Topic: Sounding horn at tickover stalls the engine?

[gp_st3]

The symptom - if I sound the horn when the bike is ticking over @ less than 1.5k revs, the engine speed falls and will actually  stall if I keep pressing the button for more than a couple of seconds… It seems unlikely that this is this ‘normal’ behaviour, but I have no basis for comparison?

Bike is a CB750K2 I’ve had for 5 yrs with good condition wiring loom (replaced by a previous owner), old/original switch gear and horn which I should say sounds very feeble but has always worked.

Have taken the switch gear apart cleaned the button contact and checked/re-soldered the cable connection to be sure that it’s grounding properly (for info, I have a measured resistance of 0.39 ohms on the light green lead from the horn back to battery -ve earth when the button is pressed but have no idea whether that’s ok or part of the problem).

Any thoughts or indeed links to previous similar threads that my searching missed would be appreciated.

[Phinn]

What does the battery read?

At idle, the generator doesn’t output much of anything, so the system is all-battery under those conditions.

[gp_st3]

Yes, should have included that info:

Battery termnials:  12.89V
At horn connection: 12.15V

[TwoTired]

And electrical load that stops the motor is most likely a spark related issue.

What remains stock of the iginition system.  And, what is the voltage to the coils before/during pressing the horn button?

Cheers,

[gp_st3]

I installed Boyer micro power ignition and coils about 6 months ago.

The motor doesn't so much cut out like it would for lack of spark, rather the revs steadily drop lower and lower (like when you switch off the tap and let it run out of fuel) and then it gently stops.

Perhaps the horn drawing power puts enough additional load on the alternator to slow the motor which in the absence of more throttle to keep it spinning just quits?

I could test the voltage at the coils but am wary of messing up the electronics by doing something wrong in ignorance!

Unfortunately I have no idea whether the situation was any different with the original points / coils - the issue only came to light when the bike went for it's annual Dept of Transport certification test and the inspector checked the horn operation while the bike was sitting at tickover. In all other respects the bike runs perfectly.

Thanks for the input.

[Don R]

 Check Voltage at the boyer while honking the horn.

[dave500]

does just the horn do it?by itself?or during the test is the high beam on etc so fully loading the alternator?

[gp_st3]

does just the horn do it?by itself?or during the test is the high beam on etc so fully loading the alternator?

The horn does it all by itself - lights off, no other significant load.

By the way, the battery seems healthy with 13V or thereabouts after a long run.

Check Voltage at the boyer while honking the horn.

Will check the voltage across both coils tomorrow and report.

Thanks.

[dave500]

that boyer system should have the coils hard wired to power and daisy chained to each other,check the power at the control unit which is powered via the ignition key and switch.

[gp_st3]

ok, so before taking off the tank to get at and check the coil/ignition voltages I connected a 55W headlamp bulb in place of the horn (drawing 4.5A compared with the supposed 2A that is stamped on the horn)... and happily all is well, I can keep the horn button pressed and the bulb lights brightly with no effect at all on engine @ tickover!

It would seem that for some reason the horn is drawing a lot more current than it's supposed to so I'll buy a replacement - a good opportunity to get one that makes a lot more noise :-)

Thanks to those who replied and my apologies for not doing such a simple test before asking the question...

Cheers.

[Deltarider]

See if you get a pair of Voxbells. It's a classic and sounds mighty good.

[dave500]

I love em delta,ya gotta have loud twin deep tone horns not high pitched air type squealers!i use the hella "fanfare"set,have a look at truck supply places,youll be surprised how cheaper they can be on common lighting/relays etc,by the way run any higher load horn via a relay.

[gp_st3]

I love em delta,ya gotta have loud twin deep tone horns not high pitched air type squealers!i use the hella "fanfare"set,have a look at truck supply places,youll be surprised how cheaper they can be on common lighting/relays etc,by the way run any higher load horn via a relay.

Thanks for the recommendations and will remember to use a relay if I go for a higher load horn.

[Yamahawk]

I know that Boyer Bransden MKIII Ignitions are extremely sensitive to voltage, the one on my Enfield Bullet 500 won't run when the voltage drops below 10vdc. It won't start at low battery voltage either. The MKIV box is supposed to cure that, and the Micro Power ignition isn't supposed to do that, so your horn must be pulling a heck of an amp draw for it to kill the engine. As has been suggested, replace that turkey with something that gobbles better
Charlie

[gp_st3]

Last question - is a horn drawing 5A ok for the existing wiring/button or do I need to use a relay?

[Yamahawk]

Last question - is a horn drawing 5A ok for the existing wiring/button or do I need to use a relay?

Definitely use a relay. 4 pole, in/out relay of 20 amp or better, I ordered two from Radio Shack online $4.95 get one half off lol...and they were 60amp relays about 1" cube. Order an extra, and use it for headlight also! You may need two for headlight, hi/lo beam... 
Charlie

[TwoTired]

If it's not power draw that kills the motor, consider vibration.
Take a good look at the connectors that power the coils.  Old bikes often have corrosion and or loose connectors.

You don't need relays for the horn or headlight.  The horn isn't a continuous draw.  And the headlight doesn't draw enough to overtax wiring or switches.  However, another warning about the myriad connectors throughout the bike.  None of the connectors were gold flashed, so oxidation is inevitable.  the oxidation equates to resistance, and that leads to voltage loss and power loss through heating.
All the connectors should be examined and cleaned if they aren't bright and shiny. And then coated with a thick silicone grease to block further oxidation.

Unless, you are adding significant power to an electrical branch, relays just add complexity and bypass lurking issues with the bikes wiring distribution.  They also mean your stock wire diagrams no longer apply, leading to longer investigation times when any electrical issues arise in a few years.  Trust me, you won't remember what you did to it last year or prior.

Cheers,

[Deltarider]

I'm not much in favor of adding relays either (which more or less has become a hype here), however for a set of horns like mine, it's a must or the switch won't live long I'm afraid. My horns draw 10A which causes such sparks at the switch that it will eventually kill it. It's the only relay I added. I really can't see the need for further relays, but that's me.

[gp_st3]

Thanks again for all the advice, I'll fit a new horn with rated 5A load using the existing connections and see whether that works ok; if not then will go the relay route.
At the same time, can check for any dodgy looking connections and clean them up as well.
Cheers.

[TwoTired]

I don't know what the stock horn draws for amperage.  Deltarider is correct that if you put on horns that draw significantly more power than stock, a relay is good practice.  Years ago, I bought dual FIAMS, and the install kit included a relay.  These are 115DB dual horns that I expected would scare mindless zombie drivers back into their proper lane.  However, I never put them on any of my bikes.  I found the front brake high pitched squeal on my regular commuter was quite effective at getting their attention, instilling corrective action.

Cheers,