Terrence,

 

These $12 intermittent hi current relays are for intermittent service.  One of the big terminals expects to see +12 VDC when the 50-60 amp breaker (Motor power) is engaged.  The S (switch) terminal, supplied with + power while the relay case is grounded, completes the solenoid (coil) circuitry and pulls in the solenoid to cause the hi current +12 VDC to pass thru to the motor and also provide the same voltage and current to the I (indicator) terminal that has been wired to the other relay I terminal and thence to the "motor in operation" panel light.  Don't let this wire short to ground since it will try to carry amperage up to the limit provided by motor relay breaker (fire! fire!).  These wires should either be fused or use mini diodes to isolate each from the other hoping that the diode will blow if a short occurs rather than ignite the wire.  Remember that the S wire passes thru the appropriate pressure switch that breaks the connection if the pressure switch has opened because the pressure requirement has been met.  This wire then passes thru the gear up/down switch and thence thru a supposedly 5 amp breaker and finally to the DC bus. 

 

As you can now see: If the master relay is on, the gear switch is in the down position and the gear down pressure has been relieved (for whatever reason), the pressure switch will cause the down relay to be selected.  If the motor power breaker is open, no power would be supplied to the motor or the indicator light.  As long as these conditions are met, the relay will be powered and, since it is not a long term relay, it will heat up, melt the coating on the solenoid winding wires and ultimately cause the resulting goop to freeze the plunger - either open or closed - depending on when things cooled or, alternatively  short the coil.

 

Some people have used a "miscellaneous" breaker for the relay coil power and are reluctant to pull this relay when the motor power relay is also pulled.  There are several solutions.  Wire things so that the relay coil power breaker is always pulled if one desires not to have the pump engaged or place an inline fuse for the relay coil after the motor breaker so that all power to the entire system is removed by merely pulling the motor breaker.

 

BTW, the  "gear up" relay is also susceptible if there is a minor leak in the up lines and ones chooses to pull the motor breaker whilst in flight in order to stop the continuous pump blips.  Arrrrgh.

 

NAPA or Lancair should have these relays or you can buy more expensive Bosch relays.

 

 

Posted for "terrence o'neill" <troneill@charter.net>:

 Just had a gear down solenoid (bottom one in teh attachment) get hot/not
work/popped a 10a CB while checking operation before first flight ...   (The
'up' solenoid still works.)
 No markings on it, except a raised S and an I on the actuating terminals.
 Looks like a cheap item, and would like to replace it with something that
will NEVER fail... and should I replace the 'up' solenoid also?
 Asking for suggestions, before going to NAPA aircraft supply.
 
 Electrically challenged Terrence
 L235/320 N211AL

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