How to Read Solenoid Valve Symbols: Hydraulic and Pneumatic Valves Side by Side

You open a catalog or a hydraulic circuit diagram and see a row of boxes, arrows and T-shaped marks. How do you tell where the spool sits when the solenoid is de-energized, and which ports are connected to each other?

A solenoid valve symbol uses boxes to show the spool positions, and arrows and blocking marks to show how the ports connect. Once you can read these symbols, you can tell the flow path in each valve position and the starting state when the power is off.

This article uses actual symbols from the Tai-Huei catalog to explain how to read hydraulic solenoid directional valve symbols and pneumatic solenoid valve symbols, so you can work out what a valve does straight from the drawing.

Sources: the spool function tables in the Tai-Huei G02 and G03 series catalog, and the AS/AD series pneumatic solenoid valve catalog. The spool functions described here have been confirmed by Tai-Huei engineers.

Key Points

1. Count the boxes. The number of boxes equals the number of valve positions. Each box shows how the ports connect when the spool is in that position.
2. Check both ends. A box with a diagonal line is the solenoid operator, and a zigzag line is a spring. A valve with a solenoid at one end only is single-solenoid; a valve with solenoids at both ends is double-solenoid.
3. On a three-position valve, read the center box first. When both solenoids are de-energized, a spring-centered spool returns to the center position. The center flow path decides whether the pump unloads and whether the cylinder can hold its position.
4. Check the ports. Tai-Huei hydraulic solenoid directional valves have four ports: P (pressure), T (tank), and A and B (work ports).

What Are the Parts of a Solenoid Valve Symbol?

A solenoid valve symbol is made up of boxes, arrows, blocking marks and operators. Once you know what each element means, you can read the number of positions, how the oil or air flows, and how the spool shifts and returns, straight from the catalog symbol.

Symbol element	Symbol	What it means	What to watch for
Box		One working position of the spool	The number of boxes is the number of positions: two boxes for a two-position valve, three boxes for a three-position valve.
Arrow		Direction of oil or compressed air flow	Shows the flow path in that position. Read it together with the ports each arrow connects.
T-shaped blocking mark		The port is blocked in that position	If every port in the center box is drawn with a blocking mark, none of the ports connect to each other in that position.
Solenoid operator		The spool is shifted by solenoid force	Read the full symbol and the catalog markings to see which side the solenoid is on. The letters a and b can also mark the operator end or the position, so do not judge the solenoid side from the letter alone.
Zigzag line		Spring return	When the solenoid is de-energized and no other force acts on the spool, the spring returns it to its designed position. Three-position valves usually return to center.
Detent symbol		Mechanical detent	Shows that a detent holds the spool in the selected position after it shifts, so the spool does not spring back when the solenoid is de-energized. Always check the actual symbol in the catalog.

Solenoid valve symbols are drawn to ISO 1219-1, the standard for graphical symbols of fluid power systems and components. When you read a symbol, check the full symbol, the port markings and the catalog for that model. Never judge what the valve does from a single element.

Two-Position Hydraulic Solenoid Valve Symbols: 2A2, 2B2, 2D2 and 2D2D

A two-position hydraulic solenoid directional valve has two working positions. Besides the flow paths, check where the solenoid, spring and detent are, so you know what the spool does when the solenoid is de-energized.

Spool code and description	Hydraulic symbol
2A2: solenoid on side a, spring return Actuation: the solenoid is on side a (the B-port side of the valve body); a spring is on the other side. When de-energized: the spring returns the spool to its normal position.
2B2: solenoid on side b, spring return Actuation: the solenoid is on side b (the A-port side of the valve body); a spring is on the other side. When de-energized: the spring returns the spool to its normal position.
2D2: double solenoid, no detent Actuation: solenoids on both sides, no detent and no spring return. When de-energized: the spool stays in the last position it shifted to. It does not return on its own. Mounting: the valve must be mounted with the spool axis horizontal.
2D2D: double solenoid with detent Actuation: solenoids on both sides with a detent and no spring return. When de-energized: the detent holds the spool in the last position it shifted to. It does not return on its own.

What Does an L at the End of the Code Mean?

Take the 2A2L and 2B2L in the Tai-Huei catalog: L stands for reverse assembly. It moves the solenoid to the other side to suit the space available on the machine.

