When a hydrostatic drive will not move, moves one way only, goes weak once it is hot, or creeps in neutral, the cause is almost always one of four things: low charge pressure, a bypass valve left open, a leaking or misset valve in the loop, or a pump that is not truly in neutral. Put a gauge on the charge pressure port before you touch anything else, because that one reading sorts most of these complaints in ten minutes.
This covers closed-loop pump and motor drives: Sundstrand and Sauer-Danfoss Series 15, 20, 40, 42 and 90, Eaton heavy-duty Models 33 through 64, and Denison Gold Cup. Every number below comes from the factory manual named beside it. Where the factory published no number, we say so.
Test charge pressure first, every time. The charge pump keeps the low side of the loop full, feeds the control that strokes the pump, and makes up for internal leakage. Danfoss is blunt about it: charge must stay at its specified pressure under all conditions or the transmission gets damaged. A weak charge system explains no-drive, sluggish response and power fade when hot, all on its own.
Why won't my hydrostatic drive move at all?
Work it in this order, the same order the Danfoss Series 90 and 42 manuals use. Cheap checks first.
- Oil level. Low oil starves the charge pump.
- Control input. Broken linkage, dead coil, bad connector or blocked pilot line. If the control never asks the pump to stroke, nothing moves.
- Bypass valves. Open bypass valves short the loop. Engine runs, machine sits.
- Charge pressure in neutral. Low points at the charge relief, charge pump or inlet filter.
- Charge pressure in stroke. Good in neutral but falling in stroke points at internal leakage or a motor-side charge relief set wrong.
- System pressure. Healthy charge but no high pressure means relief or multi-function valves, then the rotating groups.
What should hydrostatic charge pressure read?
It depends on the series, and on some units the setting is printed right in the model code. These are the factory figures. Charge pressure is a differential: read the charge gauge, read the case gauge, and subtract the case reading. Speed matters too, so take the reading at the speed the factory names.
| Series | Factory charge figure | Conditions | Source |
|---|---|---|---|
| Sundstrand Series 15 (U and in-line transmissions) | Charge relief 70-150 psi | Relief valves set at 3600 rpm pump speed. Use a 1000 psi gauge with a snubber in the charge pressure port. | Sauer-Danfoss BLN-9646 |
| Sundstrand Series 20 heavy duty (Models 20-27) | 190-210 psi in neutral, at least 160-180 psi in forward or reverse | Above case pressure. The lower in-stroke figure is set by a second charge relief in the motor manifold. Sundstrand notes some machines use special settings, so check the machine spec too. Use a 500 psi gauge. | Sauer-Sundstrand BLN-9630 |
| Series 40 (M25, M35, M44) | Standard charge relief setting 14.0 bar (200 psi) | Referenced to case, factory set at 1800 rpm in neutral | Danfoss 520L0635 |
| Series 40 M46 | Standard charge relief setting 19.5 bar (285 psi) | Referenced to case, factory set at 1800 rpm in neutral | Danfoss 520L0635 |
| Series 42, 28 and 32 cc | Standard 14 bar (203 psi), optional 10-24 bar (145-348 psi) | Nominal, at 1800 rpm, pump in neutral | Danfoss 11022637 |
| Series 42, 41 and 51 cc | Standard 20 bar (290 psi), optional 10-24 bar (145-348 psi) | Nominal, at 1800 rpm, pump in neutral | Danfoss 11022637 |
| Series 90, model code ending 20 | Measured 18.1-21.7 bar (262-315 psi) | Pump in neutral, 1500 rpm, 50°C (120°F) reservoir, charge gauge minus case gauge. Higher input speed reads higher. The last two digits of the model code are the nominal setting in bar. | Danfoss 520L0818 and BLN-9947 |
| Series 90, model code ending 24 | Measured 22.0-26.9 bar (319-390 psi) | ||
| Series 90, model code ending 28 | Measured 25.8-30.7 bar (374-445 psi) | ||
| Series 90, all | Minimum 10 bar (150 psi), maximum 35 bar (508 psi) | Operating limits | Danfoss 520L0818 |
| Danfoss IHT-M15 integrated transaxle | 5.5-9 bar (80-130 psi), at least 5.5 bar (80 psi) in forward or reverse | Normal operating rpm | Sauer-Sundstrand BLN-10098 |
| Eaton heavy-duty Series 1, Models 33-64 | Read the stamp on the charge relief valve: three digits times ten is the setting in psi (022 is 220 psi). Eaton's chart runs from 220 to 380 psi. | The setting is chosen per application, so the valve stamp is the spec for your pump | Eaton 7-606 and 6-610 |
| Denison Gold Cup hydrostatic | On a new system start, replenish pressure should rise to 200 psi (13.8 bar) within 30 seconds or you shut down | Start-up check, run at 1500 rpm or less. Denison's P24/M24G procedure also calls for an alarm below 50 psi replenish. | Denison Goldcup Hydrostatic Application Manual, SP-514 |
Series 90 owners get the easiest version of this: the charge setting is the last two digits of the model code on the nameplate, in bar. Danfoss lists codes from 20 to 34. If the plate is gone, our Sundstrand and Sauer-Danfoss identification guide covers how to place the unit.
