Three measurements separate a tired motor from a tired pump: case drain flow at operating temperature, shaft speed against the flow the pump is actually delivering, and stall pressure at the motor inlet. Case drain above the manufacturer's limit means internal leakage across the pistons, the valve plate or the shaft seal, and no adjustment will recover it. Speed that is low while inlet flow is correct means the motor is bypassing. Full stall pressure with no rotation means the motor is seized or its brake has not released. Every reading is taken with the machine blocked and the circuit relieved, and none of them requires removing the motor.
What the readings mean together
| Reading | What it points to |
|---|---|
| Case drain within limit, speed low | The motor is sound. Look at pump delivery, the control valve section, or a flow divider sending oil elsewhere. |
| Case drain above limit, speed low | Internal leakage in the motor. Seal kits will not recover worn pistons or a scored valve plate. |
| Speed correct, torque low under load | Relief set low, or leakage that only opens as pressure rises. Re-test case drain under load. |
| Will not turn, full pressure at the inlet | Motor seized, brake not releasing, or a mechanical fault downstream of the shaft. |
| Turns freely off load, stalls early | Wear that closes up when the motor is unloaded and leaks once pressure arrives. Motor replacement is the usual answer. |
How to confirm it
- Isolate the machine before anything is opened
Block or lower the attachment, park on firm level ground, stop the engine, and relieve stored pressure at the tank or the test port. A hydraulic motor stores energy in the circuit and in whatever it is driving; treat it as live until the gauge reads zero.
- Measure case drain at operating temperature
Run the circuit until the oil is warm, then route the case line into a graduated container for sixty seconds at a known speed and compare the volume with the manufacturer's limit. Cold oil under-reports leakage badly, so warm it first or the test is worthless.
- Measure shaft speed against actual inlet flow
Fit a tachometer to the output shaft and measure flow at the motor inlet with a flow meter or a flow tee. Divide flow by displacement to get theoretical speed, then compare. A large shortfall with correct flow at the inlet is leakage inside the motor, not a pump problem.
- Stall the motor briefly and read inlet pressure
Load the motor to a standstill against its mechanical stop or brake for a few seconds only, and read pressure at the inlet. Full relief pressure with no rotation isolates a seized motor or a brake that will not release; low pressure moves the search back to the valve and the pump.
- Check the brake release and the circuit valve first
On travel and swing motors, confirm that release pressure actually arrives and that the brake piston is venting. A motor condemned for a stuck brake is an expensive way to learn this, and it takes ten minutes to rule out.
- Repeat the readings and write them down
Take each measurement twice and record temperature, speed and flow alongside it. A single reading in isolation cannot be defended, and the second reading is what shows whether a fault is stable or opening up with heat.
What to do about it
Case drain high with correct inlet flow: the motor is worn internally and needs a rotating group overhaul or exchange. A seal kit only addresses external leakage and shaft seal failure, so fit one only when the drain figure is within limit.
Low speed with low inlet flow: the motor is being starved. Check pump delivery, the section spool stroke, pilot pressure to that section, and any flow divider ahead of it before ordering anything for the motor.
Low stall pressure: check the relief setting and the pump's ability to hold pressure under flow. A relief that is set low looks exactly like a weak motor on a seat-of-the-pants test.
No rotation at full pressure: confirm the brake release before dismantling. If release pressure is present and the shaft still will not turn, the motor is mechanically failed and exchange is usually cheaper than repair.
Whatever the finding, hydraulic motors, seal kits and brake components are quoted by Guangzhou Shenyue Machinery Technology Co., Ltd., with 3,000+ part types on the shelf and 100,000+ part numbers available to order, each line marked genuine, OEM-quality or aftermarket.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Motor seal kit | Genuine; OEM-quality. Cures external and shaft seal leaks only, not internal wear. |
| Rotating group and valve plate | Genuine; OEM-quality. The economical repair when the housing and shaft are sound. |
| Complete motor assembly | Genuine; OEM-quality new; remanufactured exchange with core return. |
| Brake release valve and piston | Genuine; OEM-quality. Confirm release pressure before replacing. |
| Case drain hose and fittings | OEM-quality. A restricted case line raises internal pressure and destroys seals. |
Need the parts this covers?
Send the machine model, serial number and the part numbers you need. We confirm fitment before payment and state the grade of every line — genuine, OEM-quality or aftermarket.
Request a quoteFrequently asked
Can I test a motor without a flow meter?
Yes, with limits. Case drain volume needs only a container and a timer, and stall pressure needs only a gauge. Shaft speed against flow is the one reading that really wants a meter, because without it you cannot separate a starved motor from a leaking one. If no meter is available, compare the suspect circuit against an identical one on the same machine.
Is high case drain always a failed motor?
No. A restricted or undersized case drain line, a crushed hose or a blocked return filter raises case pressure and forces oil past seals that were healthy. Check the case line and the return path before you accept a motor diagnosis, and re-measure after correcting them.
How long can I hold a stall test?
Seconds, not minutes. Stalling converts all the hydraulic power into heat inside the motor and the relief, and it can damage a healthy unit. Read the gauge, release the load, and let the oil circulate before repeating.