Two directional control valves can have the same number of spools, ports, and positions and still make a machine behave very differently. One valve may let an operator blend two functions. Another may stop every downstream function whenever a priority function is engaged.  

The difference often lies inside the valve bank: its series, parallel, or tandem circuitry.

Choosing among these arrangements affects more than hose routing. It influences operator control, actuator sequencing, available flow, pressure demand, heat generation, and the size of the pump and prime mover.  

What Does the Spool Do?

A spool is a closely fitted cylindrical component that moves inside the valve body. Raised areas called lands block passages, while grooves connect selected passages as the spool shifts. These connections direct pump flow to an actuator and route returning fluid toward the reservoir or another valve section.

In a sectional valve, each work section typically controls one cylinder, motor, or other function. Internal galleries connect those sections into a valve bank.

Manufacturers sometimes describe series, parallel, and tandem as spool options, section circuitry, or valve-bank circuitry. The implementation and ordering terminology vary, so the hydraulic schematic and manufacturer’s model code should always be treated as the final authority.

How a Parallel Spool Valve Circuit Works

In a parallel circuit, a common pressure gallery makes pump flow available to each work section. When one spool shifts, it connects that gallery to the corresponding actuator.

Because the sections share the available pump flow, an operator can often command more than one function at a time. This is one reason parallel circuitry is common on mobile equipment requiring flexible, overlapping movements.

Parallel does not mean that every active function receives equal flow. In a basic uncompensated valve, oil follows the path requiring the least pressure. If two spools are fully shifted, the lightly loaded actuator may move first while the more heavily loaded actuator slows or stops. The operator may be able to blend the functions by feathering the spools, but the result will still depend on load, spool opening, pressure loss, and available pump flow.

Pressure requirements do not normally add branch by branch in a parallel arrangement. At a given moment, the pump must develop enough pressure for the highest-pressure active load, plus circuit losses. However, the pump must provide enough total flow for the requested actuator speeds.

A parallel circuit is often a good starting point when:

• Functions must operate independently or in varying combinations.  

• The operator needs to blend movements.  

• There is no rigid priority between functions.  

• The circuit includes appropriate compensation when repeatable load-independent motion is required.  

For example, parallel circuitry may allow lift and tilt functions to be blended on material-handling equipment. Without pressure compensation, however, a large load difference can make one function dominate the available flow.

How a Tandem Spool Valve Circuit Works

A tandem circuit establishes a sequential flow path through the valve bank. Upstream sections have priority over downstream sections.

When the upstream spools are neutral, pump flow can continue through the open-center passage to the downstream sections. When an upstream spool is fully shifted, it diverts the available pump flow to that function and restricts or closes the path feeding the sections that follow it. As a result, a downstream function may slow or stop until the upstream control returns toward neutral.

Careful spool metering may leave some flow available downstream, but that behavior should not be mistaken for guaranteed flow sharing.

Tandem circuitry is useful when:

• One machine function must take priority.  

• Downstream interruption is acceptable or intentional.  

• The desired sequence can be established by valve-bank order.  

• A relatively straightforward open-center system is preferred.  

An OEM might use this arrangement when a steering, clamping, positioning, or other primary function must take precedence over an auxiliary movement. The tradeoff is reduced flexibility: valve-section order becomes part of the machine’s functional logic.

How a Series Spool Valve Circuit Works

In a series circuit, oil returning from an upstream actuator is routed back into the valve’s open-center passage instead of going directly to tank. That returning oil can then supply a downstream work section.

This makes it possible for more than one function to operate from the same stream of oil. Unlike an uncompensated parallel circuit, the downstream function is not simply competing for flow from a common pressure gallery. Its supply is the exhaust flow from the upstream function.

The pressure calculation is the critical limitation. Pump pressure may need to overcome the upstream load, the downstream load, and circuit losses. Those pressure requirements are therefore additive. The total must remain within the ratings of the pump, valve sections, actuators, lines, fittings, and relief system.

