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CAT 320C Excavator Overview
The Caterpillar 320C is a mid-size hydraulic excavator introduced in the early 2000s, designed for general excavation, demolition, and utility work. Powered by a CAT 3066 turbocharged diesel engine, it delivers approximately 138 horsepower and features a closed-center hydraulic system with two variable-displacement piston pumps. Caterpillar, founded in 1925, has sold hundreds of thousands of 320-series machines globally, and the 320C remains a popular choice for contractors seeking reliability and versatility.
The 320C’s hydraulic system is engineered to prioritize boom and stick functions, with auxiliary flow typically sourced from a single pump. For high-flow attachments such as drum mulchers, processors, or large compactors, operators often seek ways to combine both pumps to increase flow rate and power.
Terminology Notes
By default, the CAT 320C routes one pump to the auxiliary spool. The second pump is reserved for boom and stick functions, and only contributes during specific operations. This design limits the flow available to high-demand attachments. While Caterpillar offers a factory solution—adding a valve section to merge pump flows—this option is expensive and involves extensive plumbing.
Operators and technicians have explored alternative methods to achieve two-pump flow without full valve replacement. These include:
In New Zealand, a technician retrofitted a 320C to run a double-acting motor-driven attachment. He bypassed the factory suggestion of adding a valve slice and instead used a pilot-operated diverter valve and external check valve. The result was a functional two-pump merge without excessive plumbing.
In Canada, a mechanic traced the hydraulic schematic and discovered that the BM2 spool could be used to trigger pump 2 flow by manipulating the bL4 pilot line. He replaced the stop fitting in the spool cap to allow partial shift, customizing flow output. This approach mirrored CAT’s B-series configuration and proved effective on the C-series as well.
In British Columbia, a contractor installed a priority flow control valve—sometimes called a Stanley valve—to stabilize flow for a 65 GPM drum mulcher. The valve maintained consistent attachment performance even when boom or stick functions were engaged. However, the retrofit cost exceeded $10,000.
Recommendations for Technicians and Owners
Merging both hydraulic pumps on a CAT 320C for high-flow attachments is possible through creative use of pilot circuits, spool manipulation, and external valves. While factory solutions exist, field-tested retrofits offer cost-effective alternatives. With careful planning and schematic analysis, operators can unlock the full hydraulic potential of their machines and power demanding tools with confidence.
The Caterpillar 320C is a mid-size hydraulic excavator introduced in the early 2000s, designed for general excavation, demolition, and utility work. Powered by a CAT 3066 turbocharged diesel engine, it delivers approximately 138 horsepower and features a closed-center hydraulic system with two variable-displacement piston pumps. Caterpillar, founded in 1925, has sold hundreds of thousands of 320-series machines globally, and the 320C remains a popular choice for contractors seeking reliability and versatility.
The 320C’s hydraulic system is engineered to prioritize boom and stick functions, with auxiliary flow typically sourced from a single pump. For high-flow attachments such as drum mulchers, processors, or large compactors, operators often seek ways to combine both pumps to increase flow rate and power.
Terminology Notes
- Auxiliary Spool: The hydraulic valve section dedicated to powering attachments.
- Boom II Spool: A valve section that controls the second pump’s contribution to boom raise functions.
- Load-Hold Check Valve: Prevents backflow and maintains pressure in hydraulic cylinders or motors.
- Negative Flow Control (NFC): A pressure signal system that regulates pump displacement based on demand.
By default, the CAT 320C routes one pump to the auxiliary spool. The second pump is reserved for boom and stick functions, and only contributes during specific operations. This design limits the flow available to high-demand attachments. While Caterpillar offers a factory solution—adding a valve section to merge pump flows—this option is expensive and involves extensive plumbing.
Operators and technicians have explored alternative methods to achieve two-pump flow without full valve replacement. These include:
- Shifting the Boom II Spool Oppositely
By activating the opposite pilot port of the Boom II spool, the open-center path is blocked, causing NFC pressure to drop and stroking up the second pump. This flow can then be redirected to the auxiliary circuit.
- Teeing Into Pump Discharge
The discharge line from pump 2 can be teed into the auxiliary spool’s load check port, allowing combined flow. This requires careful pressure management and check valve installation to prevent backfeed.
- Replacing Internal Load Check with External Valve
On some models, the internal check valve under the attachment ports can be removed and replaced with an inline valve. This simplifies routing and avoids interference with pump 2’s supply.
In New Zealand, a technician retrofitted a 320C to run a double-acting motor-driven attachment. He bypassed the factory suggestion of adding a valve slice and instead used a pilot-operated diverter valve and external check valve. The result was a functional two-pump merge without excessive plumbing.
In Canada, a mechanic traced the hydraulic schematic and discovered that the BM2 spool could be used to trigger pump 2 flow by manipulating the bL4 pilot line. He replaced the stop fitting in the spool cap to allow partial shift, customizing flow output. This approach mirrored CAT’s B-series configuration and proved effective on the C-series as well.
In British Columbia, a contractor installed a priority flow control valve—sometimes called a Stanley valve—to stabilize flow for a 65 GPM drum mulcher. The valve maintained consistent attachment performance even when boom or stick functions were engaged. However, the retrofit cost exceeded $10,000.
Recommendations for Technicians and Owners
- Study the Hydraulic Schematic Thoroughly: Understand how pilot lines and spool positions affect pump stroke.
- Use Shuttle Valves for Bidirectional Flow: Necessary for double-acting attachments requiring two-way pressure.
- Install Pressure Reducers for Fine Control: Allows operators to adjust flow output based on attachment needs.
- Avoid Backfeeding Pump 2: Use check valves to isolate circuits and protect components.
- Consider Salvage Parts for Valve Sections: Machines from 318C to 336D share compatible valve slices.
Merging both hydraulic pumps on a CAT 320C for high-flow attachments is possible through creative use of pilot circuits, spool manipulation, and external valves. While factory solutions exist, field-tested retrofits offer cost-effective alternatives. With careful planning and schematic analysis, operators can unlock the full hydraulic potential of their machines and power demanding tools with confidence.
