So I get a call the other day from a friend of mine's wife. The was out riding with another friend of hers on another XT. Anyway, and she tells me her 2003 XT 225 was having an issue. They stopped to talk while on a trail somewhere and the bike would not start, as in it would only click and then all of the sudden as she was trying to get the bike to turn over a small amount of smoke and an electrical smell came out by the headlight area.

This was last Wednesday, and yesterday I finally got over to her house to look at the bike. This bike has always had a weak battery, but with my "run the gas out after each ride" it will start fine. However the first thing I noticed was that the battery was really swollen on the sides. I told her it was time for a new battery. I turned the key on and the bike starts fine, but after further investigating by the headlight, I discovered that the 3 prong plug to the light was melted. Not completely but enough to be the culprit of the burnt wire smell. The wires look fine. I also discovered that the headlamp does not work now. I need to get the bike to my house since I have a garage and the tools to work on this.

My question is would cranking on the bike have caused the plug to melt? Or what would have caused it to do this? I will follow the wires back to make sure they are in good shape, and will also check to see that voltage is getting to the plug. Then I need to replace it I would imagine that any older 3 prong will work. I am really hoping that the headlight somehow shorted inside and that was the cause. Any thoughts from you guys?

 

When Voltage decreases the draw (amps) increase and could easily melt the wire.

 

No, running the starter motor can't in any way harm the headlight connector, or cause the melting of it.

I can think of two real scenarios that might be going on.

a) Also the battery was "really swollen". that usually indicates excessively high charging voltage. Get the bike running a check the battery voltage while reving the engine. Should not exceed 14.5V
The too high battery voltage would also dump too much current into headlight bulb and the high current combined with bit of resistance at the bulb connector could cause enough heat to build up to cause the plastic to start melting.

b) Or, only issue really is just the melted headlight connector. If the contacts inside the connector get dirty and/or corroded that will increase the resistance of that connector. Higher resistance will cause more power being dissipated at the connector -> more heat -> connector melts.
In this case just replacing or cleaning/inspecting the bulb connector will fox that issue.

But sounds like the bike does need a new battery anyways. Get that first, then check the charging voltage to make sure the regulator has not failed. If it has it will in short order ruin also the new battery.
And fix/replace that bulb connector too.

--
Mikko

 

In a twisted way that statement is true but not applicable to this situation.

Yes, P = I x U so IF and only if, power is kept constant and voltage drops the current will have to increase.

But the headlight bulb is more closely a constant resistance device than constant power. And I = U / R so when voltage U drops (like when running the starter motor) also the current I into the light bulb will decrease.

--
Mikko

 

I agree 100%, Simple power formula for watts in a DC circuit, P=IV. The starter and light want to use the same amount of power. For a reduction in voltage, the current must increase to maintain that power. If the voltage dipped low enough, current could have increased to melt the wires.

However, the fuses should all be sized to ensure this doesn't happen. I would check the main fuse and make sure somebody didn't stick a penny in there by mistake

 

Nope, it doesn't work like that. Your formula is correct but being applied to wrong situation.

Say you have a bad battery and headlight gets brighter as you rev the engine.
When engine revs rise, battery voltage increases and headlight gets brighter.
Why is the light brighter with higher battery voltage?

Because more power is heating the filament -> it glows brighter.
Power being dissipated in the filament P = I x U.
Current through the filament I = U / R

More voltage across the filament -> higher current through the filament.

If the light somehow magically was a CONSTANT POWER device, it would always glow at same brightness regardless of the battery voltage. It doesn't do that.

--
Mikko

 

I read on some other places that the 3 prong plug was probably not making a good contact and caused some heat, I wasn't there so I don't really know the sequence of events or how they are related. But since power with the key on is sent directly to the light, I think the cranking may have drawn it down and up and it may be just a coincidence that the plug went. I will check the voltage and the fuses, and replace the battery and the light socket.

 

Hmm I see your point. I guess somehow I thought the light would always draw the same amount of power. Since the light is a resistive load and not an inductive load this makes sense now Mikko, thanks for the clarification.

As far as why that prong melted, could have been a loose connection that caused increased resistance and heat in the circuit I guess. Hey you could always fix it like the below attached!

 

I'd be checking that regulator and rectifier very closely. The rectifier converts your AC stator voltage to DC voltage and then the regulator simply regulates that voltage to 12 volts. The rectified DC voltage goes to the battery so you should expect around 14-15 volts there (right across the battery when bike is running), but the regulated voltage should not get above 13 volts for the rest of the bike (including the headlamp)...IMO.

 

Hmmm, the sentence that can start so many problems.