A friend of mine has a 1981 chevy truck that he bought new. It hasn’t been driven for quite a while. We have been working on it for about a year now, and it is close to being done. We tested the horns so I could sand blast them and paint them. One worked OK, and one did not. The one that did not work had the 12V spade broken off. I told him that I would take a stab at fixing it, so I took it home and blew it apart just for fun. It was very interesting. They are very basic, and I assume all horns are pretty much alike. So take a look at this, and try to do your own.

1. Pictures 1, 2 and 3 show a complete horn before disassembly. Please note that this is the sister horn. I had to include pictures of this horn because I did not think to take any ‘before’ pictures of the inoperable one. With the exception of the 12V spade (circled in blue) that is attached in this picture, this looks just like the inoperable horn. Note that in picture 2 you can see that the center part in the middle is black plastic and the top and bottom sides are metal. In the very center of picture 3 (circled in red) is a small screw that needs to be removed.


Picture # 1 Picture # 2 Picture #3

2. Picture 4 shows the inoperable horn after I ground off the rivet heads with a 3” pneumatic grinder and removed the screw from picture 3.


Picture #4

3. Picture #5 shows the horn after I lifted the flat sheet metal plate seen in picture 4 off. Now use a punch and tap out the rivets. If they do not come all the way out easily, get them part way out and then pull them out by grabbing the head on the other side and pulling with a twisting motion.


Picture #5

4. Picture 6 shows the same side as 5 with the rivets out. Now flip the horn over and you will see picture 7.


Picture #6

5. Picture 7 shows the other side of the horn with the rivets out (blue circles). The small bolt head circled in green is designed to change the tone of the horn. Turning it in or out puts more or less pressure on a set of points (like in an old distributor) as seen circled in blue in picture 11. This alters how long the points stay open, which alters the rapidity of the vibrations mentioned in steps 6 and 7, which alters the sound’s frequency/tone. The post where 12V goes into the horn is circled in red. Note that as mentioned in step 1, the power-in spade is missing.


Picture #7

6. When you take the metal part on top in picture 7 off of the black plastic part and turn it over, you have picture 8. Ignore the plastic part on the left side of picture for now. Look at the metal part on the right. The almost round ‘cap’ in the center (circled in red) is made of thin metal. It is that part that vibrates rapidly up and down when the electromagnet (described in steps 7 and 10) is activated. That vibration makes the horn’s noise. If you press down in the center (circled in blue) the cap will pop up and down about a 1/4” making a popping noise. After you lift off this vibrating ‘cap’, you will see picture 9.


Picture #8

7. Picture 9 has a lot of things to look at. On the right, the screwdriver is pointing to a loose wire (circled in red) and a rivet head (circled in blue). The rivet head that is circled in blue is the back side of the rivet that attaches the power-in spade circled in blue in picture 1 and which is missing and circled in red in picture 7. The power comes in through the rivet head that is circled in blue and flows through the wire circled in red (which should be attached to the rivet head circled in blue) and flows through that loose wire through windings in an electromagnet in the center (under the plate circled in green) and is then attached to bottom side of the points under the plate circled in yellow in picture 9 and as circled in blue in picture 11. When the points are closed, power can flow through to the ground and the electromagnet pulls down on the thin vibrating ‘cap’ circled in red in picture 8 and mentioned in step 6. That movement also pulls down on the bottom plate of the points, thereby opening the points and cutting off the power flow. As soon as that happens, the magnetic field dissipates, and the bottom plate springs back up to close the points. This happens time after time, probably hundreds of times a second. The rapid vibrating of that thin ‘cap’ makes the horn’s noise. This horn had two problems. First, the wire had separated from the rivet head, so power was not flowing through the electromagnet. Second, the points were badly corroded such that even when they were closed, current was not flowing through them and, hence, through the electromagnet.


Picture #9

8. In picture 10, I have drilled out the rivet, and the wire is still loose. Be careful not to damage the plastic insulation sleeve that keeps the hot rivet from grounding out against the metal housing. We will fix the loose wire in a few minutes.


