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These questions come up time and again, so I thought I'd try to put together a troubleshooting guide to the electrical problems these bikes suffer. I'm not going to go into component testing as that's covered in the manuals, but rather explain what's connected to the various circuits and where/how to look for problems. It's important to note that the '78-79 and '80-81 bikes are wired slightly different, so be sure to be aware of the changes.
Main fuse. This is a major difference between the years; the '78-79 bikes have the main fuse located in the fusebox (30 amp). The fused side of the circuit goes to the ignition switch. This fuse also protects the taillight circuit, so if you blow this fuse, that's one thing to check. If this fuse blows, nothing works. The '80-81 bikes have this fuse in a separate rubber holder (located behind the left sidecover at top left), and it only feeds the ignition switch. Circuit plug connectors to check are at the positive battery cable and the ignition switch on the '78-79 bikes; the '80-81 have an additional connector behind the rubber fuse holder.
Ignition switch. Again, there's two types; the '78-79 bikes have a three wire plug (red=power in, brown=power to fuse box, blue=taillight, running lights if so equipped), the '80-81 have a four wire plug. The extra wire on the late switches is a red/yellow, is 'hot' all the time, and feeds the hazard flashers. Note that this wire is only protected by the main fuse, so if you're blowing the main fuse, try unplugging the hazard flasher (the small flasher above the battery) to clear the problem. Another problem that pops up from time to time is the plug that connects the switch to the harness can melt, preventing the bike from turning on or even sometimes turning off, so give this a good look.
This covers the 'main' power supplies, now let's go into the various circuits after a brief explanation of the fuse panel. The '78-79 panels will have (left to right) at their top a red wire, then three browns which are all common to each other; the bigger of the three browns will be the turn signals. The '80-81 panels are the same except instead of a red they have a blue. I'll note the bottom wire colors as I go through the individual circuits.
Headlight/instrument lights. Fed from the fuse panel (10 amp fuse) by a red/yellow wire to the lighting relay. Changes from a r/y to a blue/black after passing through the relay, then goes to the reserve lighting unit and the instrument lights (where it turns into a blue wire). If you have a bike that has the 'lights, on-off' switch on the right handlebar control, these two wires also go to that. On the switch, these are red/yellow, blue (not used), and a white which is connected to the blue/black. This is supposed to bypass the headlight relay where the switch is fitted. From the reserve lighting unit to the left handlebar switch, then from there to the headlight. Plug connections to check are at the relay, reserve lighting unit, and left handlebar switch plug under the tank. Note that the main wiring diagram for the '78-79 bikes has several mistakes in how this circuit is drawn, so go to the unit repair section for accurate info.
Tail/running lights. If you have a '78-79 bike, these are fed directly from the ignition switch with a blue wire. On the '80-81 bikes, the blue wire from the switch goes to the fuse panel (10 amp fuse), then to the lights. All wires will be blue. Plugs to check: switch, main plug under the seat (the late bikes), or the individual plugs on the early models. Running light plugs will be inside the headlight shell. The Specials aren't fitted with running lights, so lack the wiring in the front harness.
Turn signals. Fed from the brown wire at the fuse box with a 20 amp fuse. This wire goes to both the turn canceller and the turn signal flasher. The remaining wires from the canceller and flasher go to the left handlebar switch, then from the switch to the lights. If the switch is working properly, you should have continuity between the chocolate and brown/white, and the yellow/red and black (ground) when the lever is pushed left, push right and it's the dark green/brown white and again the yellow/red and black. If you have a hazard switch, you should have continuity between the chocolate (left), dark green (right), and brown/white when the switch is 'on'. All these are at the plug(s) under the tank. If your turns aren't working and you suspect the canceller, unplug it; if they now work, the problem is in that or it's connection, if not, the problem is elsewhere. Note that while the switches function the same between years, the wiring plugs are different and don't interchange between the '78-79 and '80-81 bikes.
