Repairing the horns in a Porsche 964

Contributed by: Mark Schettenhelm

The Porsche 964 contains 2 horns, a high and low tone (and one more for the alarm).The manufacturer was Mixo (now Valeo) and the model name appears to be Stritone. They are located in the right front fender above the oil cooler.When the horns fail they can often be easily repaired.

The are three main steps to follow:

  1. Are the horns receiving power?
  2. Is the horn out of adjustment?
  3. Is there a continuity problem within the horn?

I will cover these in order, starting with the easiest and quickest items first.

These tools may be needed:

Jack

Jack stand

Torque wrench and socket to remove wheel

Phillips screwdriver

Sockets 6, 7, 10 mm

Socket wrench with short extension

Socket screwdriver-like handle (makes it easier to use with the sockets than a socket wrench)

Multimeter/Ohmmeter or at least a way to check continuity

Fine sandpaper

Contact cleaner spray (Not necessary, but you may want to use it if you have it)

Soldering Iron

Rosin core Solder

1. Are the horns receiving power?

The first thing to check is the fuse for the horn that is located with the other fuses in the luggage compartment.If the fuse looks good then you will have to get to the horns and see if they are receiving power.

  1. Raise right front of car on a jack stand. (Use normal precautions)
  2. Remove wheel
  3. Remove Phillips screws holding front fender liner in place
  4. Horns should be exposed as shown in picture #1
  5. Remove leads attached to the horns and connect to your multimeter to see if you get 12v power when someone presses the horn button
  6. If you don’t get power it could be the fuse, the horn buttons, the horn relay or a short in the wire. I will not cover diagnosing those problems here because that is not the problem I had, so I don’t have experience in that area.
  7. If you have power going to both sets of wires then you should proceed to “Is the horn out of adjustment?”

Picture #1 Showing horns under right front fender.

Is the horn out of adjustment?

At this point it will probably be easier to work if you remove the horns.Leave the wires off and use your 10mm socket to remove the bolts holding them in place.You can remember how they attached, but it will be easy to figure out when you have to put them back.

  1. Get out your multimeter and check for continuity between the terminals. There is no polarity to the terminals so you don’t have to worry about positive or negative. If your multimeter has a continuity beep it helps to make this easier so you don’t have to watch the readout.
  2. If you get continuity and some resistance (approx 2-3 ohm, but the amount of ohms is really not too important, continuity is) then you are close to getting it to work. It is probably out of adjustment and you will need to adjust it until you get to where the horn will sound. You will need to turn the 7mm bolt on the back of the case counterclockwise until you get no continuity, then turn a quarter turn clock wise.You should get the continuity and resistance back.Reconnect the horn and test (You can hook up some wires to a 12v source, like your battery, be careful.Horns draw about 8 amps so you need a powerful battery to get them to work, household batteries won’t do it); it should sound.You can continue to adjust the bolt to get the best (loudest) sound.

3.      If you have no continuity you can try this adjustment, but it will most likely not work – continue to turn the bolt clockwise until you get continuity, then turn it back a quarter.If this fails proceed to the next step “Is there a continuity problem within the horn?”

Is there a continuity problem within the horn?

It appears that you have a broken circuit in your horn.But there is one more thing you can try before opening up the case.You could try to clean up the contacts on the outside.If you look at picture #2 you can see that the spade terminals and nuts are prone to rust.You can see here some of the rust that was present on one of the terminals.I used a 6mm socket to remove the nut and pulled out the terminal.I used some sandpaper on both terminals and their nuts to clean them up.On this terminal the corrosion was a problem, but there was also a problem on the inside too. 

Picture #2 the contacts on the outside with one terminal removed.

How a horn works

If that doesn’t do it, then you will have to open up your horn.At this point it would help to understand how a horn works.Power comes into the horn at one terminal and travels to a pair of contacts similar to those found in old distributors.The electricity goes through the contact points and then to one coil and on to another coil.The power then goes out the other contact.These coils act as an electromagnet that pulls the diaphragm down.The diaphragm is pulled down until the point that extends from it pushes down on one of the contacts, which disrupts the circuit.When the circuit is disrupted the power to the electromagnets stops and the diaphragm moves back up – moving air.It will reach the place where the point will release the contact that will re-energize the circuit, pulling the diaphragm back down again.This will happen rapidly and the air movement will travel out the different sized horns to produce the different tones.The adjustment bolt adjusts the contact gap, which you want to optimize to produce the most volume.

Repairing the continuity problem

  1. Remove the 4 screws on the back of the horn. When it comes apart you will see that there are paper gaskets between the back and the diaphragm and between the diaphragm and the horn. Be careful with the paper gaskets, and set them aside.
  2. Now that it is apart you should start by doing a continuity test between outside spade terminals and inside posts. The inside posts are shown in photo #3. I have circled the posts in red. Connect one lead to the outside terminal and its matching post. If no continuity, then go back to the outside of the case, remove with a 6mm socket the nut and get the spade terminal out to clean up with some sandpaper.

  1. Next checking for continuity between the two posts. This will probably show that you have a break in the circuit. This is commonly caused by a problem with the contact points. You can try to get some sandpaper between them and clean them, but I found just taking them out was easier than it may appear. If you do attempt to clean the points, turn the adjustment bolt so that it is just threaded and the top contact is all the way up. Then reach down between the points with some sandpaper and clean them while they are spread. This isn’t easy. Besides you will probably end up going to the main circuit board anyway (shown in picture #3). If you try to clean the points check the continuity again. (Make sure the points are touching.) If you have continuity you can reassemble and skip to step 12.

