Contactor control circuit
From the datasheet, it appears as there are two terminals connected externally, and they are X1+ and X2-.

So say P1 is +12 and P3 is the return. Probably the hold coil is connected from P1 to P3 and the pull-in coil from P1 to P2.

When 12V is sharply applied to P1 (remember P3 is the return) the gate voltage on the MOSFET rises to 12V, turning it on, and energizing the high current pull-in coil through the MOSFET and D3 (forward biased). The capacitor (probably 1uF) discharges through R1 until the MOSFET turns off and the pull-in coil de-energizes.

The purpose of D2 and D4 is to absorb inductive spikes from the coils. D3 prevents destruction of the pull-in coil if the input polarity is reversed. D1 protects the MOSFET gate from overvoltage.

Edit: See schematic below (drawn more conventionally). 







simulate this circuit &ndash; Schematic created using CircuitLab