Downstream pressure also creates backpressure on the return side of the upstream actuator. That can change the upstream actuator’s available force and movement. With cylinders, the flow passed downstream may also differ from pump inlet flow because rod-side and cap-end displacement volumes are not equal.

Series circuitry can be appropriate when:

• Two functions are intended to operate together.  

• Reusing upstream return flow supports the desired motion.  

• The combined pressure requirement fits the system’s pressure budget.  

• Interaction between the upstream and downstream loads is understood.  

• Section order and actuator area ratios have been evaluated.  

Series circuits should be engineered from the complete operating cycle rather than selected as a general method for adding another function.

Do Not Confuse Tandem Circuitry with a Tandem-Center Spool

“Tandem” has a second common meaning in hydraulic valve terminology.

A tandem-center spool describes the connections inside a directional valve when the spool is in neutral. In the standard tandem-center condition:

• The pressure port is connected to the tank port.  

• Both work ports are blocked.  

This neutral arrangement can unload a fixed-displacement pump while keeping the actuator ports blocked. It does not describe priority among multiple valve-bank sections.

A tandem-center spool also should not be treated as a zero-leakage load-holding device. Spool valves require operating clearance and can leak internally. Suspended or safety-critical loads may require properly selected load-holding valves, such as pilot-operated checks or counterbalance valves, based on the application.

How to Select the Right Circuit

Start with the required machine behavior rather than a preferred valve part number.

• List the operating modes. Identify which functions operate alone, which must overlap, and which combinations must never occur.  

• Define priority. Determine whether any function must retain pump flow when another command is active.  

• Calculate simultaneous flow. Compare the combined actuator flow demand with pump delivery and acceptable function speeds.  

• Build the pressure budget. For a series circuit, include the combined load pressures, backpressure, and passage losses. For every circuit, verify all component and port ratings.  

• Match the pump architecture. Confirm whether the machine uses a fixed-displacement open-center system, a pressure-compensated closed-center system, or load-sensing control.  

• Specify neutral behavior separately. Work-section circuitry does not replace decisions about tandem-center, closed-center, motor, float, regenerative, or other spool conditions.  

• Evaluate metering and transitions. Consider what happens as each spool begins moving, reaches full stroke, and returns to neutral.  

• Verify the complete valve code. Flow rating and port size alone do not establish compatibility. Review the manufacturer’s schematic, section order, spool code, relief arrangement, power-beyond configuration, and allowable tank pressure.  

Common Selection Mistakes

A few assumptions frequently cause trouble:

• Treating parallel circuitry as an equal-flow divider  

• Assuming series operation provides additional pressure without a system-level penalty  

• Expecting downstream functions to remain available when a tandem priority section is fully operated  

• Confusing tandem valve-bank circuitry with a tandem-center neutral condition  

• Ignoring cylinder area ratios in a series circuit  

• Relying on a directional spool alone to hold a suspended load  

• Selecting a replacement valve solely by flow rating, port size, or number of spools  

Design the Valve Around the Machine

Parallel circuitry offers flexible access to pump flow, but uncompensated functions compete according to load and spool opening. Tandem circuitry creates clear upstream priority, but it can interrupt downstream movement. Series circuitry can support coordinated operation by passing return flow onward, but the pressure requirements and actuator interactions demand careful analysis.

No arrangement is universally best. The right choice is the one that produces the required motion throughout the complete operating cycle while staying within the system’s flow, pressure, thermal, and safety limits.

When a standard valve does not provide the needed combination of priority, metering, center conditions, and actuation, Bailey can help evaluate configurable or custom valve options for the application.

Safety note: Hydraulic systems may contain high pressure, stored energy, hot fluid, and loads capable of unexpected movement. Valve selection or modification should be based on the equipment documentation, manufacturer data, applicable standards, and qualified engineering review. Depressurize and secure equipment before service and never use a hand to search for a hydraulic leak.

When winter weather hits, your fleet needs to be ready to work. A slow lift, unresponsive attachment, or unexpected hydraulic leak can leave a truck sidelined when schedules are already stretched.

