The electrical system of this bike was somewhat complicated compared to others when the bike was released in the 70s but is really pretty straight forward if its issues are approached in an organized fashion.  All of the components of the system have testing procedures laid out in the manual and you will likely want to refer to the manual for highly detailed testing methods. The first thing to understand about the system is that even when the bike was new the charging system was not what would be described as strong. If you intend to run accessories like driving lights, radios, CBs, heated clothing etc, your going to be pushing the systems capacity to the MAX and will need to make sure that the whole system is in nothing less then in tip top condition. Understand that as wiring ages corrosion becomes a potential problem. The wiring in the harness can and does corrode from the connectors inward along the wire strands in each wire. The same thing happens anywhere that the wiring insulation has been damaged. This creates resistance resulting in added heat and both have an adverse effect on the whole electrical system. You can check for excessive corrosion within the wiring by confirming that the wire close to the connections is still flexible. Corrosion tends to make the wire stiffer at any opening in the insulation. If the wires are very stiff at the connections you can conclude that there a lot of corrosion inside the wires. Excessive corrosion inside the wiring will have an adverse effect on how well the wiring and electrical components work. If the harness is in very poor condition it can be remade from scratch but this is a long process that most will not want to undertake. In most cases a replacement harness might be the way to go but when buying used 30 year old harness’s you might have to buy a few to get a good one.  If the harness is in average to good condition what follows will be very helpful towards making the electrical system functional.

The harness. Without removing the harness from the bike start by looking the harness over closely for any breaches in the tape and or plastic coverings. Pay close attention to places where the harness moves. That would be at the hinge point of the rear fender and between the frame and the triple trees. Pay close attention to any places where it looks like a repair has already been carried out in the past. Any breaches that are found should be re-taped after confirming that the wiring inside the harness at damaged locations are not broken or missing any of their insulation. If the wiring inside the harness at damaged locations is damaged, those repairs must of course be carried out before re-taping the harness. Broken wires can ether be soldered back together but use of the proper sized and properly installed inline wiring connectors will do a good job. In ether case be sure to also tape repaired wires well or use heat shrink tubing over the whole connection before re-taping them back into the harness. Once you are content that the wiring inside the harness is in good shape check ALL the block and single connections between the different sections of the harness and all of the the components of the system. Take each block connection apart and check that the wires going into the blocks are still firmly mounted into the blocks. If any of the wires can be pulled out the back side of the connector they will need to be repaired. In most cases this can be done by re-bending the catches that are part of the terminal end of the wire that goes into the block. DO NOT MIX UP ANY OF THE SINGLE WIRES IN THE BLOCKS  IF SEVERAL IN ANY GIVEN BLOCK ARE IN NEED OF REPAIR. As you go through the harness connections smear some dielectric tune-up grease into the female faces of the blocks and onto the male ends of the single connections as you go. This will protect the connection from addition corrosion when they are reconnected.  This whole process is best done from one end of the harness to the other so no connections are missed. Do not forget the connections that are behind the tail light in the tail light mounting bracket, under the right hand side cover, under the component mounting plate and in the head light bucket.

If while doing this job you discover any heavy corrosion at the connection points give the male connection a quick sanding with sand paper and use a small diameter brass brush in the female connector. Some large auto supply / fastener shops sell brushes / kits designed for cleaning wiring connectors but you will not likely need to go that route unless the connectors are in real bad shape. During this process make sure all the ground connections between the harness and the frame / engine are tight and making a true ground. Sometime a repainted frame will have its ground wires mounted on the paint and not truly to the bare metal of the frame. Make sure the short ground wire between the back of the engine to the frame is still there.  It is often missed if the engine was ever out of the bike. PROPER GROUNDING IS INPARITIVE IF THE SYSTEM IS TO WORK PROPERLY. Most simple electrical problems can be traced to BAD GROUNDING of the ground wires.  Now that your content that the wiring harness and all of it’s connections are clean, tight and lubed with the dielectric grease you can start checking the components of the system. There no sense in testing components of the system if the harness is not good. We will start with the simple stuff and go from there. If the bike is being fully restored or reburbed it is best to refurb the harness off the bike.

