Ed,

 

The benefit is the direct #2 or #4 cabling to the starter relay (and then to the starter) while the Master relay to main bus is then a much smaller wire sized for, say, your max alternator load current (like 70 amp).

 

The disadvantage is the single point of failure.  If the starter relay should become stuck in the closed position, there is a lot of damage possible because you could not shut down power via the master sw/relay.  Did you run the starter coil circuit through a pullable breaker or is it just through the highly reliable start position on the ignition switch?  Well, at least they never fail.  Oh, that's right, the coil is fed + voltage from the hot side of the contactor and all the switch does is ground the other side of the coil - what could possibly go wrong?.....

 

Grayhawk

 

 

 

One option no one mentions is power to the start contactor directly from the battery.  When I routed 12v. from battery (Odyssy 680) to main relay then to starter contactor, I had very slow cranking (IO360 on Lancair 360).  Battery direct to starter relay and no problem.  Bingelis book shows this option.  Why run starter current through master relay?  How often does a start contactor weld itself closed?  If it does the battery will go flat pretty quick-am I correct?

Ed Gray Dallas