Where is the charge pressure gauge port?
| Series | Charge port | Case port | Gauge |
|---|---|---|---|
| Series 90 pump | M3 (M6 on some end caps), 9/16-18 | L1 or L2, or servo port M4 or M5 | 50 bar (1000 psi) on charge, 10 bar (100 psi) on case |
| Series 42 pump | M3, 3/4-16 | L1 or L2 | 60 bar (870 psi) on charge |
| Series 40 M46 pump | M3, 9/16-18 UNF | L1 or L2 | 50 bar on charge, 10 bar (100 psi) on case |
| Series 15, Series 20 heavy duty | Charge pressure gauge port on the pump | Case drain | 1000 psi (Series 15) or 500 psi (Series 20) |
Use snubbers. A bouncing needle with no snubber reads high and teaches you nothing.
How do I check charge pressure the right way?
- Make the machine safe. Wheels off the ground or the work function disconnected. The pump can stroke while you are under it.
- Fit two gauges, charge and case. Charge pressure is the difference.
- Warm the oil and match the factory speed. Series 90 ranges are quoted at 1500 rpm with 50°C (120°F) oil, Series 40 and 42 settings at 1800 rpm, Series 15 reliefs at 3600 rpm.
- Read in neutral. Low here means the charge relief, charge pump or inlet side.
- Stroke it forward, then reverse. A small drop is normal. A big drop both ways means internal leakage. A drop one way only usually means a sticking loop flushing valve in the motor.
- If charge is low, check inlet vacuum. Series 90 is rated for 0.7 bar absolute (9 in Hg vacuum) at the charge inlet, the heavy-duty Series 20 for 10 in Hg, the Series 15 for 5 in Hg.
Do not crank the charge relief up to cover a problem. If a worn pump or motor is leaking charge oil away, raising the relief setting only hides it for a few hours while the rotating groups keep wearing. Find the leak.
