Car Electrics: If there is a PD of +12V, shouldn't it trigger a relay

[tamrat]

Relay coils are not polarity conscious, but it is easy to include a diode in series to make it only operate with a specific polarity if you wanted to.

Looking at your earlier drawing L1-42  with the boxes titled 'Turn signal switch' (inside the Main dimmer switch) which then have wires going to the 'Flasher Relay box' (in the main body ECU).   I read that drawing as saying the indicator switch is actually switching an 0v signal wire back into the flasher relay box.

Earth coming into the switch at E then going out via TR->ER  or TL->EL to put an 0v signal into the flasher unit for left or right.  The 12v supply is sitting permanently at the flasher unit at the IG terminal.  Which corresponds to your initial comment that the wires you found are at 12v with respect to the body (this is coming via the flasher unit). Then when you put the switch either Left or Right the wires drop to 0v. (this completes the circuit for the flasher unit to start flashing as it already has +12v at its IG terminal)

Basically your 12v reading is across the switch not the load, which is why it disappears when the switch is operated. I think the logic for this is something to do with needing to be able to put hazards on when the ignition is off or on (via the +B terminal), but normal indicators only when the ignition is on.   I think your relay coil is just bypassing the indicator switch to activate the flasher relay.

Doc H.

This is just spot on. It all makes sense now. Thank you very much sir for taking your time and looking into this.