Standard assembly	Reverse assembly (L)
2A2: solenoid on side a (B-port side of the valve body)	2A2L: solenoid on side b (A-port side of the valve body)
2B2: solenoid on side b (A-port side of the valve body)	2B2L: solenoid on side a (B-port side of the valve body)

When selecting a valve, compare the symbol and assembly direction of that exact model to confirm the solenoid side and the spring arrangement. Do not substitute one model for another based on the code alone.

Three-Position Hydraulic Solenoid Valve Symbols: How to Read 3C Center Conditions

Spool codes starting with 3C indicate three-position valves with a solenoid on each side. When both solenoids are de-energized, the springs on both sides push the spool back to center. So the key to choosing a three-position valve is which ports are connected and which are blocked in the center box.

Spool code and description	Hydraulic symbol
3C2 Center: P, T, A and B all blocked (closed center). Both cylinder ports and the pump are cut off.
3C3 Center: P, T, A and B all connected (open center). The pump unloads, and both cylinder ports are open to tank.
3C4 Center: P blocked; A, B and T connected (float center). The pump does not unload. Both cylinder ports are open to tank.
3C5 Center: P, A and T connected; B blocked. Side A connects to the pump and tank. Side B is blocked.
3C9 Center: P, A and B connected; T blocked (regenerative center). Pump flow goes to both sides of the cylinder.
3C6 Center: P and T connected; A and B blocked (tandem center). The pump unloads and both cylinder ports are blocked. Closed crossover: all four ports are blocked for an instant during the shift.
3C60 Center: P and T connected; A and B blocked (tandem center). The pump unloads and both cylinder ports are blocked. Open crossover: all four ports are connected for an instant during the shift.

The 3C6 and 3C60 have the same center condition. The difference is the crossover, the transition state the spool passes through while shifting: the 3C6 has a closed crossover (all ports blocked for an instant), and the 3C60 has an open crossover (all ports connected for an instant). That is why their symbols have more boxes than other three-position valves. The extra boxes show the crossover states.

Once You Can Read the Symbol, How Do You Match It to a Tai-Huei Spool Code?

To see the symbols for every G02 and G03 spool, open G02-G03 Spool Symbols on our Technical Support page.

The number after the prefix names the flow path. Different numbers mean different port connections, so always go by the catalog symbol. The G02 and G03 catalog lists dozens of spools, and we also design custom spools. Tell us the center condition and the de-energized state your circuit needs, and we will match them to the right spool code.

Solenoid Valve Symbol FAQ

Which center condition keeps the cylinder in place during a power failure?

Start with a center condition that blocks both A and B, such as the Tai-Huei 3C2 (all four ports blocked). The 3C6 and 3C60 also block A and B, but they connect P to T, so check whether you need the pump to unload.

Blocking A and B cuts off the return path through the directional valve on both sides of the cylinder, but it does not lock the cylinder completely. Internal clearance in a spool valve can cause leakage, and an external load can make the cylinder creep. If you need to hold position for a long time or keep a load from dropping, evaluate a pilot-operated check valve, a counterbalance valve or other load-holding measures for your system.

Where on the symbol do I look to see if the pump unloads in center?

Look at whether P and T connect in the center box. If P and T connect directly in center, pump flow can return to tank through the directional valve and the pump unloads. The actual unloading still depends on the design of the whole circuit.

Among Tai-Huei spools, the 3C3 (all ports connected), 3C6 and 3C60 all connect P and T in center, while the 3C2 and 3C4 block P in center. So do not choose on unloading alone. Also check how A and B connect and what the cylinder needs to do.

What is the difference between 2B2 and 2B2L, and which should I choose?

Both have the same spool function. The difference is which side the solenoid is mounted on: the 2B2 is the standard assembly with the solenoid on side b, and the 2B2L is the reverse assembly with the solenoid on side a.

If there is not enough room on side b of the machine but you still need the 2B2 flow path, consider the 2B2L. Before ordering, check the catalog for how the solenoid side is defined and which way the valve mounts.

Where does a 2D2 stop when the power is off?

The 2D2 is a double-solenoid design with no spring return and no detent. Once the spool shifts to a position, it stays in the last position it shifted to even after the solenoid is de-energized. It does not return to a starting position the way a spring-return spool does.

