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
The are three main steps to follow:
- Are the horns
- Is the horn out of
- Is there a
continuity problem within the horn?
I will cover these in order, starting with the easiest and quickest items
These tools may be needed:
wrench and socket to remove wheel
6, 7, 10 mm
wrench with short extension
screwdriver-like handle (makes it easier to use with the sockets than a
Multimeter/Ohmmeter or at least a way to check continuity
cleaner spray (Not necessary, but you may want to use it if you have it)
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.
Raise right front of car on a jack stand. (Use normal precautions)
- Remove wheel
- Remove Phillips
screws holding front fender liner in place
- Horns should be
exposed as shown in picture #1
- Remove leads
attached to the horns and connect to your multimeter to see if you get 12v
power when someone presses the horn button
- If you dont 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 dont have experience in that area.
- If you have power
going to both sets of wires then you should proceed to Is the horn out of
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.
Get out your multimeter and check for continuity between the terminals.
There is no polarity to the terminals so you dont have to worry about
positive or negative. If your multimeter has a continuity beep it helps to
make this easier so you dont have to watch the readout.
- 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 wont do it); it should sound.You can continue to
adjust the bolt to get the best (loudest) sound.
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
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 doesnt 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
- 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.
- 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.
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 isnt 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.
- 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.
- 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.
- 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 wouldnt be worth the time.
Picture #5 showing coil connections
Once you have completed the circuit it is time to clean and reassemble the
- 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.
- 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
- 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.
- You have gone
through the entire circuit so it should work now. You should have
continuity between the terminals. If you dont then you will have to
recheck the contact points, and retrace the circuit.
- 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.
- 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 doesnt 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.