Diagnosis area
Valve and pilot control
Spools, pilot pressure, proportional solenoids and pressure transients: the control layer between lever and load.
Guides in this area (4)
Each guide lists the checks in the order a workshop should work through them, what each result means, and which parts usually solve it.
Spools stick or lock when cold
Valve and pilot control
A spool that sticks when cold, or grabs in one position, is telling you about oil cleanliness, varnish or bore damage.
Controls sluggish or dead
Valve and pilot control
Pilot pressure moves every control spool. When it is low the whole machine feels tired even with a perfectly healthy main pump.
Hoses burst, seals blow, the gauge needle jumps
Valve and pilot control
Hoses failing in the same place, filters splitting or a gauge needle that jumps are transient problems.
Proportional pilot control reacts late, in steps, or not at all
Valve and pilot control
Machines with proportional pilot control fail in a characteristic way: the function arrives late, arrives in steps, or does not arrive at…
The principle behind these symptoms (10)
Reading the mechanism first usually shortens the checklist.
Why a spool sticks: the forces nobody can see, and the fix that actually works
Technical Guide
A sticking spool is not a worn spool. It is held by a pressure ring, by friction and by dirt in a clearance measured in micrometres - and…
Pilot stages explained: nozzle-flapper and jet-pipe valves in hydraulic pumps
Technical Guide
Why a pump regulator needs a pilot stage, how nozzle-flapper and jet-pipe designs differ in gain and cleanliness tolerance, and the field…
Why a valve whistles or screams: the edge, the volume, or the supply
Technical Guide
A whistling valve is either cavitating at a metering edge, oscillating on its own flow force, or resonating with the oil volume and line.
One valve body, many spools: why the part number decides the behaviour
Technical Guide
Four choices define a spool: working edges, window shape, centre condition and shoulders.
Spool valve lap and metering: why the same valve body feels different on two machines
Technical Guide
Lands, working edges, lap and metering area: how the geometry inside a spool valve decides whether a machine feels crisp, sluggish or will…
Tube, sandwich, manifold or cartridge: how valve mounting decides serviceability
Technical Guide
How a valve is connected decides how long a repair takes and how many places can leak.
Why a hydraulic function overshoots and hunts: damping, correction and what you can change
Technical Guide
Every hydraulic loop needs correcting: gain buys speed but costs stability, and the damping ratio moves with temperature and air.
Measure before you replace: reading a valve load flow curve, and why you must test hot
Technical Guide
A worn pump and a worn valve look identical on site. The load flow curve plus two measurements taken hot will tell them apart before you…
Three ways a two-stage valve positions its spool - and why the failure modes differ
Technical Guide
Force feedback, direct feedback and spring centring all make a two-stage valve work, and each fails in its own way.
How clean does hydraulic oil need to be? Targets by system type
Technical Guide
Roughly 80% of hydraulic failures start at the oil supply. Here are the particle-size targets by system type, and where contamination…
Units we supply for this topic (4)
Catalogue-verified units whose internals are the ones described above.
K3V112DTP
20–25 t hydraulic excavator
112 mL/r (per pump) · 34.3 MPa
K5V200DTH
40–45 t hydraulic excavator
200 mL/r (per pump) · 34.3 MPa
K3V112S
10–12 t excavator and industrial open circuits
112 mL/r · 34.3 MPa
K3V140DTP
27–33 t hydraulic excavator
140 mL/r (per pump) · 34.3 MPa
Parts that solve this (6)
Catalogue part lines from the same families. Search the full library by part number from the parts library.
2953802048
CHECK VALVE ASS'Y · pos. 041
2953802049
CHECK VALVE ASS'Y · pos. 042
2933180-0416
CHECK VALVE ASS'Y · pos. 042
PAJ503518
NEEDLE BEARING · pos. 124
PRNA6932
NEEDLE BEARING · pos. 124
PAJ503806
NEEDLE BEARING · pos. 124
Machines this applies to (6)
Machine pages that list the units above as fitment, model by model. See every machine →
SANY SY215
Machine page
Pump, swing motor and travel assembly options for this machine, with the fitment list.
SANY SY225
Machine page
Pump, swing motor and travel assembly options for this machine, with the fitment list.
VOLVO EC210B
Machine page
Pump, swing motor and travel assembly options for this machine, with the fitment list.
VOLVO EC210D
Machine page
Pump, swing motor and travel assembly options for this machine, with the fitment list.
DOOSAN DH220-5
Machine page
Pump, swing motor and travel assembly options for this machine, with the fitment list.
HYUNDAI R220-5
Machine page
Pump, swing motor and travel assembly options for this machine, with the fitment list.
Other diagnosis areas
Pump and pressure supply
Pressure, delivery, wear and the pump-engine match: the guides that start at the source of the oil.
Swing, travel and drive assemblies
Motor and reduction-gear complaints: slow swing, weak travel, run-away, noise and metal in the gear oil.
Boom, arm and cylinder functions
The working functions: drift, creep, jerky movement and every-function-at-once complaints.
Oil, filtration and the system as a whole
Oil condition, water and air, cleanliness, climate and the system-level decisions that follow.