Debugging the electrical system of a motorbike
History:
Recently my bike (2014 KTM Duke 200) started having a crazy speedometer. While on the tarmac the speedo seems to have a mind of its own and started to show "0" reading for a few seconds and will come back alive in a few seconds.
Also when the headlight in turned ON or if the radiator switches on in the bumper-to-bumber traffic the speedo will never show any speed except "0" on its console.
Cause:
The reason for this behavior in the KTMs are due to the electrical charging circuit. Due to the high compression ration of the engine it usually runs very hot and because of that the stator coil is usually exposed to very high temperatures. This was solved in later versions of the bike with a mechanical change in the engine case to direct more oil to flow over the stator coil to keep it cool.
As the stator coil is exposed to more heat, eventually the insulation layer gets melted and the coil gets shorted. And due to this the AC voltage produces across the three windings will be shorted and become less than the nominal rated voltage. When this is given to the rectifier and regulator (aka RR unit) of the bike it fails to give 14 volt output which is usually expected to charge the battery when the engine is turned on.
Hence due to insufficient voltage the speed sensor wont be able to transmit of the speed signals properly to the ECU and so whenever the battery is not able to drive enough current the speedo reads "0".
How charging happens in bike:
The only rotating part in a bike is the engine shaft. The engine shaft has magnets attached to it which continuously rotates. Now there is a coil of wire places around this shaft with magnets. When the magnets keep rotating with coils around it, et voila you have a generator. But it generated AC voltage while the bike electricals work in DC.
There comes the RR unit (rectifier and regulator). It converts the three phase ac voltage usually in the range of 20 to 40 volts, to 14V DC which is the charging vltage required for 12V battery.
In this loop even if one of the component fails(either the coil, RR or the battery) it will be a field day to diagnose and pin point the issue.
Diagnosing the coil:
Usually a multimeter is more than enough to debug this part. Put the meter to measure ac voltage and measure the voltage across the three coils. Usually all three readings should be more or less equal to the rated voltage in the user manual.
Also check if there is a short between the three coil terminals and ground(bike frame). If there is a short you have a faulty stator coil.
Diagnosing the RR unit:
Try measuring the diode readings in the forward and reverse bias with positive leads and negative leads connected across the input and output respectively.
Diagnosing the battery:
Keeping the multimeter in DC voltage mode put the leads across the battery. It should read something like below,
When idle and engine turned off - 12.2v and above
During engine cranking - Should not drop below 10.5v to 11v
When engine is idle - 14v to 14.3v
The charging voltage above 15v means you have a faulty RR unit. Charging voltage less than 13 volt indicates either a faulty RR or a faulty stator coil.
PS: Always keep a multimeter handy. This simple device can help you solve very costly problems.