Alternator. All years have the same wiring from the coils; a brown and green from the field coil, and three whites and a yellow from the 'stator' coil. The field brown wire is tapped into the brown wire from the ignition switch inside the harness. The green wire goes to the regulator. On the other coil, the three white wires go to the regulator, and one is tapped to give a tach signal. On the '78-79 bikes, the yellow wire goes to the lighting relay to turn on the headlight when the motor starts. On the '80-81 bikes, the same white wire that goes to the tach is tapped and ran to the relay, with a in-line diode (exposed on the harness) and the yellow wire isn't used. These are the infamous plugs behind the fusebox that go bad...
Regulator. The three white wires go to one plug, along with a black (ground) and a red (output) which connects to the wire between the main fuse and the ignition switch. The other plug contains the green wire from the field coil, another black ground, and a brown that is tapped into the same brown wire as the field coil brown.
Ignition/starting system. Power comes from the fuse box (10 amp fuse) with a red/white wire directly to the right handlebar kill switch. It comes back from the switch to the TCI unit, but is tapped in two places; one goes to the ballast resistor, and one goes to the starter switch (solenoid). If you have a '81 bike, the ballast resistor and it's tap will be missing. On the larger plug at the TCI, you'll also have a black/white (to the tip-over switch), a black (ground), a red/yellow (to the starter lead at the solenoid, missing on the '81 bikes), a orange and a gray (one to each ignition coil), and the tricky one, a white/red wire. On the '81 bikes, this goes directly to the coils (and is common to both), but on the earlier bikes is tapped into a red/white wire that comes from the ballast resistor to the coils. The smaller plug at the TCI is from the ignition pickups. There is an additional plug in this located behind the fuse box by the alternator plugs. The start wire that completes the circuit for starting the bike is a blue/white from the solenoid to the right handlebar, and goes to ground when pushed.
Oil/neutral lights. Both lights get power from the brown turn signal circuit. The oil light goes to ground through the sensor (located under the carbs) via a black/red wire. There's two plugs; one right next to the alternator/ignition plugs behind the fuse box, the other inside the headlight shell. The neutral light works the same, but uses a sky-blue wire (the switch is under the shifter cover). The plug at the engine is included in the pick-up coil plug.
Horn. Also powered off the brown turn signal wire, the push button completes the circuit by going to ground via a pink wire.
Brake lights. Again, powered off the brown turn signal circuit. The wire to the brake lights is a green/yellow. Single plugs at the headlight shell and rear switch.
Of course, if your bike has been seriously 'POed' and has hacked wiring, some of this probably won't apply, but this should help you figure out what they did...
Note that pretty much everything except the head/tail lights and ignition is powered by the turn signal fuse, so if stuff doesn't work, think about what does; if the horn works but the signals don't, it's not a fuse. That brown wire from the fuse panel is tapped in multiple places, so if you lose power to a individual bit you may have to split the harness to find out why. Also pay attention to the grounds; almost any black wire you find will be a ground, and this is another wire(s) that has been tapped in multiple places. Make sure your ground wire connections at the regulator is clean and tight; for added insurance, if you run a extra wire between the spot where the negative battery cable attaches to the frame and the regulator screw where the grounds attach, that will make the frame connection much less critical.
One thing you need to be when checking for problems is to be methodical and check one thing at a time. When you have multiple issues, they may not be related and even if they are, checking one circuit at a time prevents confusion. Best bet is to start with the simplest of the problem circuits (the one with the fewest parts/connections), tracing/fixing it's problems. This may fix your other issues, but if not, move on to the next-simplest and repeat. Do this, and pretty soon all the problems will be gone...
I know that it sometimes seems like an electrical 'guru' will wave their 'magic wand' over a problem and announce from their lofty height 'there lies your problem' while pointing with the wand, but it ain't always so...
There's rarely any 'magic bullets' when fixing electrical issues, slow and steady will always get you there... 
