracing the circuit you find that it gets weird going through the steering column. Here's my adventure in wiring up, troubleshooting and fixing the horn circuitry.
Basically, the horn beeps when +12V is applied. The case of the horn is grounded to the chassis through its mounting screw. Unfortunately, it is not as simple as it seems. A relay lies inbetween the steering wheel horn button and the horn. When you push on the horn button in the center of the steering wheel, you ground one side of a relay coil which energizes. This pulls in the NO contact which applies +12V to the horn circuit. It is done this way so that you don't have high currents flowing through the steering column. High currents would spark and pit the horn ring contacts - pretty soon your horn wouldn't work!
Typical horn wiring diagram.
The above horn wiring diagram show the schematic symbol for a relay. The coil is indicated by a coil of wire, the magnetic core is shown by the parallel lines. The magnetic field is shown by the dashed line on the Common armature and NO contacts.
Two fuses are required for the horn circuit, one 10A for the horn side and one 5A fuse which can be shared with other relay control devices. A relay coil only uses a few tenths of an amp. Both should be connected to +12V Hot which is a +12V source that is always live. The alternative to +12V Hot is to connect it to +12V Acc which is +12V that is only live when the ignition key is in the Acc or Run position.
Driver's side horn and wiring.
Here's the driver's side horn mounted in the space behind the grill between the radiator and the driver's side fender. I've routed the electrical wires to the passenger side horn in a plastic split wiring loom through the radiator cross brace. The main electrical wiring loom passes through the rad support through a rubber grommet to protect the loom. Rule of thumb, everytime a wire or wiring loom passes through a metal hole, place a rubber grommet in the hole to protect it!
Wiring loom in the engine compartment along driver's side wheel well
In the engine compartment I ran the wiring loom across the driver's side wheel well over the master cylinder and through the firewall - again through another grommet. I used special black tie wraps with screw mounting holes to mount it to the wheel well. I used black tie wraps as opposed to white because they don't show and make a cleaner install. I also use tie wraps at the point whenever a wire enters or exits the wiring loom.
Over the master cylinder and through the firewall
After entering the dash area, the horn wire is routed to the horn relay. The coil side of the horn relay, is routed to the steering column. I used a 1980s GM Van steering column as it didn't have a ignition key. It has a GM color coded standard connector where the black wire is the horn button. The black wire runs up the steering column to the 1st stop which is a spring loaded contact.
Hint: Before you remove your steering wheel, put the wheel in the straight ahead position. This way you'll know which position to install it later.
From the wiring harness connector the black wire goes to the first spring loaded contact.
This is a good time to take your continuity meter (ohm meter) and make sure that you have a connection between the wiring harness' black wire and the spring loaded contact. Also clean the contact with contact spray (pick it up at Radio Shack for $5) and you can use a pencil erasor to clean it too. While your at it, spray contact cleaner on the turn signal contacts. You can also grease the steering shaft bearing while you are working in this area. Just make sure that no grease gets on any electrical contacts because grease acts as an insulator.
This ring sits on top of the steering shaft. The bottom of the ring makes electrical contact with the spring loaded contact.
The next piece in the puzzle is a plastic insulated ring which makes a constant electrical connection with the spring loaded contact as the steering wheel turns. This is a good time to clean the ring's contact ring. There is a second spring loaded contact which is part of the contact ring. Now's a good time to clean it too!
The contact ring taken apart to clean the second spring loaded contact.
I got into this mess because the second spring loaded contact was intermiddent and the horn wouldn't work. It ends up that it is very easy to take apart. There are 4 bent tabs that are easily straightened and it comes apart. Clean all 4 pieces including the spring. I stretched the spring and cleaned the ends with a pencil erasor. Put it back together, double checked for continuity and installed it.
Steering wheel installed in the right position and the second spring loaded contact circled
The next piece is an plastic insulated flat spring steel ring. It consists of two parts: a white plastic insulator ring and a spring steel disk. The insulator keeps the spring steel disk from making contact with the center of the steering wheel. The second spring loaded contact makes contact with it. The spring steel disk is the part that the horn button presses on when beeping the horn.
Spring steel disk with insulator
Place the insulator side down onto the steering wheel.
Spring steel disk in place.
At this point, you should have good continuity from the wiring harness connector's black wire to the spring steel disk but you should not have any continuity with chassis ground. The steering shaft and center of the steering wheel should have continuity to ground.
Horn button mount
The last piece to go on before the horn button is the horn button mount. It has 3 insulated mounting screws. It makes direct electrical contact with the spring steel disk but is insulated from the steering wheel which is grounded.
Horn button mount installed - knotch to the top
Some things to check at this point. The knotch of the horn button mount should be at the top. This is to align your horn button if it has an emblem on it. Notice that the steering wheel nut is in place and a safety snap ring is present to stop the steering wheel nut from loosening completely. Now you can test for continuity from the black wire at the wiring harness connector to ground by pressing the horn button mount. Or you can press it to test to see if the horn works. Snap the horn button on and it's done.
The steering column that I used is a 1980s Chevy van, the steering wheel is from a 1976 Camaro. All the parts from the Camaro worked perfectly fine with the van's steering column. I had to use the Camaro's contact ring as the second spring loaded contact from the van was too short. So if you plan on mating up steering wheels to other model steering columns, keep all the pieces for the horn mechanism from both - you may have to mix an match to get it to work.
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