900 GTS wires getting hot

[mizike77]

I have two wires going into my ignition switch that get very hot when the lights are turned on. The red wire and one of the greys. No heat with lights off but hot enough to melt things when the lights are on. The bulb is a worldlight 12v h4 60/55 Anyone have any ideas?

[Craig in France]

<snip> Anyone have any ideas?

Yup. It means you've either got too much resistance in the circuit and/or you're pulling too much current. To explain ...

Electrical components and circuits basically get hot because of a phenomenon known as 'Joule heating'.

This phenomenon is described by the formula, P (joule) = Isquared x R where P (joule) represents the heat generated, I is the current and R the resistance. From this, you can see that excessive heat is caused either by:
a) the resistance in the circuit being too high
b) the current flowing thru it being too high.

Start with checking the resistance in the circuit. The pucker way would be to isolate the individual components of the circuit and check each using use an Ohmmeter. But life is short, so you may chose to simply clean all the cable and component connections you can get to, not forgetting those in the ignition switch and the handlebar switch (and the battery, both terminals, and don't forget the fuse box). Any build-up of corrosion anywhere in the circuit will lead to an increase in resistance and, from the formula above, extra heat. But I bet it's the ignition switch contacts ...

(It is also possible that after all these years the cables themselves have some broken strands. The smaller the cross-sectional area of the cable, the more resistance there is.)

Then it's down to current ...

A 60w watt bulb pulls 5 amp at 12 volts, so check to see if the size of cable that's feeding it is up to this. (I can't believe it isn't: a puny 16 strand, 1mm2 cable has a nominal rating of 8 amp. Go, for example, http://www.energy-solutions.co.uk/cable_conductor.html). Also, I'm pretty sure this was the original bulb specification anyway?

So look at what else is on when you're running with the lights on. I can remember my old GT used to blow it's fuse when I had the front and rear lights lit, an indicator flashing and I then put the brake light on. The ignition circuit was also wired thru that fuse, so the result was total electrical failure ...

Another solution, of course, is to re-wire the headlamp, putting it on a new circuit that incorporates a relay that is switched using the existing circuit . My Laverda Corsa runs like this, these days.

HTH

Craig

[mizike77]

Thanks for the great info! The directionals have been wired in as running lights on this bike. I have already disconnected the fronts, but I have yet to disconnect the rears. They are on all the time that the lights are on with dual filament bulbs....now , that being said, I have taken the bulbs out altogether to see if the ignition wires would still get hot and they did. Also forgot to mention that this bike came to me without an ignition switch. All this is happening when I temporarily tied the wires together for test purposes. I have a switch coming but am reluctant to hook it up because I dont want to cook it!