Here's a link of what to look for in terms of the 'quality' of your connections: Troubleshooting XS wiring, Checking for Poor connections
Here's how to 'read' your meter: Troubleshooting XS Wiring, Meter Types
Main fuse. This is a major difference between the years; the '78-79 bikes have the main fuse located in the fusebox (30 amp). The fused side of the circuit goes to the ignition switch. This fuse also protects the taillight circuit, so if you blow this fuse, that's one thing to check. If this fuse blows, nothing works. The '80-81 bikes have this fuse in a separate rubber holder (located behind the left sidecover at top left), and it only feeds the ignition switch. Circuit plug connectors to check are at the positive battery cable and the ignition switch on the '78-79 bikes; the '80-81 have an additional connector behind the rubber fuse holder.
Ignition switch. Again, there's two types; the '78-79 bikes have a three wire plug (red=power in, brown=power to fuse box, blue=taillight, running lights if so equipped), the '80-81 have a four wire plug. The extra wire on the late switches is a red/yellow, is 'hot' all the time, and feeds the hazard flashers. Note that this wire is only protected by the main fuse, so if you're blowing the main fuse, try unplugging the hazard flasher (the small flasher above the battery) to clear the problem. Another problem that pops up from time to time is the plug that connects the switch to the harness can melt, preventing the bike from turning on or even sometimes turning off, so give this a good look.
This covers the 'main' power supplies, now let's go into the various circuits after a brief explanation of the fuse panel. The '78-79 panels will have (left to right) at their top a red wire, then three browns which are all common to each other; the bigger of the three browns will be the turn signals. The '80-81 panels are the same except instead of a red they have a blue. I'll note the bottom wire colors as I go through the individual circuits.
Headlight/instrument lights. Fed from the fuse panel (10 amp fuse) by a red/yellow wire to the lighting relay. Changes from a r/y to a blue/black after passing through the relay, then goes to the reserve lighting unit and the instrument lights (where it turns into a blue wire). If you have a bike that has the 'lights, on-off' switch on the right handlebar control, these two wires also go to that. On the switch, these are red/yellow, blue (not used), and a white which is connected to the blue/black. This is supposed to bypass the headlight relay where the switch is fitted. From the reserve lighting unit to the left handlebar switch, then from there to the headlight. Plug connections to check are at the relay, reserve lighting unit, and left handlebar switch plug under the tank. Note that the main wiring diagram for the '78-79 bikes has several mistakes in how this circuit is drawn, so go to the unit repair section for accurate info.
Tail/running lights. If you have a '78-79 bike, these are fed directly from the ignition switch with a blue wire. On the '80-81 bikes, the blue wire from the switch goes to the fuse panel (10 amp fuse), then to the lights. All wires will be blue. Plugs to check: switch, main plug under the seat (the late bikes), or the individual plugs on the early models. Running light plugs will be inside the headlight shell. The Specials aren't fitted with running lights, so lack the wiring in the front harness.
Turn signals. Fed from the brown wire at the fuse box with a 20 amp fuse. This wire goes to both the turn canceller and the turn signal flasher. The remaining wires from the canceller and flasher go to the left handlebar switch, then from the switch to the lights. If the switch is working properly, you should have continuity between the chocolate and brown/white, and the yellow/red and black (ground) when the lever is pushed left, push right and it's the dark green/brown white and again the yellow/red and black. If you have a hazard switch, you should have continuity between the chocolate (left), dark green (right), and brown/white when the switch is 'on'. All these are at the plug(s) under the tank. If your turns aren't working and you suspect the canceller, unplug it; if they now work, the problem is in that or it's connection, if not, the problem is elsewhere. Note that while the switches function the same between years, the wiring plugs are different and don't interchange between the '78-79 and '80-81 bikes.
