This subject comes up now and then on most aircraft lists. The conventional type of solenoid (or contactor, if you prefer) looks the same whether it is used for the master or starter, but there is a big difference. The starter contactor has a stronger spring and a different coil. It is rated (guessing here) at about 500 amps compared to maybe 50 amps for the master contactor. One cannot be used for the other application, but every once in a while a master contactor gets installed in the starter circuit. But, you say, the starter current goes through both, so how does that work if the master is only rated at 50 amps (the starter probably pulls 300 amps)? The answer is buried in the rating - the current rating is about how much current can be interrupted,
not how much it can carry. The master will not interrupt the high starter current reliably. Has nothing to do with the battery being low.
Hope this clarifies the situation.
Gary Casey
Just thought I would throw in a data point here amidst all the relay talk.
Two weeks ago I had what turned out to be
a stuck starter solenoid. I had
been troubleshooting my noise issue and had done a series of startups and
shutdowns. Four over 20 minutes or so. On the last shutdown, I momentarily
grounded the mags as normal and pulled the throttle and mixture. Prop kept
going..thought it was what we used to call dieseling. Turned off mags.still
running. Fuel off.. still
running. Master off killed it. Master back on
turned prop again, and quickly blew the fuse on my Vertical Power system at
the main battery. I thought I had a problem with the dreaded starter
adapter, but that looked OK.
I'll save the whole troubleshooting story, but it was indeed the starter
solenoid. 2007 kit, about 90 hours total time. It was a Hersee solenoid
that I got directly from Lancair in the FWF kit for the starter circuit.
The part number was 24144 if my memory is correct (I do have a pic). I
looked it up on their website. It was a continuous duty solenoid like you
would use for a master, and not an intermittent as needed for the starter
circuit. It seems to really make a difference. Someday I will cut it open
to see what it looks like.