The time to identify those problems is before the first cold morning.

Winter Readiness Goes Beyond Starting the Engine

Getting a vehicle running is only part of the job. Its hydraulic equipment also needs to respond reliably, from dump bodies and liftgates to snowplows and other work attachments.

Cold-weather preparation raises questions that a routine vehicle inspection may miss:

• Is the hydraulic fluid suitable for the temperatures your equipment will encounter?

• Have seasonal attachments received the same attention as the vehicles carrying them?

• Do operators know when sluggish performance requires stopping and reporting a problem?

The answers matter whether your fleet will work through winter or park equipment until spring.

Give Your Team a Consistent Way to Prepare

Bailey’s Hydraulic Maintenance Checklist for Winter Fleet Preparation gives fleet managers and technicians a practical tool for organizing inspections and documenting what needs attention.

The printable checklist covers preseason preparation, daily winter operation, and storage. It also includes a corrective-work log and release-to-service signoff fields, helping your team carry findings through to repairs.

Use it alongside your equipment manufacturer’s instructions to make winter preparation more consistent across trucks, mobile equipment, and hydraulic attachments.

A hydraulic function stops working. Is the pump failing, or did the controller never command the valve to shift?

Replacing a hydraulic valve will not repair a broken wire, and installing a new solenoid will not free a spool jammed by contamination. Guessing can add parts costs and labor while the equipment remains out of service.

Electrohydraulic equipment makes diagnosis especially challenging because the electrical and hydraulic systems work as a chain. A joystick, switch, sensor, controller, wiring harness, and solenoid may generate and transmit the command. The valve then directs pressurized fluid to a cylinder or motor that performs the work.

A problem anywhere in that chain can produce the same symptom: nothing moves.

The reliable way to distinguish an electrical fault from a hydraulic one is to stop treating symptoms as proof. Instead, trace the command and hydraulic response through the system until you find the point where expected behavior stops.

Why Electrical and Hydraulic Failures Look Alike

In an electrohydraulic system, electricity controls when and how the hydraulic circuit operates. The hydraulic system supplies the force and motion.

A typical command path may look like this:

If an attachment will not raise, for example, several different failures could be responsible:

• The joystick is not producing the expected signal.  

• A safety interlock is preventing the command.  

• A connector has corroded or a wire has broken.  

• The solenoid coil is open, shorted, or receiving insufficient voltage.  

• The solenoid operates, but the valve spool is stuck.  

• Pilot pressure is unavailable.  

• The pump is not delivering adequate flow.  

• A relief valve is bypassing fluid.  

• Internal leakage prevents the cylinder from developing force.  

This overlap is why “it sounds electrical” or “it feels hydraulic” is not a diagnosis. Even fault codes require interpretation. A code may identify an abnormal circuit or sensor value without proving which component caused it.

Symptoms can help establish a starting point, but each clue should be confirmed with a test.

Electrical failures are often associated with abrupt or intermittent operation, warning messages, communication faults, or one function failing after a switch is activated. Hydraulic problems more often appear as weak force, slow movement, heat, noise, leakage, or performance that changes with load.

Those patterns are useful, but neither is universal. A loose electrical connection can fail consistently, while contaminated hydraulic valving can stick intermittently.

A Step-by-Step Diagnostic Process

1. Record the Failure Before Resetting Anything

Begin with the operator’s description and the conditions present when the problem occurred:

• Which function failed?  

• Did it stop suddenly or deteriorate gradually?  

• Was the equipment cold or at operating temperature?  

• Does the failure occur under load, at a particular position, or after a certain sequence?  

• Do other functions still work?  

• Were any alarms, fault codes, unusual noises, leaks, or odors present?  

• Was maintenance or wiring work performed recently?  

Capture active and stored fault codes before disconnecting power or clearing the controller. Also record fluid temperature, engine or motor speed, operating mode, and other relevant conditions. An intermittent fault may disappear before the technician reaches the machine.

