750 SF Horn issue

F

fatmale

New member
Location
London UK
Hi All
I hope you doing well. I put this in General by mistake- moved it here as it's Technical...

I have an issue with the twin horns switch and relay on my 1972 750 SF. Not working in situ. Horns are fine if I bi-pass the switch and headlamp and go direct to the actual horns from battery with live wire.

The bike has a headlamp with the big push down/pull up plastic key.  The ignition switch is in a plate which holds the speedos. Its therefore a crossover bike between the one that had Smiths clocks ( I think ) and the next which had Nippon Denso clocks. Headlamp from the older bike.

The horn button is a small one in a chrome switch on handlebars. Wires x 4 come down from that into headlamp. Two go to headlight bulb and two go up into circuit under headlamp key.  (this key seems to only control headlights and tail lights). I thought the horn relay switch was buggered as rusty etc so replaced with four pin equivalent. No joy yet.

Should there be a live wire to the relay as well as wires from the switch via headlamp?
Does the relay work via handlebar switch and then allow full power to horns?
If yes where's the full power supposed to come from? There doesn't seem to be a wire to carry it.. Can I run a power wire from battery to relay switch? Currently horn handlebar switch button not activating relay switch. I fear the headlamp circuit has a problem maybe as wires go through there. If I bi pass that the relay switch activates but still no horns working.
I know this is a bit hard to advise on. Any pointers much appreciated.
Very best
Matt
 
I have spent a lot of time trying to get the horns working on my SF1. The two FIAM horns are working fine testing to a battery. The horns earth through the hanger brackets and there was no issue with a good earth to frame.
I tried two FIAM 3 pole relays and one Royal Enfield 3 pole relay, all to no success.
I eventually installed a standard 4 pole relay. I ran a 14 gauge wire with inline 30 amp fuse from battery positive to the 4 pole horn relay. Then power up from relay to horns. I used the horn button and the switched 12 volt wire to the relay switch connections. That way the stock Nipon Denso horn button (connects to earth circuit) operates the horns as intended. The bonus is the horns are powered direct by a fused heavy gauge feed and are really loud!

I hope this is of some help.

Cheers

Jim
 
Canuck750 said:
That way the stock Nipon Denso horn button (connects to earth circuit) operates the horns as intended. The bonus is the horns are powered direct by a fused heavy gauge feed and are really loud!
Click to expand...
Direct connection from battery to relay to horns for the heavy gauge wire is exactly what relays are designed to do, whether they be headlight, horn, whatever. As long as you get 12 volts exiting the handlebar switch (destined for the relay 'actuator') when you push the button and an earth wire from the other side of the relay 'actuator', the system is pretty much bulletproof.
 
I suspect by "3 pole relay" you mean a relay with 3 connection pins. A true 3 pole relay has 8 pins and 3 separate sets of switching points inside it to switch 3 devices at the same time.

A 3 pin relay should operate the horns just as well as a 4 pin relay if it has the same high current power supply direct from the battery. Only issue with a 3 pin relay is that it uses the same power supply to operate the switching coil as it does to honk the horn, so it will still work even when the ignition switched off. A 4 pin relay uses a separate 12V feed (via the ignition switch) to operate the switching coil.
 
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