Great discussion regarding contactors.

Little more to say about using the correct continuous and/or intermittant
power contactors in the correct service. All you electrically knowlegeable
guys have  explained their correct use and why what's likely
happening when one fails. However, there are some more things to think about.

To recap, The starter contactor needs to - and does because of it's
construction - pull a quite large coil current when activated to produce the
high magnetic force (flux) needed to pull in and hold in the contactoer - way too much
to be a continuous duty relay. The reason is that when you hit the starter,
the starter pulls a really large current to get things moving. When that
happens, the terminal voltage at the starter bus drops precipitouly -which
is usually connected to the contactor coil. For a 12 volt starter this can
be as low as 3 - 4 volts. Because of this, the starter contactor must be
able to remain pulled in at that low voltage until the engine starts. This
means few but heavy coil turns which means high very high current in the
coil. The current and voltage can vary a lot depending on ambient
temperature so it must handle the worst case situation. The lowest expected
voltage at the starter and therefore the starter contactor coil must be able
to hold the contactor engaged. Hold it in too long though, particularly
after the engine starts, the starter is disengaged and the bus voltage
returns to normal and it doesn't take long to overvoltage the coil and burn
up the contactor's coil often causing the contacts to weld.

On the other hand, the continuous duty main contactor has a significantly
larger number of smaller wire turns and therefore requires a much lower
current to operate. As such, it can remain actuated indefinitely without
overheating.

Interestingy, my O-235 has operated on automotive contctors with diodes across the coils to spare the operating switches, starter and main,
from NAPA for 20 years without a glitch. Back when building my 235 ('87 to '93) they were the recommended contactor and they've worked fine.

The only thing I did for the starter contactor was to mount it up-side down to let gravity help it drop out.

It's possible that the lower cranking force for the 235 goes easier on the starter contactor than a 320 or 360 - but I've never measured it.

Just for safety, I plan to R & R the relays this summer as 20 years is probably pushing the envelope.

Dan Schaefer
N235SP