2. Control Every Energy Source

Hydraulic equipment can retain dangerous energy after the engine or electric motor is stopped. Suspended loads, charged accumulators, trapped fluid pressure, electrical power, and mechanical motion must all be addressed.

Before disconnecting wiring, removing a valve, or opening a hydraulic connection:

• Lower or mechanically support raised equipment.  

• Shut down the machine according to its instructions.  

• Follow the employer’s equipment-specific lockout/tagout procedure.  

• Isolate electrical and hydraulic energy.  

• Relieve stored pressure according to the manufacturer’s procedure.  

• Discharge accumulators using the prescribed method.  

• Verify the zero-energy state before beginning service.  

OSHA’s hazardous-energy guidance specifically includes electrical, hydraulic, mechanical, pneumatic, and other energy sources. Never use your hand to search for a hydraulic leak; high-pressure fluid injection is a medical emergency.

Some diagnostic measurements require an energized machine. Those tests should be performed only by trained personnel using the equipment manufacturer’s procedure, suitable instruments, and controls that protect everyone from unexpected movement.

3. Use Both the Electrical and Hydraulic Schematics

The electrical diagram shows what must happen before the controller energizes the output. The hydraulic schematic shows what should happen after the valve shifts.

Trace the entire affected function:

1. Start with the switch, joystick, or controller input.  

2. Identify relevant emergency stops, limit switches, neutral switches, and other permissives.  

3. Follow the output circuit to the solenoid or proportional-valve electronics.  

4. Locate the corresponding valve on the hydraulic schematic.  

5. Trace the pressure, pilot, work, drain, and return paths.  

6. Identify the component responsible for holding or moving the load.  

A schematic turns troubleshooting into a sequence of testable conditions. Without one, technicians may need to trace the physical system before meaningful isolation can begin.

4. Determine the Scope of the Failure

Before testing individual components, ask whether the fault affects one function or the entire machine.

If several unrelated hydraulic functions are weak, look for something they share, such as the pump, reservoir, suction circuit, main relief valve, system-enable signal, controller power, or common ground.

If only one function has failed, focus on its local input, wiring, coil, valve section, hoses, and actuator. This simple division can prevent unnecessary testing across the whole machine.

5. Perform a Visual Inspection

Check the accessible, easily verified conditions first.

On the electrical side, inspect for:

• Loose, corroded, wet, or heat-damaged connectors  

• Backed-out or bent terminals  

• Chafed, pinched, or stretched wiring  

• Poor ground connections  

• Blown fuses or tripped protective devices  

• Damaged sensors or switches  

• Signs of an overheated solenoid coil  

On the hydraulic side, inspect for:

• Low, aerated, contaminated, or overheated fluid  

• Restricted or collapsed hoses  

• External leakage  

• Damaged filters or restriction indicators  

• Loose pump couplings  

• Abnormal vibration, cavitation, or mechanical binding  

• Metal or other debris in the oil or filter  

Do not replace a fuse and assume the problem is solved. A blown fuse is evidence of an overcurrent condition that still needs a cause.

6. Verify the Electrical Command at the Valve

The solenoid-operated valve is often the best dividing point between the two systems.

When the function is commanded, determine whether the solenoid receives the voltage or current specified by the manufacturer. Measure at the valve connector with the circuit connected and operating under its normal electrical load when the manufacturer’s procedure permits. An unloaded circuit can appear healthy even though excessive resistance causes voltage to collapse when current flows.

Verify both the supply and return paths. A good voltage reading to chassis ground does not necessarily prove that the coil has a sound ground or return circuit.

With power isolated and the coil disconnected, a technician may also check coil resistance or continuity against the manufacturer’s specification. An open circuit can indicate a broken winding, while abnormally low resistance can indicate a shorted winding. Exact acceptable values vary by coil design and temperature, so a generic resistance value should not be used.

Also remember that a controller may intentionally withhold the output because an interlock or sensor condition has not been satisfied. Never bypass a safety circuit simply to make the function operate.