             The Light bulbs. Check all the bulbs to confirm that they are the correct type and of the correct wattages for their locations. All the bulb types and wattages are listed in the specs which are part of the photo album where spec sheets for the bike are posted.  While checking the bulbs also check that the bulb sockets are in good shape. They can become badly corroded if the rubber gaskets under the lens are missing. Check that the gaskets are also still in place where gaskets are used. If there’re grounding wires inside the light boxes confirm that the connection is good. Bulbs can still be obtained from the dealer if you need them. Improper bulb types and or wattages will cause problems.

            The fuse box. Confirm that the fuse box in in good shape and that all the glass fuses are of the correct size and type. The fuse boxes on these bikes are known to be WEAK. If the fuse box is in questionable condition replace it outright with an after market type that will fit under the side cover. Newer type fuse boxes that use blade type fuses would be the way to go if changing the fuse box out. In this photo only the fuse holder have been updated.


           The battery. If the bike has been sitting for more then a year without the battery being maintained or if the battery is dry or very low of fluid, I would strongly suggest that you replace it outright. Just about any battery will accept a charge but if it’s in poor condition it will not hold a charge for long. They can be load tested but the cost of that test might not make it worth the effort. Some battery suppliers will load test the battery for you right at the battery sales counter and this is an option if you intend to attempt to use a batter that is in question. In any case a poor condition battery WILL let you down sooner rather then later and have an adverse effect on the charging system. If the charging system is weak you will be kill the battery just to operate the bike. Typically what happens is everything seems OK while the engine is running and the bike is being used with RPMs in the normal running range. However, after the engine is turned off, it will not restart because all the charge from the charging system was being used to run the bike combined with the discharging of the battery. THE ENGINE WILL NOT START UNLESS THE BATTERY IS DELIVERING A FULL 12 VOLTS TO THE IGNITION BOX EVEN IF THE STARTER OR KICK START LEVER IS TURNING THE ENGINE OVER!   If you find yourself stranded you might get lucky and get the engine to start with a push or by making use of a slope to get the bike rolling before “popping” the clutch. Good luck with that. If you’re replacing the battery be sure to replace it with the correct size and amp hour type. Specs on the battery are listed in the electrical specs which are part of the photo album where spec sheets for the bike are posted.  The gel type batteries that are now available are said to be a very good choice but of course must still be the right size and amp hour type.

            The charging system. This is the most critical part of the system and it MUST be in tip up condition to provide enough charge for the battery to be charging while the bike is being operated. If it is not in tip top condition, the battery, even if new, will become discharged  or over charge damaged in due time and leave you pushing the bike to wherever it is you intended to go. An indication that the charging system is in trouble is a bouncy or unsteady tachometer needle. The tachometer is operated by the amount of charge that is being produced by the charging system. The faster the engine is turning the more charge it creates the higher up the scale the tachometer needle travels to indicate the RPM of the engine. If the tachometer needle is bouncy or unsteady, it can indicate that the charging system is not producing a consistent charge and should be checked. This is of course only true if the tachometer is not damaged or sticky which is rarely the case but can happen. Test the charging system as per the manual to confirm that the values are correct. If they are not, you will need to closely check the wires of the AC generator. You will likely have to remove the generator cover to inspect the inner assemblies that are under the generator cover.  


Note that under the generator cover is a DRY zone of the engine. This area should be DRY and free of water and oil. Heavy rust or oil on the inside parts are bad signs. Heavy rust indicates a potential leak at the generator cover to engine gasket. In any case that gasket should be replaced as part of generator service. Rust can be cleaned up but if you find oil under the generator cover it likely means that the crank end seal is leaking. If not the crank end seal then perhaps a leak is present at the O-ring under the oil gallery plug that is inserted into the block under that cover. DO NOT RUN OR CRANK THE ENGINE OVER WHILE THE GENERATOR COVER IS REMOVED. The generator cover holds the gallery plug in place only when the cover is installed on the engine. If you do run or crank the engine over with the generator cover removed, the oil gallery plug will be dislodged by oil pressure force and you will have a mess of oil to clean up. If the rotor is very badly rusted it may become unbalanced.

