A multimeter allows you to “see” what is going on in the electrics and track down a problem. An inexpensive meter can be used to measure the voltage between two terminals, but beware of inaccurate readings if the meter’s battery begins to run low. Select an appropriate AC or DC voltage range depending on whether the measurements are before or after the rectifier. For a voltage reading, make sure that your leads are plugged into the “COM” and the “V” sockets (not “A,” which is for measuring current).
The AC voltage coming from the alternator should be about 70% of the DC voltage of the battery or inverter. (The rectifier will output only the peaks of the AC.) So it is normal to see readings of 17 to 20 VAC in the wiring of a turbine that is charging a 24-volt battery bank (at 24 to 28 VDC).
Normally you will find three AC wires. Check that the voltage between each pair is the same. If the voltage is higher on one wire, there may be a break between this point and the rectifier, or some blown diodes. If there is no voltage between two of the wires, they are probably shorted together somewhere.
Some turbines (the Bergey XL.1, for example) have the rectifier on the turbine, so you will see DC voltages between the two wires. Also be aware that the XL.1 has a controller that boost voltage to improve performance in low winds. So do not expect to see full battery voltage at times.
They can be pricey, but clamp-type current meters (or clamping probes for multimeters) are the ideal troubleshooting tool. Select one with a DC range to trace short-circuits in small wind systems where the turbine is turning very slowly. Clamp the tongs over each wire and you will “see” slow pulses of current in the ones leading to the short-circuit. If the turbine is running fast and vibrating, your clamp meter can reveal which wire is not conducting current.