Picture #10

9. Pictures 11, 12 and 13 detail cleaning and testing the points. The points can be seen in picture 11. Unfortunately, the picture does not show the green corrosion. I tested for current flow through the points in picture 12, and there was none. So I cut a strip of 400 grit wet/dry sandpaper and folded it over so I could sand both sides of the points at the same time (picture 13). After a few seconds, the points were clean and the power was flowing. I had to push down on the spring loaded bottom plate of the points to open them up a bit for sandpaper insertion.


Picture #11 Picture #12 Picture #13

10. In picture 14, I have replaced the rivet with a 6x32 machine bolt. The bolt passes down through a replacement power-in spade on the top (circled in red in picture 16), through the plastic shield that keeps the rivet/bolt from grounding out against the metal housing, through two washers with the loose wire sandwiched in between, through a split lock washer and finally into a 6x32 nut. Once the nut is tightened, the wire is tightly squeezed between the washers and there is now a good connection between the new spade and the wire. Also please note that I scraped the decaying gasket off and carefully blasted the housing in my blasting cabinet. I covered the center parts with tape for extra protection. This is a good time to explain something important that I unfortunately did not take a picture of. Look at the thin metal ‘cap’ circled in red in picture 8. As mentioned in steps 6 and 7 this thin piece of metal vibrates up and down due to pulsing of the electromagnet. Picture 14.5 shows the bottom side. On the bottom side there is a metal post that extends below the thin metal cap by about 3/4”. It is an extension of the round metal post circled in blue in picture 8. The metal post hangs below the ‘cap’ and fits down into the hole circled in green in pictures 9 and 14. There was about a 1/16 - 1/8” gap all around between the metal post attached to the ‘cap’ and the hole circled in green in pictures 9 and 14. The magnet in the bottom of the hole circled in green in pictures 9 and 14 pulls down on the metal post and then releases it to cause the cap to vibrate. Now that the points are clean, it is important to test the continuity between the power-in spade and the metal housing. With the points closed, your multimeter should show continuity. If not, there could be a break in the windings in the electromagnet, but that is very unlikely.


Picture #14 Picture #14.5

11. Then I made a new gasket (picture 15), and placed it on the metal housing in picture 14. Place a few dabs of silicone or something to hold it fast to the housing while completing the next step. Then I placed the thin metal cap carefully on top of the gasket/housing and moved it side to side to side to center the post in the hole circled in green in pictures 9 and 14. If the post touches the side of the hole, the horn will not work. You will now have a part that will look like the part on the right-hand side of picture 8. Then, CAREFULLY place the black plastic piece (left hand part in picture 8) on top of this assembly. If you move things around too much, the metal post can touch the side of the hole. Then carefully install the flat metal piece as seen in pictures 3 and 4. I purchased 8x32 machine bolts 1-1/4” long to replace the rivets to reassemble the horn. Carefully insert the bolts and carefully snug them down. Then test the horn by touching +12 volts to the power-in spade and grounding the mounting tab. If you did everything properly, it should work. If you did everything properly and it does not work, the metal post mentioned a few sentences above is likely hitting the side of the hole. Back up a few steps until you can slide the ‘cap’ around again and be sure the post isn’t touching and then assemble it again. You may want to just insert two bolts opposite each other and snug them down at this point and then test it. Once it is working, and it will if you checked everything, insert and snug down all the bolts and test it again. It SHOULD still work fine. Then I took the nuts off the bolts one at a time and reinstalled them with blue Loctite. Then I replaced the screw circled in red in picture 3. Then I used a three-inch cutoff wheel to cut/grind off the excess threads sticking above the nuts. Then I painted it with Krylon Satin Black (pictures 17 and 18). Again, you could easily use rivets if you wanted that perfect appearance.


Picture #15 Picture #16


Picture #17 Picture # 18

I hope this was helpful. It should work for you.