Here's headlight circuit. Key ON, will
not turn on headlight. Very ingenious design where headlight is off until engine is running.
1. With key ON, power from ignition-switch goes to headlight-relay's power-input terminal (top-right)
2. BUT, NO power goes into headlight-relay's activation-coil (top-left). Note 2x diodes pointing to relay's activation coil input.
3. One diode (left Y wire) takes AC pulses from alternator and rectifies it to +DC pulses when engine is spinning
4. +DC pulses then activates headlight-relay's coil, relay-switch closes and sends +12v to output terminal, which then turns ON headlight
5. output of relay
also comes back around on 2nd diode and self-powers (latches ON) headlight relay, headlight stays on for good once relay turns on.
6. to turn OFF headlights, power is removed from headlight-relay's power-input terminal when ignition-switch is turned off. So when you stop engine with kill-switch, headlight-relay stays on (self-latching), thus headlight stays on. Have to remove ALL power to everything by turning off ignition-switch.
NOTE: Where's ground for headlight-relay coil? Other side of coil goes to starter-relay B+? If you aim +12v --> <-- +12v, nothing would flow, how does headlight-relay coil get activated??? Ground of headlight-relay coil actually doesn't go to power side of solenoid that powers starter, but power side of activation coil.... which then connects on other side of coil....
to GROUND!. So you have two coils in series. What happens is starter-solenoid's coil is of much lower impedance and drops most of voltage in series circuit. Higher impedance coil in headlight-relay then sees connection to starter-solenoid's coil as pretty much ground and enough current flows to activate headlight-relay.
