Restoring a Horn

On the 1971 FJ40 there are two horns, one mounted to each apron. There is a low tone horn and a high tone horn. The low pitched horn is on the drivers side, the high on the passengers side. There is a raised L or H cast into the bottom front of the top aluminum cover to tell you which is which. See Fig. 1.

On the previous page I got the horns working electrically but wanted to spruce them up a bit. Upon inspection I realized the bottom of the horn was a steel shell (rusted) and the top was aluminum painted black. I started by disassembling the horn.

Start by removing the 23 mm nut that holds the ground strap on. A 15/16 socket will work. The solenoid stud may turn out also. See Fig. 3. Next remove the 6 small 4mm phillips screws holding the horn halves together. Use your JIS screwdrivers for this or plan on replacing the screws! Separate the halves. See Fig. 4. Separate the sound plate from the upper horn half. See Fig. 5. You will notice there are paper gaskets between these parts. These gaskets are not available from any source so you will have to make your own. I provided a template below. Make sure to replace BOTH gaskets! They also act as electrical insulators for the diaphram!

Referring to Fig. 3, remove the screw holding the +12V connector insulator in place. Unsolder the wire from the solenoid coil to this connector. If you remove the tone adjuster (point gap) nut and screw make sure to note how many threads are showing so you can put it back the same way! Remove the screws holding the coil and armature in place. Watch how all the little small pieces come out... (Make sure to only do one horn at a time. You want to use the other as a guide to putting this one back together! ) See Fig. 6-7.

How does the Horn actually Work?

Since we have it all the way apart lets look at how it actually works. When you press the horn button you ground one side of the horn relay. The other side of the relay is connected to +12V through a fuse. The relay contacts are a simple switch. One side is connected to a fused +12V circuit and the other to the connector on the horn. When the relay contacts close current flows from the battery through the relay contacts to to the connector in the horn. From there it flows into the solenoid coil then to ground through the fixed upper and lower movable point contacts (Which are now closed) energizing it. When a solenoid is energized it creates an intense magnetic field in it's center. If you noticed when you took the horn apart the metal diaphram has a metal stud that goes into the center of the solenoid coil. This stud is now drawn into the coil further due to the magnetic field. As it moves further in the diaphram edge contacts the edge of the lower movable point contact eventually opening the points. The instant the points open, the current stops flowing through the solenoid coil. This causes the magnetic field to collaspe allowing the stud to move back up which closes the points again. The whole cycle repeats very fast. This movement of the diaphram at high speed creates a series of sound waves that are directed through a small opening in the aluminum top cover. This hole leads into a spiral expanding cavity that allows the sound wave to expand rapidly eventually exiting the horn.

At this point clean up the upper and lower shells and paint or powder coat them. I powder coated the steel half with gloss black and the upper aluminum half with Star Dust Silver. Use a 4mm x .075 tap to clean out the threads of all screw holes then reassemble the parts using your other horn or these pics as a guide. Before putting the points back together use a points file to clean them up.

Use the template in Fig. 11 to make 4 identical gaskets. Click on the image to enlarge it to full size. This is a full size uncompressed image! Select print from your browser. Make sure your printer is set to print 100% or 1 to 1. Print the image on the thickest paper your printer will handle. Cut out with scissors then cut out the center. Next make the holes with whatever you can. I used a set of gasket maker hole punches from Harbor Freight. Apply a thin coat of RTV to both sides, let dry for maybe 5 minutes or until no longer sticky then assemble. Use a bit of anti-sieze on everything.

Next up: Rear Heater Fun

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