The two green wires are usually not on a stock GS which is what contributes to the GS charging problems. The R/R and the battery are not tied together very well on the ground side increasing resistance and lowering charging voltage.
You can see current traveling in the frame, needs to crawl up one B/W ring lug wire to the B/W crimp in the harness then come out the other B/W ring lug to get to what is typically a solenoid ground. The R/R(-) ground wire is usually close to or stacked under the same mounting bolt.
By realizing all current coming out of R/R(+) the red wire, needs to return to its source on R/R(-), it is perhaps self-explanatory why that mounting bolt become the focal point for collecting up all currents so they can get back to the R/R(-). That final jump from SPG to R/R(-) has to support 14A and so care should be taken in making sure those connections are very clean and you are using a substantial(14 AWG) but short wire (<1 ft).
You should notice (if all connections are clean) that the only current flowing from the battery(-) to the SPG is the battery charging current (3 amps). Anything else like harness returns or anything through the frame (like Dyna-S) will get picked up by another path.
If you ground the R/R(-) to the battery (-) then all the 14 amps comes to that point before making what is usually a 2 ft wire to the R/R(-). It can work but it is far more likely to develop a larger voltage drop that way.
Comment