-6 degrees is a very cold start! Well done mate. Great result.
I agree the negative voltage spike is better run to ground than through our bikes electrical system.
Dario I would like to make a diode shunt harness similar to yours. I've no experience doing this type of electrical work so need some advice. I've ordered some pcb board. How is diode and wiring soldered on? You don't have a pic of the reverse side of your diode soldered up? Also conformal coating. Would silicon glue be OK?
I will make a new one and will take pictures (step by step).
For now this:
The conformal coating is a dielectric coating, that protect the electronics components against moisture or contaminants. Is kind of a clear coat, usually you apply the coating to all the components, pcb and wires. If you don`t have a specific electronics conformal coating I think you can use a transparent nail enamel. You have to clean the assembly very well before applying the coating (cleaning with isopropyl alcohol is ok).
For the diode, you have to form the leads to be able to fit the part in to the little holes of the PCB. When forming the leads, try to not bent them to close to the body (cause you will cause stress and the part will crack eventually), then insert the part in the pcb, then solder the part on the bottom side, then cut the leads. Now, prepare the wires (strip the insulation), thin the wire using your iron and some solder, then solder the wire to the pcb and the lead. You can be generous with the amount of solder here. Do the same with the other lead, clean the flux residue with isopropyl alcohol, you can stake the wires to the PCB using some silicone glue, them apply the coating, then use some shrink tube to cover the assembly and you are ready to go!