I had two problems with my GT550a when I got it: 1) the regulator was overcharging the battery and 2) The neutral warning light was always on when the ignition was switched on. From posts in this forum, it seems these are common issues. I offer no explanation about why the wiring loom is wired the way it is, or whether the bike ever worked (I assume it came out of the factory working), but here's what I did to fix these issues.
Background: The wiring on the bike as I inherited it had been chopped and patch really badly so I know I am not the first to try to resolve these fault. So I replaced the front and middle harnesses with NOS. This did not fix the issues but did lead me to assume that the old wiring was at least the same as the new wiring, and was able to open the old loom and check what was going on. Here's what I found...
Overcharging: There are two power circuits. An orange circuit (power from the ignition switch) and an orange/white circuit (power from the run/stop switch). The orange circuit powers the ancillaries; lights, indicators, horn etc. and the orange/white circuit powers the ignition circuit; ignition and coils. Contrary to what is drawn in the wiring diagram, the regulator is powered by the orange/white circuit in the standard loom. The orange/white wire coming from the run/stop switch is spliced inside the loom to the coils and to the lead running to the regulator. The issue with the overcharging is due to this i.e. the regulator voltage is dragged down (in my case to about 9V) by the coils. This causes the regulator to think the battery voltage is too low and sends current to the alternator field winding which cause the alternator to charge the battery at up to 17V when it should not. The cure here is to disconnect the orange/white wire from the back of the rectifier connector plug (just tape it up) and instead run a new orange wire from the orange lead that powers the indicator unit. This then powers the regulator with a higher voltage (in my case about 11.5V). If necessary, also open the regulator and slacken the screw tensioning the points spring until you get between 13.5 and 14V on the red charging wire going to the battery. Note you cannot do this with the replacement solid state RRs availability on the market so stick to the original if you can.
Neutral always on light: The neutral light is driven through a pole on the gear sector switch (behind the clutch pushrod cover on the left hand side of the engine). The selector switch earths the blue wire (in the separate gear position wiring loom) when in neutral, and opens the circuit when in gear. The blue wire breaks-out of the loom at one end and is connected with bullet connector to the blue wire that goes through the loom all the way to the headlight shell and then onto the dashboard. With the alternator connector UNPLUGGED, this should work as designed. The problem with the light always on is there is a second blue wire coming out of the main harness which is connected to the back of the alternator plug (i.e. connected to the alternator earth black/white when the alternator is PLUGGED IN). This 2nd blue wire is for some reason spliced to the blue wire connected to the neutral indicator wire inside the loom! I can offer no explanation for why this is or what this wire is supposed to achieve, but the upshot is, when connected as standard, this blue wire earths though the alternator and therefore makes the neutral light burn all the time. The cure here is to disconnect this blue wire from the alternator plug (just tape it up) and run a new black/white wire to earth (I connected mine to the earth wire going to the regulator). After this the neutral light will behave as expected. Note this has no effect on how the alternator functions.
So there you have it. If anyone can explain why the stock Suzuki wiring is so f**ked up, please let us know. I suspect the parts available from NOS do not match the 550ab models. Good luck if you have these issues and hope you solve it, as I have.
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