Responding to John's question below:

 

Starter solenoids switch hundreds of amps of surge current when the starter starts to crank.  To assure that they make the contact with minimum resistance, the contact is pulled into the electrical circuit with a very strong magnetic field that requires LOTS of current through the coil.  It is a real "wham."  In turn, if there is a bit of welding at the contact surfaces, the circuit is broken by contactor bar being pushed out breaking the circuit using a big spring (which the magnetic field must also overcome).  The high current draw through the coil causes a lot of resistance heating which is why the duty cycle is so restricted.  A few seconds on, then minutes off to cool.

 

Starter solenoids are wired differently than continuous contactors even though they look the same.  But look closely and they are marked differently.  On the little terminals, starter contactors will have "S" (for Start, where you feed power to engage the contactor) and "I" (for Ignition) which feeds power to the ignition system (throwback to old distributor ignition systems) during the  start. 

 

The starter solenoid is energized by connecting the start switch to the S terminal, and the current flows through to the coil to solenoid frame which is presumed to be mounted on a ground thus completing the circuit.  The small "I" terminal is connected to the high current outlet terminal that goes to the starter.  It feeds battery power to the ignition system.  Why?  When distributors operated normally they were supplied with power via a resistor that limits the current when the ignition points are engaged.  During the start, current from the "I" terminal goes around the resistor to the distributor to make a stronger spark when the battery voltage sags because the starter motor is engaged. 

 

The continuous solenoid is wired differently.  Feed power to one small terminal and ground the other small terminal, and the thing will energize and pull in the contact allowing current (rating typically 60-85 amps max) from one big connection to the other big connection.  Nothing is connected to the case.

 

I just read the words above and they are too complicated.   So I quickly made the ugly sketch attached.  Hopefully the differences become clear.

 

KEY POINT: USE STARTER CONTACTORS FOR STARTERS (INTERMITTENT USE) AND CONTINUOUS CONTACTORS FOR SHIP'S POWER (BATTERY CONTACTORS) AND DON'T MIX THEM UP.

 

How do I know this?  I replaced the el cheapo starter contactor on my Lancair IV hydraulic pump with a continuous Cole Hersee contactor rated at 85 amps, and I got fooled by the wiring change.  No instructions in the box.  Had to put an ohm meter on the two different contactors to figure it out.   Duh.  And to think I once knew the difference long, long ago.  It all slowly came back as I tried to figure it out with the ohm meter.   The mind is going, going, going.....

 

Fred