The 2D2 must be mounted with the spool axis horizontal. If you need a detent to hold the spool in position, choose the 2D2D. For both the 2D2 and the 2D2D, the state after the power is off depends on which position the spool last shifted to.

Pneumatic Solenoid Valves: How to Read 5/2 and 5/3 Symbols

Pneumatic solenoid valve symbols are read the same way as hydraulic directional valve symbols: count the boxes for the number of positions, then use the arrows and blocking marks to see how the ports connect.

The Tai-Huei AS/AD series uses five ports: P is the supply port, A and B are the working ports, and E1 and E2 are the exhaust ports. So two boxes mean a 5/2 valve (five ports, two positions), and three boxes mean a 5/3 valve (five ports, three positions).

Model and description	Pneumatic symbol
AS15/AD15: single solenoid, two-position (5/2) Actuation: solenoid on one side, spring on the other. When de-energized: the spring pushes the spool back to its return position, and the port connections switch with it.
AS25/AD25: double solenoid, two-position (5/2) Actuation: solenoids on both sides, no spring return. When de-energized: with both solenoids off, the spool stays in the last position it shifted to. It does not return on its own.
AS35/AD35: double solenoid, three-position (5/3) Actuation: solenoids on both sides, spring-centered. When de-energized: with both solenoids off, the spool returns to center. If all five ports in the center box are drawn with blocking marks, it is a closed-center valve with all five ports blocked.

How do I choose between a 5/2 and a 5/3 valve?

Start with whether the cylinder needs to stop mid-stroke. If it only needs to extend and retract, choose a 5/2 valve. Choose the single-solenoid AS15 or AD15 if you want a spring to return the valve to a set position when the power is off. Choose the double-solenoid AS25 or AD25 if you want two signals to control the valve separately and keep the last position when the power is off.

If the cylinder needs to stop partway, choose the 5/3 AS35 or AD35, which returns to closed center when both solenoids are de-energized.

What do 302, 303 and 304 at the end of the model number mean?

These numbers give the port size: 302 is PT 1/4 (Rc 1/4), 303 is PT 3/8 (Rc 3/8) and 304 is PT 1/2 (Rc 1/2), with effective areas of 31, 40 and 42 mm² respectively.

What are the operating pressure and temperature ranges of the AS/AD series?

According to the Tai-Huei catalog, the operating pressure is 0 to 10 kgf/cm² (approx. 0 to 0.98 MPa, 9.8 bar or 142 psi) and the operating temperature is 5 to 60°C. The response time is 30 ms or less, and the operating frequency is 360 cycles per minute.

Will the cylinder return on its own when an AS25 or AD25 loses power?

Not necessarily. In the Tai-Huei AS/AD lineup, the AS25 and AD25 are double-solenoid two-position valves. When both solenoids are de-energized, the spool stays in the last position it shifted to and does not return on its own.

If you want a spring to push the valve back to a set position when the power is off, consider the single-solenoid, spring-return AS15 or AD15. Keep in mind that a returned spool does not guarantee the cylinder returns. What the cylinder actually does still depends on the valve port connections, the air supply and the load.

What state are the five ports in when both AS35 or AD35 solenoids are de-energized?

The AS35 and AD35 are double-solenoid, three-position (5/3) valves. When both solenoids are de-energized, the springs push the spool back to center, where all five ports are blocked. This is called a closed center.

A closed center can stop the cylinder mid-stroke, but it is not a mechanical lock. If the machine has a vertical load, a risk of a falling load or a personnel safety concern, add a mechanical lock or other safety measures.

For the difference between direct-acting and pilot-operated pneumatic valves, plus port size and flow selection and how to troubleshoot a sticking valve, see Comprehensive Selection Guide for Pneumatic Solenoid Valves: Principles, Specifications & Troubleshooting.

Further Reading

• How to Select Hydraulic Solenoid Valve Size: Flow Rating Chart and ISO 4401 Mounting Interfaces
• Comprehensive Selection Guide for Pneumatic Solenoid Valves: Principles, Specifications & Troubleshooting
• Hydraulic Workholding Solenoid Valves: A Case Study from a Korean Machine Tool Builder

References

ISO 1219-1:2012 Fluid power systems and components — Graphical symbols and circuit diagrams — Part 1

ISO 4401:2005 Hydraulic fluid power — Four-port directional control valves — Mounting surfaces