Excavator Control Valve Secondary Pressure Adjustment — What Most Operators Get Wrong
Everyone talks about main relief pressure. Nobody talks about secondary pressure, and that is exactly why so many machines feel heavy, drifty, or inconsistent under load. Secondary pressure — sometimes called pilot control pressure or balance pressure — sits behind the main spool in the control valve and does the quiet work of keeping every cylinder move smooth and proportional. When it is off, the machine fights you. When it is right, the levers feel like they are reading your mind.
What Secondary Pressure Actually Does Inside the Valve
The main spool in a load-sensing control valve has pressure pushing on both ends. One side gets the load pressure from the cylinder. The other side gets the secondary pressure, which acts as a counterbalance. This balance determines how much effort you need to move the lever and how fast the spool shifts.
If secondary pressure is too low, the load pressure overwhelms the spool. The lever feels heavy, the machine responds slowly, and the cylinder crawls even when you push the lever to the stop. If secondary pressure is too high, the spool shifts too fast. The machine lurches, the bucket slams into the ground, and the seals take a beating every time you change direction.
Secondary pressure also affects how the load-sensing signal works. The pump adjusts its output based on the difference between main pressure and secondary pressure. If that difference is wrong, the pump either starves the system of flow or dumps excess flow back to the tank, wasting fuel and generating heat for no reason.
This is why secondary pressure adjustment matters just as much as main pressure — maybe more — and why skipping it is a mistake.
How to Find and Adjust the Secondary Pressure Valve
Locating the Secondary Pressure Relief Valve
The secondary pressure valve is usually a small cartridge or poppet-style valve mounted on the side of the main control valve block or on the pump compensator. It looks different from the main relief valve — smaller, with a finer adjustment screw. On some machines, it is tucked behind the pump housing and you need to remove a cover to reach it.
Check the service manual for the exact location. Do not guess. The secondary pressure valve is often confused with the pilot relief valve, and adjusting the wrong one gives you the wrong result. They look similar but they control completely different circuits.
The adjustment screw is usually a small hex or slotted screw with a locknut. Mark the locknut position before you touch anything. A paint pen works fine.
The Adjustment Procedure That Actually Works
Start the engine and let it idle until the hydraulic oil reaches operating temperature. Secondary pressure changes with temperature, so a cold adjustment will be wrong the moment the machine warms up.
Connect a pressure gauge to the secondary pressure test port. If there is no dedicated test port, you can measure it at the pilot line coming off the valve block. Make sure the gauge is rated for the pressure range — usually 20 to 50 bar for secondary circuits.
With all levers in neutral, the secondary pressure should sit at the factory specification. If it does not, turn the adjustment screw slowly. Clockwise increases pressure. Counterclockwise decreases it. Quarter turns only.
After each adjustment, move the boom lever slowly through full travel while watching the gauge. The pressure should stay steady throughout the stroke. If it climbs as the cylinder loads up, the secondary pressure is too low — increase it slightly. If it drops when the cylinder hits resistance, the secondary pressure is too high — back it off.
The goal is a flat, consistent reading from neutral to full travel. No climb, no drop. Just steady pressure doing its job.
Why Secondary Pressure Drifts and How to Catch It Early
Temperature Is the Biggest Enemy
Secondary pressure is extremely sensitive to oil temperature. When the fluid heats up, it thins out, and the pressure drops. When it cools down, the fluid thickens, and the pressure climbs. A machine that feels perfect at 8 AM can feel like a different machine by 2 PM — not because anything broke, but because the secondary pressure shifted with the oil temperature.
This is why factories set the secondary pressure based on a specific oil temperature range. If your machine runs hotter than normal — due to a dirty cooler, low fluid level, or heavy continuous work — the actual secondary pressure will be lower than the gauge reading suggests. The valve behaves as if the pressure is low even though the screw is set correctly.
Fix the root cause first. Clean the cooler. Top off the fluid. Let the machine cool down. Then readjust the secondary pressure at the correct oil temperature.
Wear Changes Everything Over Time
The secondary pressure valve has a small spring and a precision poppet. Over thousands of hours, that spring loses tension. The poppet seat wears. The valve starts leaking internally, and the pressure it holds drops without any visible sign.
This is why a machine that felt great for two years suddenly starts feeling heavy. The spring is tired. The adjustment screw cannot compensate for a worn spring — it can only mask the symptom for a while.
Check the secondary pressure every 500 hours. If the reading is more than 5 bar off spec and the adjustment screw cannot bring it back, replace the valve cartridge. It is a small part and a cheap fix compared to the damage caused by running with the wrong pressure for weeks.
How Secondary Pressure Affects Each Function Differently
Boom and Arm Circuits
These circuits carry the heaviest loads, so secondary pressure has the biggest impact here. If the secondary pressure is low, the boom drops when you release the lever, even though the main relief valve is set correctly. The cylinder cannot hold position because the spool is not balanced.