Picture #4 showing inside of horn

  1. To remove the contacts take a 7mm socket (I put it on a screwdriver handle to make it easier to use), and remove the adjusting nut on the back of the case. You will not have to take it all the way out, just unthread it from the plate. The other end of this bolt is visible on the right in picture #4.
  2. Then from the inside of the case remove the other bolt that holds the contact points in place. It is shown in picture #4 on the left with a red circle around it. When you remove the bolt the various pieces will come apart. Put them all aside for now.
  3. You will now see the circuit board where the problem probably lies. Looking at picture #5 you will see the four metal areas that form the path from one post to the other. The electricity flows from one terminal into a coil, through the other coil and then through the contact points. The problem is probably with the wires that come from the coils. If you look closely you can see where the copper wire is soldered to the circuit board. This is shown in picture #5 with the red circles indicating the connections. With the vibrations they can shake loose and the connection is lost. Testing from plate to plate you should be able to find at least one plate that does not connect with the others. You should examine the connections to see if there is a loose wire. Resolder the bad connections and test again. There should be continuity on both ends of each coil wire, not just the wire itself, but with the plate it is soldered to. In picture #5 you should have continuity between the upper two contacts and also between the lower two contacts. If everything is right you should be able to go from the post shown in the top left all the way to the plate that has the adjustment bolt running through it. You should be able to connect everything but the final post on the lower right. In each of my horns I found loose wires. The only other failures could be in the contact points or the coils, but failure in the coils would be very unusual. To fix that would require further disassembly and probably wouldn’t be worth the time.

Picture #5 showing coil connections

  1. Once you have completed the circuit it is time to clean and reassemble the contact points.
  2. Before you reassemble them you should take a minute to clean the points – often this is the problem. Brushing some sandpaper on the points should clean them enough to insure conductivity. As I said before you might be able to slip some sandpaper through them if it is assembled, but this seems easier.
  3. You will need to get all the parts in order. For this look at picture #6. There is the shorter bottom contact point that serves as a spring. Then the insulator, then the upper contact point. This is where you then place the small insulated washer in the large hole of the upper contact point. Lastly the larger insulated washer, the metal washer and the bolt. This is where it gets tricky. You need to keep this all together and bolt it in. It is important that the upper contact is electrically isolated from the lower contact. They should only touch at the points.

Picture #6 showing disassembled contact points

  1. Make sure that the adjusting bolt is sticking up with the spring on it. (The spring is not shown in picture #6. It is necessary to provide a current path to the upper contact.) Then carefully put the contact assembly with the bolt through it in place and carefully thread the bolt a few turns. Then you can switch to the adjusting bolt and thread it until it makes contact. From there you can finish tightening the fastening bolt. Alternatively you can start with the adjusting bolt and once it is threaded you can switch to the other end. It may take a few tries but this is doable. (Something that occurred to me after was that I could tie some string or a rubber band around everything to hold it in place then cut it when finished.) Once it is in place you will want to make sure that everything is in line – you have to have the points touch. Loosen the fastening bolt a little and maybe use a thin blade screwdriver to make sure the bottom smaller contact point is in line, then retighten.
  2. You have gone through the entire circuit so it should work now. You should have continuity between the terminals. If you don’t then you will have to recheck the contact points, and retrace the circuit.
  3. To reassemble the horn place one of the screws through the back of the horn. Place a paper gasket on the screw to position it. Then the diaphragm, making sure that the rectangle plate runs from one coil to the other. If it is hard to lay the diaphragm flat you may have to turn the adjusting bolt clockwise to relieve some of the spring tension. After the diaphragm, attach the other gasket then the horn. I placed the horn to the opposite side of the fastening bracket to make it easier to install. I forgot to look how they were originally; you may want to check before disassembly. Put the first screw in and then the others.
  4. Once it is reassembled you should check the continuity and readjust as mentioned above. Get to the point where you get the resistance, then turn back about one quarter turn. You should be ready to hook it up to a 12v power source (car battery) and test. It should sound. If it doesn’t you will need to turn the adjustment screw a little to get it to sound. At this point, if your ears and neighbors can stand it you can adjust for maximum volume. Turn the adjustment bolt until it is the loudest.

When satisfied, remount the horns.First connect the wires, and then remount the horns.Do the inside horn first; the horn bracket should slide on top of the mounting bracket until the bolt lines up.After it is bolted in place put the outside horn in place. Two things I noticed that may affect you as well.One is that the alarm horn (US car, factory alarm) had fallen down when the bolt fell out.When I took off the fender liner the bolt fell out and I could see the horn hanging from the wires.I refastened the horn using a split washer to lock the bolt in place.I also noticed that the bracket for the horns was a little loose from the body.The two nuts where it fastens to the inside fender had loosened.I tightened them.The bracket and alarm horn are shown in picture #7.This is a good time to look for leaks around the oil cooler, rust, etc.

Picture #7 showing horn bracket and alarm horn.

Once everything is in place it is time for a test and then reattach the fender liner, the wheel and bring the car down.

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