Horn wont blow

[Akgriffith]

Ok here's my story. The horn is good and will work when tested. When I take off the steering wheel and run a jumper wire to both copper tangs the horn blows. When I put steering wheel back on and push the horn button it blows the fuse. I cannot figure out why it works as should with the jumper wire but blows the fuse when the button is pushed. I have to imagine it's a ground issue but this has me stumped. The steering shaft spline is showing as a ground when I put my test light to it, is that normal? Any ideas? It's been like this since I bought the car and nothing has changes recently.

[spider2081]

I agree jumping the brass contact tabs should be no different than using the horn button.
My thoughts on this are: The wire diagram I have shows fuse 9 is the correct fuse for the Horn and the cars coolant fan. Is it fuse 9 that fails?
The horn button only activates the horn relay by supplying a ground to one side of the relay coil. Fuse 9 supplies power to the relays coil and the contacts. Anything done in the steering column should not be able to blow the fuse.
Most of the Spiders call for a minimum of 16 amp fuse for the horn/coolant fan fuse. Is yours at least 16 amps?
Could someone have modified the original horn wiring, possibly bypassing the original relay?

[18Fiatsandcounting]

Can you measure the voltage between ground (car body) and each of the copper tangs (with a good fuse)? It should be zero for both of them, and if it's not, something is funny with the wiring.

The horn relay has power to it all the time, but it only activates when its ground is connected, which is what you're doing when putting a jumper between the two tangs. One tang goes to ground, and one goes to the relay. If it's wired right.

-Bryan

[spider2081]

? It should be zero for both of them,

I think one should have battery voltage on it (around 12.6 vols) and the other should be zero. The one connected to the relay coil should measure what is called " open circuit voltage" in this case battery voltage. As mentioned the other side of the relay coil is connected to fuse 9 a "hot all the time fuse"' The other copper tang is the ground and measured with a volt meter should display zero as there is no difference of potential between ground and ground.

[18Fiatsandcounting]

? It should be zero for both of them,

I think one should have battery voltage on it (around 12.6 vols) and the other should be zero. The one connected to the relay coil should measure what is called " open circuit voltage" in this case battery voltage.

Yes, you are right, and I flubbed this one. One tang does indeed go to ground (or should), and the other goes to the "ground" side of the relay which as you point out will have battery voltage on it when no current is being drawn through the relay.

-Bryan

[Akgriffith]

Thanks for the insight guys! I'll grab my multi meter and check the tangs. As far as wiring goes, the entire car has been rewired by the PO. And he seems to have done a good job, even uses the correct colored wires. This is the only electrical gremlin I have. I suspect the relay may be worth looking at here. I'll report back with my findings.

[Akgriffith]

I had a chance to check voltage on both tangs. 1 is 0 and the other is 12.35. Seems right. It's got a 20 amp fuse. Does the wiring at the steering column need to be getting power from the relay? It appears to be getting power from the fuse block. Would that have something to do with this?
Thanks again!

[aj81spider]

I'm not sure if it has anything to do with your problem, but your switch is wired differently than a stock 1977 if the fuse goes directly to the horn button. The stock wiring has power going from fuse 9 to one side of the relay coil. The other side of the relay coil goes to one side of the horn button. The other side of the horn button goes to ground. When pressed the button basically closes the circuit on the coil which energizes the relay.

It occurs to me that if the wire goes directly from the fuse to the horn button then every time you press the button you are grounding the fuse. That would explain why the fuse blows. It would not explain why the horn works when you short the tangs together. I think there's some more wire tracing to be done.

[spider2081]

It appears to be getting power from the fuse block.

It might appear to be getting power from the fuse block when measuring voltage at the tang in the steering column. Measuring "open circuit" voltage, on the tang, connected to the relay's coil will give the appearance of being connected to the fuse. To test that appearance out remove the horn relay by unplugging it. With the relay removed the voltage should not be present. If there is no horn relay in the car, or the voltage is present with the relay removed from it's socket the cars wiring has been modified.

A 20 amp fuse should be ample for the original pair of horns. I think air horns might have used a 25 amp fuse.

[Akgriffith]

Got it fixed. The relay was wired incorrectly. One side of the relay was going to ground. Rewired it and now it works as it should. Thanks again for the help guys!

[spider2081]

The Bosch style cube relay that many auto manufacturers use is manufactured in two styles. One is called the "a" type and the other is called a "B" type. The difference between the 2 types is that pins 30 and 86 positions are swapped.
Installing the A type relay in a Fiat that is configured for the "B" type relay can cause this type of issue.

The physical characteristics for the 2 types are the same so the wrong type will easily pulg into either socket.