Hydrostatic troubleshooting table
| Symptom | Likely cause | Test | Fix |
|---|---|---|---|
| Won't move either direction, charge pressure normal | Bypass valves open, multi-function valve or check/relief valve stuck, control not stroking the pump | Check bypass actuators are closed. Check control input. Read system pressure at M1 and M2 while stroking. | Close bypass valves. Repair or replace the control or the faulty valve. |
| Won't move, charge low in neutral | Charge relief leaking or set low, worn or wrongly clocked charge pump, plugged inlet filter | Charge and case gauges in neutral. Inlet vacuum. | Clean or reset the charge relief. Replace filter. Repair or replace the charge pump. |
| Charge good in neutral, falls away in stroke | Internal leakage in pump or motor, motor charge relief set wrong | Charge in forward and reverse. Motor case flow with loop flushing defeated. | Reset motor charge relief. If case flow is high, the unit needs rebuilding. |
| Moves in one direction only | Control or servo orifice blocked on one side, multi-function or relief valve faulty on one side, loop flushing valve sticking | Swap the two multi-function valves. Read charge in each direction. | If the fault changes direction, replace that valve. Clean orifices. Repair the flushing valve. |
| Loses power when hot | Charge falling as oil thins, worn rotating group leaking more when hot, cooler not keeping up, relief set too low | Read charge and system pressure at full operating temperature, not cold. Check cooler airflow. | Clean or repair the cooler. Correct relief settings. Rebuild a worn pump or motor. |
| Creeps in neutral | Pump mechanical neutral or control neutral off, binding linkage, leaking multi-function valve | Servo gauges at M4 and M5, or system gauges at M1 and M2, with the control centered. | Neutral adjustment per the manual. Free up the linkage. Swap valves to find a leaker. |
| Runs hot | Low oil, dirty or undersized cooler, low charge, high inlet vacuum, relief valves set too low, motor internal leakage, overloading | Oil level, charge pressure, inlet vacuum, relief settings, motor case flow, system pressure | Fix the cause. Heat is a symptom. |
| Low torque, stalls under load | Pressure limiter or relief set low, variable motor stuck at minimum displacement, internal leakage | System pressure at the motor under load | Reset the limiter. Repair the motor displacement control. Repair the leaking unit. |
Rows follow the Danfoss Series 90 (520L0818) and Series 42 (520L0638) troubleshooting tables, rewritten for the shop floor. Noise is covered in hydraulic pump making noise.
Is the bypass or tow valve open?
This is the number one no-drive call that turns out not to be a hydraulic failure. Somebody towed or pushed the machine, opened the bypass, and nobody closed it.
- Series 90: the bypass lives in the two multi-function valve cartridges on the pump. To engage it, the actuator is backed out three turns, and Danfoss warns against going past three and a half or it will leak externally. Close both before you condemn anything. On the newer 030-100 frames the actuator torques to 41 N·m (30 lbf·ft), on 130-250 frames to 100 N·m (75 lbf·ft).
- Series 20 heavy duty: a rotary bypass valve on the motor manifold. Closed for normal operation, open to move the vehicle when something has failed.
- Series 42: the bypass is built into the check and high-pressure relief valves. Danfoss notes that a partially activated bypass causes cross-port leakage, which feels like a weak pump rather than no pump at all.
Bypass valves are not tow valves. Danfoss says so in the Series 90 manual. With the engine off there is no charge flow lubricating the pump and motor, so bypass is for moving a machine a very short distance at very slow speed. Towing a hydrostatic machine any real distance on the bypass is a reliable way to buy a rotating group.
Why does my hydrostatic creep in neutral?
Creep means the pump still pushes a little oil when the control says zero. There are two neutrals and they have to agree: mechanical neutral, the swashplate at zero angle with no control signal, and control neutral, the control centered so it ports no servo pressure to either side.
The Series 42 manual sets mechanical neutral first. Bridge servo ports M4 and M5 with a hose, put gauges on M1 and M2, turn the neutral screw until one gauge rises, back it the other way until the other rises, and set it halfway. The Series 90 non-linear MDC and the Series 40 M46 EDC use the same split-the-difference method on the servo gauges. On the M46 EDC, Danfoss accepts up to 1.5 bar (22 psi) difference and says zero is usually not possible.
- Linkage drag. The IHT-M15 transaxle's neutral return needs less than 2 N·m (20 in·lbf) of linkage friction to center the swashplate. A sticky cable holds the pump on stroke.
- A standard Series 90 MDC has no adjustment. Centering springs hold the spool, so look at linkage, control and valves.
- Gold Cup pumps have no centering springs in the stroking chambers. Shut one down on stroke and it will likely stay there. Denison offers an automatic brake and neutral bypass control for drives that cannot tolerate creep.
- A leaking multi-function valve makes Series 90 neutral hard to find. Swap the two valves and see if the behavior changes direction.
- After a rebuild, the Eaton Models 33-64 manual has you check swashplate zero angle during assembly. Skip it and you build creep into a fresh pump.
Why does my hydrostatic lose power when it gets hot?
