"A pre-charge resistor is a device that slowly charges the capacitors inside a electronic speed controller before it is powered up. Without this resistor, closing the contactor would generate a large amount of inrush current causing the contacts to arc. This arcing can permanently weld the contacts in the "on" position. The inrush current from not pre-charging can also cause the contact tips to get pitted resulting in a poor connection and diminish vehicle performance."
"A coil spike suppression diode is a protection device across the contactor coil. Energizing the contactor coil creates a magnetic field that forces the solenoid arm to engage the contacts. When the contactor is turned off, the magnetic field collapses causing a back “voltage spike” that can damage the controller. The diode safely suppresses this energy."
"Note: Pre-Charge Resistors and Coil Spike Suppression Diodes are required with all Alltrax controllers but are not used with Curtis AC controllers or Sevcon controllers."