1989 mustang horn problem

Have you checked that the horn fuse is good? You can remove the wire to the horn and place a 12v test light in place of the horn to verify you are getting voltage to the horn when the button is pressed. If the light lights when the button is pressed you can make sure the horn has a good ground. If it still doesn't sound I would link you need to replace the horn itself. If the light doesn't light when the button is pressed, I would look close at the wiring to the horn and continue looking at other circuits like the clock spring or the connection under the steering wheel at the horn button,

suggest checking if 12 volts goes to horn wire at horn end when horn pressed from inside vehicle. A 12 Volt probe {google it} can be a cheap checking tool for this.
If 12v volts is switched when button is pressed {could be earth being switched if it is then place probe on constant 12 volts and negative lead on switched earth wire} then the horn itself is faulty.
If no voltage switched then horn relay needs to be checked. check if voltage at horn relay coil contact when button pressed.
if yes then relay may be faulty. if no then horn switch could be faulty.
you can check switched voltage also at horn relay switched contact when relay plugged in. To see if relay is doing what it is meant to do

I am not an expert on auto horns, but can solve most electrical problems. Do you have a multimeter? Using the ohm-meter, measure the ohm value between the two wires, and from each wire to the horn mounting bracket (you need contact with un-painted metal. The value between the wires should be low (10-100 ohms) but not zero. This represents the horn coil. The value between each wire and the bracket should be very high (10 kilohms or more). This means the horn coil is not grounded to the bracket.

Now, using the voltmeter, measure the voltage between each wire (from one wire to another). The value should be zero before pressing the horn button, and 12 vdc when the horn button is pressed. I would expect one of the wires will also be zero ohms (ohmmeter) when measured to the car frame or body.

Ok, you are ready to hook up the horn. The auto ground wire(s) are connected to a horn wire that is common with the horn bracket. Or, connected to a black horn wire if both horn wires are isolated from the bracket. Connect the auto switch wire (the one that reads 12 vdc when the horn button is pressed) to the remaining horn wire.

see the diagram attached.God bless you

You may have a short circuit in your horn button causing the
fuse to blow when you press the button or in the horn itself.

1. Locate the horn under the hood normally in the front of
the engine compartment.
2. Once you locate the horn, disconnect the wire from the
horn.
3. connect a VOM (Volt Ohm meter) to the horn's wire.
plus(+) side to the wire and negative (-) side to ground
or some metal part on the car.
4, set the VOM to dc voltage and have someone press the
horn button. You should read 12 volts on the VOM, if
not you have a short in the horn button, if you do mread 12 volts
you have a short in the horn. Replace the horn....

Need to check for either a 12V feed at one of the horn terminals with an assistant pressing the horn button. May be an earthed switch so if no 12V then switch your multimeter to resistance (ohms) and check same wire/s for earth when assistant presses button.

horn is usualy located behind the grill pull the wire off the horn using a
test light have somone push the button + check wire for power if theres
power try using a jumper wire to put 12v to the horn if no toot bad horn
I should have said check fuse 1st but do that if no pwr at horn wire
if the fuse is good and the horn works with the jumper most likly the
horn button is bad. hope this helps good luck

You can remove the horn relay and use a Ohm meter and check the resistance between the horn relay connector terminal No. 86 (Black Wire) and ground. If the ohms are less then 5 ohms replace the horn relay. There are a lot more tests but as Polarcycle states you can do what he suggested other then I believe you have two horns in this vehicle.

Diagram should be on the inside of the fuse box or relay cluster lid/cover. If Test light has 1 wire and a sharp tip? Yes, but only if you push the horn button too ;-)

My guess is that the horn lost its ground, but you can check this with the light. The horn only beeps when both GROUND and Pos 12v are present. The horn button gives the relay 12v to close, and the relay gives the horn 12v to operate. The ground is always connected.

Connect tester clipwire to 12v source (battery, headlight (has to be on) and touch its tip to metal of car. Does it light? Good. the tester is working and the 12v is a good source.

Touch one of the 2 horn connection or wires (poke it thru the plastic - why tip is sharp). Does it light? Touch the other, does it?

If both light, (1 will be brighter) we can assume the horn coil is good and it is getting a good ground and I was wrong (it happens... but not often). The side that is BRIGHTER is the ground wire side.

Now have someone mash the horn button. Do you hear a tiny click? That is the horn relay. If not, look for the horn relay fuse and replace it. If you DID hear the click, the relay coil is good and is trying to switch 12v to drive the horn.

Now is time to locate and replace the horn fuse or the relay may have bad contacts. Look at the other relay part numbers in the cluster and see if there is a match for the horn one. Swap it out and try the horn.

Hope this all helps. To 'test' the horn, touch the dim side again anf listen for a scratching sound. That is the 12v going thru the tiny bulb in the tester (a resistance) and trying to drive the horn.

Good Luck!

Usually a wire straight from the fuse box, check the fuse box for a blown fuse, if this is not the case, follow the positive wire leading to the horn back to where it hits the wiring harness, it should tie off into another system, such as the interior lighting, radio, or cigarette lighter