Hydraulic starter fault finding: pre-charge, pressure and no crank
When a hydraulic starting system gives one weak crank and then nothing, the cause is almost always a lost accumulator nitrogen pre-charge rather than a failed starter. The accumulator, not the starter motor, is what stores the energy. If the nitrogen behind the bladder has leaked away, the accumulator will still show system pressure on the gauge while holding a fraction of the usable oil it was sized to deliver.
It is invisible unless you go looking for it, and in our experience it is the first thing worth checking on any hydraulic starting system that has stopped delivering the cranks it used to. This guide covers how to diagnose it, along with the other faults that stop a hydraulic starting system doing its job. If you want the mechanism explained instead, see how a hydraulic engine starter works.
Before you start. A charged accumulator holds a large amount of stored energy at pressures typically around 200 to 210 bar. Isolate and fully discharge the oil side before disconnecting any component or touching the gas valve. Use nitrogen only. Never charge an accumulator with compressed air or oxygen.
The accumulators, not the starter motor, are where a hydraulic starting system holds its energy.
Why the accumulator is the thing to suspect first
A bladder accumulator holds nitrogen on one side of a flexible bladder and hydraulic oil on the other. The nitrogen is compressed as the system pumps oil in, and it is that compressed gas that pushes the oil back out through the starter when you crank.
The pre-charge is the nitrogen pressure in the accumulator with no oil in it. On a starting system it is normally set to around 90 per cent of the starter's minimum working pressure, so that the accumulator delivers usable oil right down to the point where the starter can still produce cranking torque.
Nitrogen migrates slowly through the bladder over time. That is normal and it is why pre-charge is a scheduled check rather than a fit and forget item. What it does to the system is not obvious:
- The gauge still reads full. The pump will still charge the system to its normal pressure, so a routine glance at the gauge tells you nothing is wrong.
- The usable oil volume collapses. An accumulator normally delivers roughly a quarter to a third of its nominal volume as usable oil. Lose the pre-charge and that fraction falls away, because there is less compressed gas to push the oil back out.
- You get fewer cranks. A system sized for three consecutive cranks may manage one, and it will do it weakly.
So the classic symptom set is: pressure looks normal, first crank is short or slow, second crank does not happen.
The bladder holds the nitrogen pre-charge. Lose it and the gauge still reads full while the usable oil collapses.
How to check the pre-charge
- Isolate the accumulator from the system and fully discharge the oil side, so there is no oil pressure acting on the bladder.
- Fit a charging and gauging rig to the gas valve. Reading it any other way is guesswork.
- Read the nitrogen pressure and compare it against the specified pre-charge for the system, which is set from the starter's minimum working pressure.
- Note the oil temperature. Gas pressure changes with temperature, so a reading taken on a cold morning and a reading taken after a run are not directly comparable.
- Recharge with nitrogen to the specified figure if it has dropped, then bring the system back up and test.
- If it drops again quickly, the bladder has failed and the accumulator needs replacing, not recharging.
Symptom: no crank at all
| Likely cause | How to check |
|---|---|
| System pressure at or near zero | Read the system gauge. If there is no pressure, the question is why it will not build or where it went. |
| Isolation valve closed | Trace from accumulator to starter and confirm every valve position. |
| Start valve not opening | Operate the control and listen. On a manual valve, check the linkage and the detent. |
| Reservoir low on oil | Check the level. A low reservoir will also cause the pump to draw air. |
| Charging pump not running or not delivering | On an electrically driven pump, check supply and rotation. On an engine driven pump, check the drive. |
| Starter internally seized | Only conclude this once full system pressure is confirmed at the starter inlet. |
Symptom: one weak crank, then nothing
This is the pre-charge signature and it is worth treating as pre-charge loss until proven otherwise. Check it first, before removing anything.
Other causes that produce a similar pattern:
- Accumulator undersized for the duty. If the system has always managed only one crank, it may never have been sized for more. Our guide to sizing a hydraulic starting system works through charge pressure, oil used per crank, cranks before recharge, and accumulator capacity.
- Cold oil. High viscosity oil moves more slowly through the circuit and the crank looks weak. If the fault disappears when the installation is warm, this is where to look.
- Internal leakage in the starter, which bypasses oil instead of converting it into torque.
- A check valve leaking back, which lets the accumulator discharge into the reservoir rather than through the starter.
Symptom: pressure will not build
- Relief valve set too low, or stuck open. Check the setting against the system specification before assuming a pump fault.
- Pump not delivering. Worn, or drawing air on the suction side. Look for foaming in the reservoir.
- Suction restriction. Blocked strainer or a closed suction valve.
- Low reservoir level.
- Failed accumulator bladder, which lets nitrogen into the oil circuit. Air or gas in the oil makes the system feel spongy and pressure build erratic.
Symptom: pressure bleeds off between starts
A hydraulic starting system should hold its charge for long periods. That is the entire reason for choosing one on a standby engine. If pressure is gone by the next morning, find out where.
- External leak. The easiest to find and the easiest to overlook on a dirty installation. Clean the unit down and look again.
- Check valve leaking back into the reservoir.
- Accumulator gas valve leaking. Soapy water at the valve will show it.
- Starter internal leakage past seals, discharging the accumulator slowly through the motor.
- Control valve not seating.
The emergency hand pump. Work it periodically or it is an assumption, not a failsafe.
Symptom: the hand pump will not build pressure
The emergency hand pump is the reason a hydraulic starting system can start an engine with no power of any kind available, so it needs to work when everything else has failed. It is also the component least likely to have been used since commissioning.
- Check valves in the pump stuck or dirty after long standing.
