Horn keeps blowing fuse

[grannyknot]

When I hit the horn I can hear the relay click and a little chirp from the horns, if I keep the horn switch down there is no sound from the horns then it blows the 10amp fuse in about 4 seconds.

I have jumpered power from the battery to the aftermarket horns and they honk loudly so I know they work and are grounded. There is continuity between the green horn wire and the H terminal on the relay, as well as continuity between the S terminal on the relay and the horn contact switch. There is 12v at the B terminal on the relay, all is as it should be except this short that is happening.

Not sure how to proceed,

Thanks,

Chris

[Bruce Palmer]

Better get it fixed, 'cause living in Canada without working horns is sheer suicide...... Just a little hands across the border humor.

[Mikes Z car]

Only guess I could make is that the connector on the aftermarket horns is causing a short to ground right at the horns (maybe the connector plug on the aftermarket horns is a little different from the plug on the stock horns?). If that makes sense is there a way to jury rig a different connection between the horns and the wire that plugs into them? Or try unplugging the horns to see if the fuse still blows when the horn relay is held on for more than four seconds?

Edited June 21, 2013 by Mikes Z car

[grannyknot]

The wire coming out of the horn switch at about 1 o'clock is a 12v hot lead, it is normally attached to the screw at 3 o'clock, when I push the horn pad down that hot lead is grounded out on the steering housing which is connected to the steering shaft and that is the short, that is where the fuse blows. What am I missing here? Shouldn't the horn switch connect a hot lead to a neutral that becomes hot to complete the circuit?

[Dr. 240Z]

I'm not an electrician. I did look this up in my Nissan Service Manual (1972). It suggests connecting the horns to a 12V battery and adjusting each horn to read 3 Amps. That said, there needs to be a volume adjustment screw on your aftermarket horns.

Hope this helps.

George

[Mikes Z car]

That wire at one o'clock has 12 volts on it because it is connected to the coil of the horn relay and the other end of the coil of the horn relay is connected to 12 volts unless it is wired wrong somehow. Grounding that wire on the steering wheel throws the horn relay. The horn relay on my car (1/1970 240z) is right next to the driver's left knee, you can see it in the picture here. There are three wires on my relay I hope yours is like that. My theory based on what you have said is that there is a partial short to ground on the wire that feeds 12 V from the relay to the horns. That would explain why they barely work briefly when you hit the horn button I think. An idea here to follow up on that would be to unplug one wire at a time on the horn relay always having at least two wires connected. You are looking for a wire that can be unplugged and the relay will still click. Checking for relay clicking should not cause a blown fuse as you only need to hit the horn button briefly to see if the relay clicks. Two of the three wires when disconnected will stop the relay from clicking when you hit the horn button. The third wire from the relay that goes to the horns can be unplugged from the horn relay and the horn relay will still click. Leave that one unplugged for now. Remembering a recent incident on my car I just realized the bottom wire is not the one you want to leave unplugged for the test if your car is like mine. On my car unplugging that wire stops the clicking so it isn't the wire you are interested in. Once a wire is disconnected and the relay still clicks hold down the horn button until it would normally blow a fuse. If it doesn't blow a fuse that would indicate there is a short to ground on the wire that was disconnected, the one that runs from the horn relay switched 12 volt output to the horns. If no obvious place comes to mind where some work has been done lately that might have damaged a wire you could run a new wire I imagine, going from the contact on the horn relay where you unplugged the wire over to the horns, re-using the existing connectors on both ends. Someone else may have a better troubleshooting technique or might think of something else seeing these words, or just plain refute my whole idea.

Is it possible the wires on the relay were switched around?

Egads, what a long post.

Edited June 24, 2013 by Mikes Z car

[grannyknot]

Mike, thanks, I'll try that today when I get home.

[grannyknot]

Mike, BINGO, your theory was bang on. I ran a new wire out to the horns and they work beautifully.

Thanks so much.

Now I just have to get both of the brake lights working and the engine running and I can go for a drive again.

Chris

[Mikes Z car]

Terrific! Glad you got er fixed.