If this is the case it should be rebalanced by a machine shop. An out of balance stator will cause engine vibration and damage the cranks seals. If all the values of the generator tests are good the next component to check is

            The voltage regulator / rectifier. This is the square alloy component that is mounted in the middle of the upper frame tubes under the fuel tank. It has fins on its upper surface and no other part looks like it.



The regulators job is to control the voltage in the system by diverting excess voltage from the charging system to ground as necessary.  It therefore MUST be well grounded to the frame to work properly. If it is mounted on new paint that might not be the case so check it for good ground. To confirm that it is working properly refer to the manual for detailed tests of the unit but do note that a quick test can be carried out as follows. First check the voltage across the battery posts with a volt meter.  You should be seeing between 12 and 13 volts with the engine not running. With the engine running at an RPM of 2500 or faster, the voltage across the battery posts should increase to just under 14 volts but not go above 14 volts. If more then 14 volts are seen at the battery it will become over charged and this indicates that the voltage regulator should be tested in detail. If only barely above 12 volts is seen across the battery, the battery will discharge over time. This condition would mean that the voltage regulator is diverting voltage to ground at soon or that the generator is not producing enough voltage.

When the generator is working the intension is to trickle charge the battery at about 2 volts while 12 volts is used to operate the bike. Everything above this should be diverted to ground.  In other words, the batter is for starting the bike while the generator is for trickle charging the battery and running the bike.

            The rectifier part of this component converts the generators AC output to DC current for use by the bikes DC system.




                                                                                                                               o – connected

                                                                                                                               x – Disconnected


The handlebar switches. The handlebar switches can become corroded inside and become difficult to use. The return function of the turn signal switch can fail to return to the center position and the horn and or start buttons can wear to the point that they fall out the front of the switch bodies.  This is seen more often on the start button then on the horn button due to the difference in amount of use. These switches can be cleaned up easy enough but there are lots of small parts inside the switches and if you forget how they go back together you might get into trouble.  If you decide to fully rebuild them take photos of the insides of the switches as you take them apart.


You can refer back to your photos if you have difficulty reassembling them. In any case the switches can be removed by removing the 2 screws that hold the upper and lower halves of the switch bodies together.  These screws are threaded up from the bottom halves of the switches. The switches  will come off the handlebar like a clam shell with those screws removed. The insides can be cleaned with spray contact cleaner. Apply liberally and operate the various switches as the cleaner is being sprayed. The contact cleaner will leave no residue. Apply a small amount of light machine oil to the pivot points of the moving parts and check that the lips of the horn and starter buttons are still intact so the buttons will not pop out of the switch bodies. Other then a full rebuild this is about all you can do with the handlebar switches.  While the throttle side switch body is apart it is a good idea to lube the twist grip throttle parts that are inside that switch body. 

If new switches are desired after market units can be found on-line for various other Yamaha bike from the 70s and they are not too costly. The XS 650s were and remain very popular and aftermarket switches for them will fit the 1100 but the block connector wiring locations will most likely have to be rearranged.

The ignition system. This is another lengthy section that will include the coils, plug wires and caps, vacuum advance, mechanical advance, pick-up coils, ignition box and ignition timing.


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  1. Woody Gall says:

    What do you know about the lamp checker switch. I have a 79SF and recently the the lamp checker warning linght in the center console has begun to flicker on and off, and over about a week of time and say about 300 miles of riding it now remains on full time. I have checked all the connctions and I seem to have faith that all are making proper contact. Do you know the method for testing the switch to see if its faulty..
    Woody Gall
    Jacksonville, florida

    • Lamp checker switch? Are you referring to the light on the instrument pod that tells you when your headlight had a burned out high or low beam filament? If so start by confirming that you do in deed still have both the high and low beams of the head light still working. If both still work and the indicator light is telling you that one of them is burned out then check the reserve lighting device. It is a sealed unit and there is no test for this item. Most would check the reserve lighting system by making use of a unit that is known to be good.

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