- Pump seals dried out from years without operation.
- Isolation valve in the wrong position for hand pumping.
- Reservoir low, so the pump is drawing air.
- Lost pre-charge again. If the accumulator has no nitrogen behind the bladder, hand pumping will feel endless because you are trying to fill a vessel with nothing pushing back.
Operate the hand pump as part of routine testing. A hand pump that has never been used since installation is an assumption, not a failsafe. Working it periodically keeps the seals serviceable and confirms the emergency route actually exists.
Symptom: it turns but does not engage the flywheel
The starting circuit is doing its job and the drive end is not. The checks are the same as for any engine starter:
- Ring gear wear, inspected through a full revolution of the engine rather than at one position
- Pinion wear, looking for chipped, rounded or hooked teeth
- Engagement mechanism contaminated or sticking
- Pinion to ring gear clearance against the engine manufacturer's figure
- Loose or distorted mounting, which changes the mesh geometry
On hazardous area installations the unit will be a pre-engaged design, meshing the pinion before the motor turns. See inertia vs pre-engaged starter motors for why that matters, and ATEX vs IECEx engine starters for the certification.
A diagnostic sequence you can work through
- Read system pressure with the system charged and standing.
- Attempt a crank and watch the gauge. How far does pressure fall for one crank, and how quickly does it recover?
- Check the accumulator pre-charge properly, with the oil side discharged and a gauging rig on the gas valve. Do this early, not last.
- Leave the system charged and isolated overnight. If pressure has gone by morning, you have a leak or a valve seating fault.
- Check the reservoir level and condition. Foaming, milky oil or a low level all point somewhere.
- Test the hand pump. It proves both the pump and the accumulator's ability to accept and hold a charge.
- Inspect the pinion and ring gear if the motor turns but the engine does not.
- Confirm the engine turns freely before condemning any part of the starting system.
Preventive checks worth scheduling
| Check | Why it matters |
|---|---|
| Accumulator nitrogen pre-charge | The single most common cause of reduced cranking capacity, and invisible on the system gauge |
| Overnight pressure hold | Proves the system will still be charged when the engine is actually needed |
| Hand pump operation | Confirms the no-power starting route is real rather than assumed |
| Oil level, cleanliness and condition | Contamination damages seals and valve seats, and air in the oil makes the system spongy |
| Ring gear inspection whenever the starter is off | Cheapest inspection available, most expensive one to skip |
| A full crank test under realistic conditions | A warm test on a warm day proves less than you think |
When to replace
Pre-charge that will not hold means a failed bladder, and the accumulator is replaced rather than repaired. A starter with internal leakage past its seals, or one that has been run on contaminated oil, is usually an exchange. A system that has never delivered the number of cranks it needs to has a sizing problem rather than a fault, and re-specifying is the honest answer.
Powerstart's M-Series covers roughly 30 to 216 Nm across the M10, M16, M22, M28, M33, M38 and M66, supplied with accumulators and as complete mini-pack systems. Full specifications are on the hydraulic starter motors pages.
Frequently asked questions
Why does my hydraulic starter give one weak crank and then nothing?
Almost always because the accumulator has lost its nitrogen pre-charge. The pump will still bring the system up to normal pressure, so the gauge looks correct, but with less compressed nitrogen behind the bladder the accumulator holds far less usable oil. A system sized for three consecutive cranks may manage one, weakly. Check the pre-charge with the oil side discharged and a gauging rig on the gas valve before suspecting the starter.
What should the accumulator pre-charge be set to?
On a hydraulic starting system the bladder accumulator is normally pre-charged to around 90 per cent of the starter's minimum working pressure, so it continues delivering usable oil right down to the pressure at which the starter can still produce cranking torque. Set it against the specification for your system and take account of oil temperature when reading it.
Can I charge a hydraulic starting accumulator with compressed air?
No. Accumulators on these systems are charged with nitrogen only. Compressed air and oxygen must never be used, because oxygen in contact with hydraulic oil under pressure creates a serious explosion risk. Always isolate and fully discharge the oil side before touching the gas valve.
Why does my hydraulic starting system lose pressure overnight?
Look for an external leak first, then a check valve leaking back into the reservoir, a leaking accumulator gas valve, internal leakage past the starter seals, or a control valve that is not seating. A hydraulic starting system should hold its charge for long periods, so pressure decay overnight is a fault, not a characteristic.
The hand pump will not build pressure. What is wrong?
Common causes are check valves stuck or dirty after long standing, dried out pump seals, an isolation valve in the wrong position for hand pumping, or a low reservoir. If hand pumping feels endless with no pressure rise, suspect a lost accumulator pre-charge, because there is nothing behind the bladder pushing back. Operating the hand pump as part of routine testing keeps it serviceable.
How often should accumulator pre-charge be checked?
Nitrogen migrates slowly through the bladder over time, so pre-charge is a scheduled check rather than a fit and forget item. Build it into the routine inspection for the engine it serves, and always check it before relying on the system after a long period out of use. Check the interval specified for your installation.
Should I repair or replace a hydraulic starter?
A pre-charge that will not hold means a failed bladder, and the accumulator is replaced rather than repaired. A starter leaking internally past its seals, or one run on contaminated oil, is usually an exchange. If the system has never delivered the number of cranks the duty requires, that is a sizing problem rather than a fault and the system needs re-specifying.
Need a replacement starter, accumulator or a full mini-pack?
Send us the engine make and capacity, the system pressure, the mounting flange and the number of cranks you need in reserve, and we will confirm the specification. Request a quote or call +44 (0) 121 818 5080.
