Last week the battery on my XS1100 died, like it does at the beginning of summer every two years. I replaced the battery but then found that my alternator would not charge the battery with the headlight on. With the light off everything seemed fine, output voltage went up to 14.5 volts like it should but when the light was switched on the voltage dropped to 12 volts and wouldn't go any higher no matter how high the engine speed. The alternator coils ohmed out OK so I concluded that the bad battery must have damaged the rectifier diodes by overheating or something.

I connected an external rectifier bridge to see if that would solve the problem and it seemed to work normally. The rectifier and regulator sections of the stock unit are actually two separate units inside one housing that share only the ground wire so either part can be replaced individually by wiring up the needed part on the outside. Having two circuits wired together was quite a cludge though so I decided to make a new unit with modern components. Three years ago there was a web page by SLHanson that showed how to wire in a new regulator if the stock one fails. The web page isn't there now but I saved a copy that I printed out. He used a Wells VR751 external regulator module for a VW/Audi alternator. It only has 2 wires to hookup and I bought one at the time. They used to be available from Checker, AutoZone, etc. but are not stocked anymore since they haven't sold any for 15 years so I ordered it from Wells but don't remember what it costs. Instead of buying a complete regulator I could have built one from individual parts but used this unit that I already had on hand.

The new regulator wires in series with the field coil but on the battery side, not the ground side like the original, so all you do is disconnect the wires in the white plug from the coil and rearrange them. In my case I wanted to add a new rectifier diode bridge too. It takes a 3-phase bridge which is not too common and has to be ordered so I used 2 readily available full wave bridges from the electronics store instead. They are part # MB356, 35A, 600volt from Circuit Specialists in Mesa, AZ for $3.50 each and probably have higher ratings than the stock part. To use 2 individual bridges all you have to do is wire the plus and minus terminals together and connect the 3 alternator coil output white wires to 3 of the A/C inputs on the bridges.

To make a neater installation I cut out an aluminum panel to mount the parts on that fits nicely under the side cover on my XS1100. I also put an 8 terminal screw type terminal strip on one side of the panel to connect all the wires to. That way I was able to get rid of the broken white alternator connector with it's burned contacts by cutting the old one off and crimping terminals on the wires to connect to the terminal strip. I put some heat sink grease under the 2 bridge rectifiers for better heat transfer, since they get hot. Their terminals are flat and can be soldered or they will accept spade type push on terminals. I had some pieces of wire from an old washer or some other appliance that had push on connectors on all the ends that I used to make the connections by cutting them to length and crimping a screw terminal on the other end that goes to the terminal strip. To make sure the housing of the regulator module was properly grounded I connected a black wire from one of it's mounting screws to the ground terminal on the T.S.

Now the alternator works normally again at a cost of less than $10.00 to build a new one. It only took about an hour to build and allowed me to clean up the alternator connectors and I also added a new blade type fuse holder to the battery. It still doesn't charge at idle same as before but I found that I have about 1.0 volt being dropped in the wiring that goes up to the ign. switch from the battery fuse and back to the other fuses so I'm going to look into that next. Maybe a Harley style ignition switch on the side panel? Here's a picture of the completed unit in case anyone wants to see what it looks like: