OK so I've had a few issues with my charging system lately so here is a run down on checks specific to a VTR, other bikes will be different:
Cold checking the Alternator - engine off cold checks -
the plug for this is under the seat just behind the fuel tank, the alternator is under a cover on the left hand side of the engine - follow the black wire up! Theres three wires coming from the alternator that go to a plug and socket - unplug it. The wires running to the alternator are all yellow on mine, as its a three phase ac generator it doesn't matter which one is which. The resistance between any of these on a good alternator should be pretty low (ie less than 0.5 ohms). This makes it hard to tell if your windings have gone short circuit. A better check is to use a multimeter that can measure inductance. The inductance between the three phases should be around 1.12mH. Its hard to explain the difference but basically a straight piece of wire or a shorted out coil will have almost no inductance but will give the same resistance reading as a coil of wire.
So if the resistance/inductance checks between the yellow wires is OK then you need to check bewteen each wire and ground - ie the frame of the bike. This should be high - ie no connection between the frame and any of the yellow wires (this is with the rectifier unplugged).
Hot Checks for the Alternator.
With everything plugged back in start the bike and check the AC output from the Alternator - between any of the yellow wires you should have around 12VAC, depending on your multimeter and other factors. If you have a frequency reading function on your meter you can use this to read off the revs like a Tachometer - as you blip the throttle you should see the frequency change. The output voltage won't change with engine revs but the frequency will.
Rectifier Cold Checks - Engine OFF!
Unbolt the rectifier and check underneath where the green earth tag goes onto to see if theres any corrosion between the rectifier and the frame (there was on mine) - I also put some heatsink compound (from Jaycar) under the new rectifier.
Rectifier Hot checks - Engine running
With everything put back together check that you have the AC voltage from the generator at the plug going into the rectifier - again all three on mine were yellow and you should have the same voltage as before (around 12VAC). Then check for DC output between Green earth tag and the red and white wire, you should have between 13.8 and 14.4 volts, if you raise the revs to around 6K it shouldn't raise the voltage by much, if it goes to 15V I'd say it was a problem, if it doesn't go above 12.8 volts then you've got a problem too. The output from the rectifier also goes through a fuse on the starter block under left hand seat cowl. When the engines running the rectifier should be getting warm/hot. Not so hot you can't put your hand on it though.
I had a blown rectifier which took out my alternator, I didn't find out my alternator was stuffed until I replaced the rectifier! I got both replacements from Electrex in the UK - cheap and fast service. I've heard of other people getting it worse than me - recitifiers crapping out and taking out pretty much all the other electrics on the bike - so I think I'm lucky!
If this isn't easy to follow send me a message and I'll give you a few more clues. Make sure you put everything back together before you fire up the engine!
Cheers
John
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Originally Posted by xDishHog
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