Alternator. All years have the same wiring from the coils; a brown and green from the field coil, and three whites and a yellow from the 'stator' coil. The field brown wire is tapped into the brown wire from the ignition switch inside the harness. The green wire goes to the regulator. On the other coil, the three white wires go to the regulator, and one is tapped to give a tach signal. On the '78-79 bikes, the yellow wire goes to the lighting relay to turn on the headlight when the motor starts. On the '80-81 bikes, the same white wire that goes to the tach is tapped and ran to the relay, with a in-line diode (exposed on the harness) and the yellow wire isn't used. These are the infamous plugs behind the fusebox that go bad...
Regulator. The three white wires go to one plug, along with a black (ground) and a red (output) which connects to the wire between the main fuse and the ignition switch. The other plug contains the green wire from the field coil, another black ground, and a brown that is tapped into the same brown wire as the field coil brown.
Ignition/starting system. Power comes from the fuse box (10 amp fuse) with a red/white wire directly to the right handlebar kill switch. It comes back from the switch to the TCI unit, but is tapped in two places; one goes to the ballast resistor, and one goes to the starter switch (solenoid). If you have a '81 bike, the ballast resistor and it's tap will be missing. On the larger plug at the TCI, you'll also have a black/white (to the tip-over switch), a black (ground), a red/yellow (to the starter lead at the solenoid, missing on the '81 bikes), a orange and a gray (one to each ignition coil), and the tricky one, a white/red wire. On the '81 bikes, this goes directly to the coils (and is common to both), but on the earlier bikes is tapped into a red/white wire that comes from the ballast resistor to the coils. The smaller plug at the TCI is from the ignition pickups. There is an additional plug in this located behind the fuse box by the alternator plugs. The start wire that completes the circuit for starting the bike is a blue/white from the solenoid to the right handlebar, and goes to ground when pushed.
Oil/neutral lights. Both lights get power from the brown turn signal circuit. The oil light goes to ground through the sensor (located under the carbs) via a black/red wire. There's two plugs; one right next to the alternator/ignition plugs behind the fuse box, the other inside the headlight shell. The neutral light works the same, but uses a sky-blue wire (the switch is under the shifter cover). The plug at the engine is included in the pick-up coil plug.
Horn. Also powered off the brown turn signal wire, the push button completes the circuit by going to ground via a pink wire.
Brake lights. Again, powered off the brown turn signal circuit. The wire to the brake lights is a green/yellow. Single plugs at the headlight shell and rear switch.
Of course, if your bike has been seriously 'POed' and has hacked wiring, some of this probably won't apply, but this should help you figure out what they did...
Note that pretty much everything except the head/tail lights and ignition is powered by the turn signal fuse, so if stuff doesn't work, think about what does; if the horn works but the signals don't, it's not a fuse. That brown wire from the fuse panel is tapped in multiple places, so if you lose power to a individual bit you may have to split the harness to find out why. Also pay attention to the grounds; almost any black wire you find will be a ground, and this is another wire(s) that has been tapped in multiple places. Make sure your ground wire connections at the regulator is clean and tight; for added insurance, if you run a extra wire between the spot where the negative battery cable attaches to the frame and the regulator screw where the grounds attach, that will make the frame connection much less critical.
One thing you need to be when checking for problems is to be methodical and check one thing at a time. When you have multiple issues, they may not be related and even if they are, checking one circuit at a time prevents confusion. Best bet is to start with the simplest of the problem circuits (the one with the fewest parts/connections), tracing/fixing it's problems. This may fix your other issues, but if not, move on to the next-simplest and repeat. Do this, and pretty soon all the problems will be gone...
I know that it sometimes seems like an electrical 'guru' will wave their 'magic wand' over a problem and announce from their lofty height 'there lies your problem' while pointing with the wand, but it ain't always so...
There's rarely any 'magic bullets' when fixing electrical issues, slow and steady will always get you there... Here's a link of what to look for in terms of the 'quality' of your connections: Troubleshooting XS wiring, Checking for Poor connections
Here's how to 'read' your meter: Troubleshooting XS Wiring, Meter Types