Combined-operation faults are flow-allocation faults. When two functions run together the pumps either sum their flow into one circuit or split it between two, and the control decides which based on demand signals from the pilot circuit. If both functions slow only when used together, check in this order: whether the machine is in a flow-summing mode at all, whether the straight-travel spool shifts and holds, and whether pilot pressure reaches both pump regulators at the same moment. A worn pump makes every function slow all the time, including single-function work, so it is the last thing to suspect here. Measure cycle time for one function alone, then repeat with the second function held at the same time. The difference between those two numbers is the fault, and its size tells you whether you are losing a whole pump's share or only a signal.
What you will notice
| Symptom | What it usually points to |
|---|---|
| Digging is normal until the travel pedal is pressed | Flow is being taken away from the front functions. Points at the straight-travel spool or at pump summing, not at wear. |
| Both tracks and the boom slow by a similar amount | A shared supply problem. Both pumps are being asked to do more than the circuit is being given. |
| One function slows and the other keeps its speed | Not a sharing fault. The losing function has its own restriction: spool, relief valve or pilot signal. |
| Slow only when the oil is hot, normal when cold | Leakage that grows with temperature. Look at the summing valve and spool clearances before the pump. |
| Engine note rises but the machine does not speed up | The pump is being asked to stroke and the flow has nowhere useful to go. Suspect a valve holding oil over relief. |
Most likely causes
| Cause | How to confirm it |
|---|---|
| Straight-travel spool not shifting or not holding | Travel and front functions are supposed to receive a fixed split. Confirm spool travel with the pedal pressed and watch whether the split changes as load changes. |
| Pilot signal reaching only one regulator | One pump strokes and the other stays near standby. Tee gauges into both regulator inlets and compare while both functions are held. |
| Flow-summing valve stuck closed | The machine behaves like a single-pump machine in combined work and normally in single-function work. Compare cycle times for the same function alone and combined. |
| Main relief passing on one circuit | Oil that should be doing work is going over relief, and the loss appears only when that circuit is loaded together with another. |
| Negative-control or load-sensing signal out of adjustment | The pumps do not receive the demand they should. This needs the specified test procedure and a gauge, not a guess. |
Combined-operation results, and what each one eliminates
| What you measured | What it rules in or out |
|---|---|
| Slow with travel and boom together, normal separately | Supply allocation. Straight-travel spool, summing valve or regulator signal. |
| Slow in every combination, including single functions | Pump delivery or a main relief. This is the wear case, and it is not what this article covers. |
| Normal cold, slow hot, only in combination | Internal leakage in the summing valve or in the spool bores. |
| One side of the machine slow in every mode | That side's pump, regulator or relief. Split the two pumps before touching any valve. |
| Slow after a hose or a valve was replaced | A crossed or restricted pilot line, or a spool fitted the wrong way round. |
How to confirm it
- Measure single-function cycle time first
Time a full boom raise from ground to relief, and a 20 m travel in each direction, with the oil at operating temperature. A number written down is the only way to know later whether a repair changed anything.
- Repeat with both functions held at the same time
Hold the boom raise and press the travel pedal together, and time the same boom movement. Compare against the baseline. A drop of more than a few tenths of a second is a real allocation fault rather than operator feel.
- Read both regulator inlets while both functions are held
Tee gauges into the inlet of each pump regulator and watch them together. Both should move when both functions are commanded. One that stays at standby pressure means the signal is missing, not the pump.
- Confirm the straight-travel spool moves and then holds
With the pedal pressed, check spool travel and whether it stays put as load changes. A spool that shifts and then drifts back is the classic cause of a machine that pulls to one side under load.
- Prove pump delivery last, and separately
Only when both regulators receive correct signals and the split is still wrong does pump delivery become relevant. Measure flow at the pump outlet rather than at a function, so valve losses do not confuse the reading. Guangzhou Shenyue Machinery Technology Co., Ltd. supplies regulators, straight-travel spools and confluence valves matched to the pump code rather than to the model name alone. Stock is held as 3,000+ part types on the shelf, with 100,000+ part numbers available to order.
What to do about it
If one regulator never receives its signal, repair the pilot line first A crushed or crossed hose after a repair is common, and it produces exactly the symptom of a failed pump while the pump is fine. Follow the line from the manifold to the regulator with the machine running and both functions held.
If the spool shifts but does not hold, replace the spool and springs as a set A spool that holds cold and drifts hot has worn bores, and a new spool in a worn bore will not hold either. Measure the bore before ordering, or the second repair costs as much as the first.
If the summing valve is stuck closed, clean it and then find the contamination A valve that sticks once will stick again, and the second failure usually happens with a full bucket at height. Cut the return filter open and look at what it captured before returning the machine to work.
If single-function cycle times were already long, the machine has two faults The allocation problem is the smaller one. Fix delivery first: a machine that cannot make flow cannot share it. Guangzhou Shenyue Machinery Technology Co., Ltd. can confirm the pump and valve configuration from the machine serial number, which is the only reliable way to order the correct variant.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Pump regulator assembly | Genuine; OEM-quality. Matched to the pump code stamped on the housing. |
| Straight-travel spool and springs | Genuine; OEM-quality. Supplied as a set; bore wear must be measured first. |
| Confluence (flow-summing) valve | Genuine; OEM-quality. Seal kit quoted with the valve where available. |
| Pilot hose assemblies | Genuine; OEM-quality; aftermarket. Length and end fittings confirmed from the drawing. |
| Main relief valve | Genuine; OEM-quality. Setting verified on a gauge before dispatch where adjustable. |
Need the parts this covers?
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Request a quoteFrequently asked
Why is the machine fine on its own but slow when I travel and dig at the same time?
Because the two conditions ask different things of the same oil. Alone, one pump can serve one circuit completely. Together, the machine has to decide how to split or combine two pumps, and that decision is made by the straight-travel spool and the summing valve. A fault there costs nothing in single-function work and everything in combined work, which is why the symptom looks so selective.
Could a worn pump cause this?
Only if single-function cycle times are also long. A worn pump loses delivery across the board and usually loses more when the oil is hot. If the machine meets its cycle time for one function and misses it only when two are combined, the pump is being asked a question it can answer; the answer is being taken away upstream of it.
Is it safe to keep working with this fault?
Short term, yes, with one caution. Oil that cannot reach a function passes over a relief valve, and relief flow turns directly into heat. A machine working in combined operation with an allocation fault runs its oil hotter than the gauge suggests, and hot oil accelerates every leak already on the machine.