bike dies with lights - help

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Roberts850

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May 5, 2015
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Hello all. I recently installed a high output alternator and new battery, and did a little rewiring. I must have miswired something. The bike, a 74 850, runs very well but the ignition cuts off when i turn the key to the far right position. I'm pretty sure it's a wiring issue, maybe in the headlight wire cluster. Any suggestions about what to check would be greatly appreciated.
 
Roberts850 said:
The bike, a 74 850, runs very well but the ignition cuts off when i turn the key to the far right position.
Click to expand...

I take it you mean "key to the far right" as you look at the face of the ignition switch (so, key at ignition & lights) and not far right viewed as you sit on the machine (parking lights)?

What type of ignition system does it have? Electronic (if so which) or points?

Does the ignition cut out immediately the key is turned or does it take a short while for the engine to cut out?
 
What kind of rewiring did you do? It could be a bad battery, does the bike die when you blow the horn?

The horn, at least the stock one, is a better battery test than the headlight. In bright sunlight it can be hard to tell if a headlight is dim. The stock horn will kill the engine at idle with a weak battery, or a least sound really puny.
 
If you have a multimeter, measuring the battery voltage when it does this may give some indication of what is happening.
Is your battery good and strong - and fairly new.
Does this happen with the engine running - or only with the engine stopped.
If the alternator is not providing charging, it just might be this problem.

Unfortunately, failing this, you are going to have to go through ALL the wiring connectors until you find the problem.
If it doesn't blow the fuse when doing this (?), at least its not the result of a dead short - a wire into a wrong connector somewhere.

Ultimately, finding this problem may involve discconecting all the wiring connectors, and reconnecting them according to the wiring diagram.
I'd start by carefully disconnecting power to the headlamp, wrapping things so nothing can short out, and trying the switch again and see what happens.
You may need to proceed through the whole loom until the problem shows itself.
Don't ignore the taillight/wiring either...

You may need to test the switch too, to be certain it is actually providing a connection.
Wouldn't be the first switch that has failed connections inside. ?

Good Luck !
 
If it is a wiring problem it's almost certainly a short to ground in the headlight circuit (if it does it only with the headlight shell toggle switch on) or in the pilot light circuit if it does it with the toggle switch is off.
 
ewgoforth said:
If it is a wiring problem it's almost certainly a short to ground in the headlight circuit
Click to expand...

Why doesn't it blow the fuse then ?

Thats why I noted that a multimeter on the battery would be a useful observation.
Volts will dip, markedly, if its a short.

Can recall taking a guzzi for its mot equivalent, and when the tester asked for lights, it went 'phut'.
Blown fuse, caused by the indicator bulb having a trapped wire in its holder.
Took a bit of tracing....
But replacing the fuse let it be ridden home - with no lights.
Amd no mot :cry:
 
Agreed about the fuse, I was going to ask if he still had the fuse and whether it was 35 amps US rated as opposed to 35 amps UK rated (about 15 to 20 US rated). I would imagine a 35 amp US rated fuse might allow a short to drop the system voltage too low for the ignition to work, killing the engine before the wiring melted.
 
On some ignition switches the extreme right position is for parking lights only, perhaps you are simply turning the ignition off because that circuit is not internally connected in the switch.
 
Thanks for all the ideas. First, it happens when the key is turned to the far right, while looking directly at the switch. It was doing it last week with old battery so I thought it was a battery voltage issue. But it does it with a new battery, immediately turns the engine off as soon as the key is turned. It's like the switch connection in that position looses continuity. So maybe it's the switch. Should I try contact cleaner? I will try turning the key with the headlight toggle switch in the different positions to see if it responds the same. Does anyone have a diagram showing which wires, which colors, connect to the various posts of the headlight toggle switch? (the wiring diagram is really tough to read) FYI all the connections in the headlight look competent -no obvious loose wires.
 
Can you actually get the lights to work in any fashion at all ?

What about a hotwire direct from the battery to the high beam connection say...
This machine is +ve earth, so use the correct wire !
This wire may need a fuse in it also, if there is a bad or wrong connection in there someplace.

Have you tried the multimeter probing on the back of the switch, to see that power is actually being switched to the headlamp.
 
Hi Roberts850.
Re your: "...When the bike is running the tun signals and horn work as expected..." and "...was doing it last week with old battery so I thought it was a battery voltage issue. But it does it with a new battery...".
I suspect there is no problem unless you turn the key to that position i.e. don't turn the key to that position.
Ta.
 
needing said:
I suspect there is no problem unless you turn the key to that position i.e. don't turn the key to that position.
Click to expand...

Unless we have missed something, that is the key position that gives lights.
So, do YOU often ride your bike at night with no lights ??

Or is this more trolling, ta = Trolls Associated (?).
 
bike dies with lights - help
 
Aha rohan
Thank you for that explanation.
Ta.

I have searched accessnorton for the word 'troll' and your name is most associated with that search.
I am slow to ire but asked that you desist from your self appointed role of 'troll catcher'.
A chess player may perceive your actions as attack being used as a diversion to defence.
Ta.
 
You keep signing your silly posts with this 'ta'.
What else are we to assume ??

Am I the really only one to spot nonsense posts, and question them ?
Sometimes they come thick and fast, but I let the minor ones go.....
 
Roberts850 said:
immediately turns the engine off as soon as the key is turned. It's like the switch connection in that position looses continuity. So maybe it's the switch.
Click to expand...

If the horn, direction indicators, brake light, also stop working then the switch is the likely suspect.

Roberts850 said:
Does anyone have a diagram showing which wires, which colors, connect to the various posts of the headlight toggle switch? (the wiring diagram is really tough to read)
Click to expand...

There are only two wires.
A blue/yellow [UY] "IN" (from Ign. Sw. '4') and blue "OUT" (to the handlebar dip switch).

I suggest you connect UY to terminal '4', and U to '8', at the toggle switch, although in practice it doesn't matter which terminals are used-as long as the headlamp is 'On' at the appropriate switch position.

bike dies with lights - help
 
My bike was doing the same thing, it was the ignition switch. leading up to this, the lights would intermittently stay on with the switch in the ign position.
 
My 74 Commando had the same problem.Started with ignition intermittently cutting out above 3000 rpm with ign+lights.Progressed to the bike going completely dead when switched to ign+lights.Replaced with an aftermarket 3 position switch and has been trouble free ever since.FWIW-Mike.
 
I don't know what this troll business is all about but the problem is that when the key is turned to the ignition + lights position the engine dies. And yes, it makes driving at night problematic.
 
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