139-3990 Caterpillar Hydraulic Main Pump Signal Pressure Solenoid (also known as the Main Pump Proportional Solenoid) is a core electro-hydraulic control component within Caterpillar excavator hydraulic systems, responsible for regulating the main pump's flow and pressure signals.

I. Core Parameters

Model: 139-3990 (1393990)

Type: Signal Pressure Regulating Solenoid (Proportional)

Voltage: 24V DC

Interface: 2-pin circular connector; cable length: 350mm

Rated Pressure: 4900 kPa (approx. 49 bar)

Control Flow: 2 L/min

Port Size: No. 6 (9/16-18)

Universal Replacement: 5I-8368 (Fully interchangeable)

II. Function and Application

Core Function: Receives electrical signals from the ECU to regulate the output hydraulic signal pressure, thereby controlling the main pump's swashplate angle to optimize excavator operating speed, compound movements, and power matching. 

Compatible Engines: 3054, 3306, 3406, 3406B

Applicable Models (Primarily Excavators):

Medium: 311, 312, 315, 317, 320, 320B, 320C, 322, 325

Large: 330, 330L, 350, 350L, 375, 375L

Other: 5080 Face Shovel

III. Replacements and Pricing

Replacement Models: 5I-8368, CA1393990, CA5I8368

IV. Common Faults and Troubleshooting

Weak Operation / Slow Speed: Solenoid spool stuck, coil damaged, or insufficient signal pressure.

Uncoordinated Compound Movements: Abnormal signal pressure regulation; requires cleaning or replacement of the valve spool.

Installation Notes: Install in the vicinity of the main pump; ensure the hydraulic circuit is clean and the seals are leak-free.