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Protection diode..............In this case/config, it's designed to clamp any negative going(polarity) induced voltage spikes coming off solenoid's drive coil (#3) to around -0.6V...........w/o these clamping diodes when the ig switch's start position contact opens (and in similarly configured circuits), a very fast rise time ( dv/dt > nS - pS) and very high potential [R(air gap) * -I(series) = -V(poof)] flyback/back EMF voltage spikes from solenoid/relay coils and the like is induced and can migrate throughout interconnected circuits and play hell on ECM's and other such sensitive electronics. TMI? :twitch:
