The Ducati singles came with a very large and ominous voltage regulator positioned under the seat. If your Ducati fails to charge the battery the fault could be in the regulator. I have yet to see a service manual explain what is inside the black box let alone how to troubleshoot it. For that reason I threw caution to the wind and opened mine to look in side. The innards are amazingly simple as can be seen by the schematic. The weight of the box comes from 3 large coils all sharing a common laminated iron core. The gray H shaped thing on top is actually a heat sink for two large diodes, D1 and D2, which are mounted underneath. Diodes D3 and D4 are also located under the heat sink but are not attached to it. The principle is simple. The AC voltage from the stator is rectified by the Diodes, D1 and D2. As current passes through coils 1 and 3 it induces a magnetic field in L2 which through a saturation principle keeps the voltage output near 6 volts. Notice that L2 is double with one winding delivering a heavy current to the ignition system and a smaller coil which serves to charge the battery. For starting, battery current flows to the ignition through L2. The advantage of this system is that it is simple and with no moving parts it is quite robust. A second smaller set of diodes, D3 and D4, supplies voltage to turn off the charge light. One side of the charge light is connected to the 6 volts in the headlight housing and the other to the regulator terminal. If the stator is not putting out AC voltage then the charge light pin will be grounded through the 28 ohm resistor and the light will turn on. If the stator is functioning then there will be 6 volts on the charge light terminal so there would be zero voltage difference across the lamp.

So the battery is not charging. Are you sure? The best way to test for charging is to measure the voltage across the battery. When the engine is started the voltage should increase to about 6.5 to 7 volts. In the absence of a voltage test the headlight should brighten.when the motor is raced and dim when it is killed. Assuming that does not happen, the next test should be to see if the stator is putting out AC voltage. Locate the heavy black wire which exits the engine just above the kick start. This is the stator wire. It will plug into the regulator in the two end terminals. Unplug the wires and using a pair of jumper leads connect the two stator wires to a 12 volt car headlight bulb. Start the engine. The bulb should burn brightly. If it does not then the trouble is in the stator rather than the regulator. Using an ohmmeter each lead of the stator should read 2 or 3 ohms to the engine case. If an open circuit is read then a wire has probably come off. A low resistance with no output suggests a short.

Assuming that the stator is healthy lets turn our attention to the regulator. Lets look for the obvious things first. Be sure that all wires are attached correctly. The wiring order on the terminal block should be starting at the right side: the 2 stator wires, the wire to the ignition (usually gray), a ground wire (black), the charge light wire (brown) and finally the positive battery wire (red). First of all, notice that a fuse is located on the top of the regulator under a black plastic cover. Check to see if the fuse is ok and that it is making good contact in its holder.

The regulator can be checked almost completely with just an ohmmeter. The diodes act as one-way valves for electrons. They have low resistance for forward current but very high resistance for backward current. Before any tests are made disconnect all wires from the regulator. Failure to do so may damage your ohmmeter if the 6v from the battery were to get across the sensitive ohmmeter. Put the ohmmeter in the x1 position. Place one probe of the ohmmeter on one of the stator terminals and the other on the battery terminal. Note the reading. Now reverse the probes on the same terminals. You should have read infinite resistance in one direction and a very low resistance in the other direction. If the test is OK then repeat the measurement on the other stator terminal.

If the resistance is infinite in both directions then either Diode D1 (or D2) is bad or a wire has come loose. In the case of an infinite reading repeat the test form the stator to either side of the fuse. If an infinite reading persists then check the metal link running to the screw terminal just at the right side of the fuse. Notice that this strip is connected to the diode's heat sink by a small brass rivet. The heat sink is actually aluminum with a gray plastic coating on it to insulate it. The positive side of D1 and D2 are actually connected to the heat sink. Rust under the rivet can act as insulation and open the circuit. Repeat the tests around this rivet. I had to replace the rivet with a small flat head screw to get a good connection. If this still fails to establish continuity then either the diode is bad or L1 (or L3) is open. If you read a low resistance in both directions then the diode is likely shorted. The diodes D1 and D2 are pressed into the heat sink and are not easily replaced and you would have to improvise something to replace them. A replacement diode should be rated for 10 amps 50 PIV.

Next test for a short to the case. Put one terminal of the ohmmeter on the case and then test for resistance on each of the 6 terminals. You should have an open circuit reading on all of the terminals. If you do get a reading then there is a short to the case and it must be found (probably a bare or broken wire inside).

Finally, the charge light circuit can be tested. Make a front-to-back reading with the ohmmeter between each of the stator terminals and the charge light terminal. Again resistance should be low in one direction and infinite in the other. Next measure the resistance from the charge terminal to the ground terminal. That should be 28 ohms. If all of the above tests pass then there is little likelihood that the regulator is faulty.

Should you have to disassemble the regulator it can be done by removing the 4 nuts holding the gray heat sink in place. Note that the nuts must come off before the cover can be removed. If the nuts are rusty the stud may turn and unscrew the locking nut at the bottom. The cover, however, cannot be removed until all 4 top nuts are removed due to shoulders on the studs inside the case. Next remove the 4 screws next to the fuse. These are the connections to the diodes. You should be able to pry the heat sink off. Be careful not to lose the plastic insulating washers in the heat sink. Next, remove the case carefully passing it over the terminal block. Using the schematic trace each of the wires and try to locate the problem.