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My 04 WR450F lighting system has failed. I have no lights. I traced it either the Stator or the Rectifier. Both are pricy and I do not want to buy both with the knowledge that only one may be bad. I used the manual and it appears I still have resistance in the Stator test, but I do not show an increase in voltage in the Rectifier test. I am leaning towards the Rectifier, but these two components are linked to each other to a point that one could cause the other to fail. Does anyone know a sure way to test out the problem?

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1st: Determine if stator is OK.

A good stator should produce voltages between 10 and 60 VAC, depending upon RPM. Something not going over 12VAC is a red flag. A bad stator could be caused by a wire shorted out where the floated ground lead was lifted.

To measure stator voltage, disconnect the two yellow leads (or yellow and white) coming from the stator. Start the bike and measure AC voltage on these two lines. Look for ~10VAC at idle and up to 60 VAC at higher RPM. Low voltages at RPM indicate a bad stator. If good, go to Step 2.

Check for ground faults in the stator - measure continuity on each lead between yellow (or white) and frame ground. Any continuity indicates a ground short. Find and eliminate the short circuit.

One can also measure stator resistance across the two yellow stator leads. It should be about 0.2-0.6 ohms (depending on the stator). If it is less, there may be a short in the stator. If it is more (or open), there is a broken wire in the stator. Again, stators can fail from vibration and/or heat.

2nd: Check regulator/rectifier.

Measure voltage at the back of the stock lights. If the voltage is over 12VAC, then the reg/rec is bad.

Note for testing stator systems: one very good way to test the stator and regulator/rectifier combination is with a capacitor. Reason for this is that a battery muddies the testing waters: if the battery is bad, it acts as a power sink and sucks up all the power that the stator can produce; when you are hunting for problems with stator or regulator/rectifier, the battery will run the load for quite a while before you realize that it isn't being charged.

With a capacitor on the system, one can check the rpm required to power something (like a light). As the capacitor only contains what the stator produces at any given moment, it is a true reflection of the power coming from the stator via the reg/rec.

3rd: Check wiring.

Trace the red or positive lead from the regulator/rectifier to the battery and look for a blown fuse or open circuit. Do the same for the black or negative lead.

4th: Check if battery is charging

Put a current meter in line with the positive lead of the battery, and look for current flow (amps) into the battery when the bike is running and the lights are off. Then, turn on lights/fan/… and see if the current ever goes negative. If it goes negative, the battery is discharging with the load. If the stator is good, and no wiring problems exist, the battery may be bad, or the load is too much for the system.

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