7. Use a Manual Override Only When Authorized

Some directional valves include a manual override. When the manufacturer allows its use, it can help separate an upstream control problem from a downstream hydraulic problem.

• If the function operates normally with the manual override but not with the electrical command, investigate the input device, controller logic, interlocks, wiring, connector, and coil.  

• If the function still does not operate, investigate the spool, pilot circuit, hydraulic supply, work circuit, actuator, load, and mechanical condition.  

This is not an absolute verdict. A partial spool shift, inadequate pilot pressure, or a load-dependent failure can complicate the result.

A manual override may also bypass normal interlocks and cause immediate equipment movement. Use it only under the OEM procedure, with the work area controlled and personnel clear of hazards.

8. Measure Hydraulic Pressure and Flow

Hearing the pump run does not prove it is delivering the required hydraulic power. Likewise, seeing pressure on a gauge does not prove adequate flow is reaching the actuator.

Use the manufacturer’s specified test points and expected values to check:

• Pump inlet condition  

• Pump outlet flow  

• Standby and working pressure  

• Pressure before and after the directional valve  

• Pilot pressure  

• Work-port pressure  

• Return-line backpressure  

• Pressure and flow with the oil at operating temperature  

• Internal leakage where an approved test exists  

Pressure represents resistance to flow; it does not, by itself, establish pump condition. A system may show low pressure because the pump is worn, a relief valve is open, a valve is not shifting fully, or fluid is escaping through internal leakage. Flow testing and pressure readings taken at carefully selected points help distinguish those possibilities.

All gauges, hoses, fittings, and flowmeters must be rated for the system and installed according to approved procedures.

Common Diagnostic Mistakes

Replacing the Most Obvious Component

A failed function does not automatically mean the cylinder, pump, solenoid, or directional valve has failed. Test the suspected component or circuit before ordering a replacement.

Trusting an Indicator Light Alone

An LED may confirm that a connector received some electrical signal. It does not prove the correct voltage or current reached the coil under load, nor does it prove the spool shifted completely.

Ignoring the Ground or Return Path

Technicians often test the positive supply and overlook corrosion or resistance on the return side. Both sides of the circuit must carry the required current.

Assuming a Click Means the Valve Worked

A click indicates magnetic or mechanical activity, but the spool may have moved only partially. Debris, blocked pilot passages, distorted valve bodies, or inadequate pilot pressure can prevent full shifting.

Adjusting Valves Before Measuring

Changing a relief, flow-control, or compensator setting can conceal the original failure and create a new hazard. Record existing settings and compare measurements with manufacturer specifications before making adjustments.

Changing Several Things at Once

Replacing multiple components or changing several settings destroys valuable diagnostic evidence. Make one controlled change, record it, and retest.

Confirm the Repair and Find the Root Cause

After correcting the fault, operate the complete function through its normal range and under representative working conditions. Verify:

• Correct direction and sequence  

• Normal speed and force  

• Stable system pressure and temperature  

• Proper operation of interlocks and protective devices  

• No external leakage or abnormal noise  

• No recurring fault codes  

• Correct neutral and load-holding behavior  

Then ask why the component failed. A burned coil may have been subjected to low voltage, excessive voltage, an incomplete spool stroke, or improper duty. A stuck valve may point to contamination. A damaged wire may indicate poor routing or inadequate strain relief.

Correcting that underlying condition is what prevents the same downtime from returning.

Diagnose the Boundary Before Replacing Parts

The fastest route to a reliable repair is usually through the point where the electrical command becomes hydraulic action.

Confirm the command. Confirm the valve response. Measure pressure and flow. At every stage, compare what the system is doing with what its schematics and manufacturer documentation say it should do.

That process does more than separate an electrical problem from a hydraulic one. It identifies the specific failure, preserves useful evidence, and helps maintenance teams correct the cause instead of repeatedly treating the symptom.

For application-specific component information, Bailey’s technical resources and support teams can help you review hydraulic and electronic-control requirements. Have the equipment model, component identification, schematic, operating conditions, measurements, and fault history available before requesting guidance.

‍