Clean the ground connections including the bus inside the fairing.
Its at the lower right looking in through the headlamp hole and has all black wires with spade connectors.
Also , as its a 91 its probably high time to clean the bar switches.
The easiest way is to disconnect the bat. then spray a cleaner of your choise into the switch while cycling the switch ., do this until what comes out is clear then blow out with compressed air or canned air or even a straw and lung power .
Bar switches are very stout but do get dirty as the years go by., kind of like us guys.

 

Next time you have to "clean it up", once it is re-connected, spray the connection with an ignition sealer or clear paint. Either can be found at your local auto parts store. Cheap, Quick, PERMANENT FIX!

 

I'm having this same issue it seems. Any resolution OP?

Mine only works at high RPMs. I've hard mounted the horn directly to bare frame to get rid of any ground issues. The hot wire to the horn has 12V with the horn switch pushed and no horn connected. With the horn connected, it gives a short single toot and that's it through the switch at idle or engine off. Directly wired to the battery it will honk all day long. Once that single toot is finished, there are not enough volts to the horn. It drops to .2V and stays there.

It seems like the 18awg wire to the horn is too small to carry the load through the circuit? Can the switch be causing this issue? Is there an Ohms check on the horn to see if maybe the horn might be bad, even though it works? I'm just grasping at straws and dreading rewiring the circuit.

 

A component that seems to work when connected directly to power source, and a good voltage reading when the component is not in the circuit...

The problem is to properly check the system, you must test it with a load on it.

What happens is when you have excessive resistance in a circuit, it only affects the circuit when the component tries to draw amperage. When you remove the component and check the wires with a voltmeter, there is no amperage draw, so the voltage reading is normal.

The way to find the problem is by doing voltage drop tests.

To check the ground circuit, touch the voltmeter (+) probe to the body of the starter, the (-) probe to the negative battery post.

If the ground circuit is good, the voltage reading will be zero.

If you read any voltage during this test, it means the ground circuit has excessive resistance, correct this before moving on to the hot side circuit test.

To check the hot side of the circuit, touch the (+) probe to the terminal on the horn (don`t touch the probe to the wire or the terminal on the wire).

(-) probe to negative battery post.

Press start button, you should read system voltage.

Less than system voltage means excessive resistance somewhere in the circuit, check all connections. If nothing is obvious, you will need to get to the connector for the right side control switches.

You can do a voltage drop across the connectors and also do a voltage drop test on the switch.

 

I had the wimpy horn problem.Solved it by using a automotive relay.Tuck the relay and fuse under the seat in a cavity at the battery-Softail in my case.Find the relay wiring diagram on the internet.Your problem will be solved.

 

You don`t know that a relay will solve a problem...If there is a problem in the switch or the switch wiring, adding a relay and more wiring won`t do a thing.

Before you start adding parts and wiring, you must have a system that is operating correctly.

And if you are getting 12v at the horn terminal (while the horn button is pressed), you are already getting all the power the system can provide.

 

I tested my Ohms across everything.

Negative battery to frame got 0.5
All horn switch wires got 0.5 in and out of the fuses

The unusual thing was the Ohms at the horn. Horn ground to horn power lead got 2k+ Ohms. Is this normal? I checked my truck horn and it gets the standard 0.5 to 1 Ohms just like the rest of the circuits on the bike. It appears the horn is faulty, correct?

 

Try connecting the + probe to the horns ground wire and connect the - probe to the frame or - side of the battery. This will tell you the resistance in the ground circuit for the horn. Anymore than 1 ohm ya need to repair that circuit. Either by cleaning connectors or replacing all or part of the wire.

Let us know what reading ya get

WP

 

No, the horn works when it gets 12 volts, you already tested that.

The problem is that the horn is not getting 12 volts when it is connected to the horn circuit.

Just do the voltage drop tests as I outlined in post 14 and you will know where the problem is.

Ohmmeters are typically unreliable for this type of thing, they are really for small circuits in electronic devices.

If you have excessive resistance in the circuit, the voltage drop test will show it.

 

I'll throw a redneck method out here to see if it helps ya. One wire going to the horn should be a + (voltage) coming from the switch, the other should be a - ground going to the frame.

Get ya about 2-3 ft of wire, at least 18ga 14 or 16 ga would be better. take the - grnd wire from the bike and connect it to the horn - terminal. Take the piece of wire and connect it to the horn+ terminal. Then take the other end of the piece of wire and touch it to the + side of the bikes battery. If it blows as it did with it connected to the truck battery ( I think that was what you mentioned) Your ground wiring is OK. If it doesn't blow well then you need to trace the ground wiring and look for a bad connection or bad wire.

If the horn blew good then you need to ck the + (positive) wire(circuit) coming from the switch.
I'll bet ya have ground problem from what I have read here
Good luck WP