Hot oil is thin oil. Worn running surfaces leak more as viscosity drops, the leakage steals charge oil, and the loop cannot hold pressure. Drives fine cold, fades hot: that is wear until proven otherwise. First rule out what makes it run hot: low oil, a plugged or undersized cooler, a relief set too low and dumping oil, high inlet vacuum.
| Series | Temperature limits | Source |
|---|---|---|
| Series 90 | 104°C (220°F) rated, 115°C (240°F) maximum intermittent, measured at the hottest point in the loop, normally the motor case drain | Danfoss 520L0818 |
| Series 15 | 220°F (104°C) maximum intermittent, -30 to 180°F (-34 to 82°C) normal operating range | Sauer-Danfoss BLN-10006 |
| Denison P24/M24G | Keep the loop below 180°F during break-in, alarm on case drain temperature above 180°F | Denison SP-514 |
Take readings hot. A morning reading tells you nothing about a machine that quits at three in the afternoon.
Loop flushing and the hot oil shuttle
A closed loop recirculates the same oil, so a shuttle valve and low-pressure relief in the motor bleed hot oil from the low side to the case and cooler, and charge oil replaces it. On the heavy-duty Series 20 the pump charge relief is 190-210 psi and the motor relief 160-180 psi, so the motor relief takes the flow whenever the pump is on stroke. If the shuttle sticks, Danfoss's Series 90 table gives the symptom: charge decays in one direction only.
What should case drain pressure be?
High case pressure blows seals and gaskets, and because charge is measured above case, it also distorts your charge reading.
| Series | Case pressure limit | Source |
|---|---|---|
| Series 90 | 3.0 bar (40 psi) rated, 5.0 bar (75 psi) maximum | Danfoss 520L0818 |
| Series 42 | 3.4 bar (50 psi) rated, 10.5 bar (150 psi) maximum at cold start | Danfoss 520L0638 |
| Series 20 heavy duty | 40 psi maximum | Sauer-Sundstrand BLN-9630 |
| Series 15 | 10 psi continuous, 25 psi maximum at cold start | Sauer-Danfoss BLN-10006 |
| Denison P24/M24G | Should settle below 50 psi at operating temperature | Denison SP-514 |
Case drain flow is the better wear test. Danfoss has you measure Series 90 motor case flow with loop flushing defeated (defeat spool fitted). Excessive flow means a worn motor. The manual gives no single limit, so compare with your unit's service data or a known good unit.
High-pressure relief and multi-function valves
Each Series 90 multi-function valve combines the system check, pressure limiter, high-pressure relief and bypass. The A and B settings are in the model code in bar, the relief sits about 35 bar (500 psi) above the limiter, and all are referenced to charge, so subtract the low side from the high-side gauge.
- Swap test: interchange the cartridges. If the fault changes direction, replace that valve.
- Series 20 heavy duty: pilot-operated relief cartridges in the motor manifold, first two digits of the setting stamped on the end. Change the setting by changing the cartridge.
- Eaton Models 33-64: a three-digit stamp on the power limiter times ten is psi. Two-digit codes map to 4000-6000 psi in Eaton's table.
A failed pump usually takes the motor with it
In a closed loop the pump and motor share the same oil all day. When a pump comes apart, the debris goes straight through the motor and back. Fit a new pump to a contaminated motor and it gets the same treatment. The Sauer-Danfoss troubleshooting charts for small transmissions in our library end their dead-end branches with one instruction: replace the transmission, pump and motor.
- Inspect the oil for discoloration, foam, sludge or fine metal, the signs Danfoss lists.
- If contaminated, replace every filter, drain and flush the system, and flush the lines before refilling. That is the factory instruction.
- Rebuild or replace both units. Tear the motor down even if it still turns.
- Clean or replace the cooler and loop hoses. Coolers trap debris and release it later.
- Flush on start-up. Danfoss cycles a Series 90 forward and reverse for at least five minutes. Denison's P24/M24G procedure runs about two hours at 1800 rpm and half displacement through a flushing filter in the loop.
Cleanliness targets: ISO 4406 22/18/13 or better for Series 90 and 42, ISO 18/13 per SAE J1165 for the Eaton heavy-duty pumps.
Eaton heavy-duty and Denison Gold Cup notes
Eaton Series 1, Models 33-64
The charge relief setting is stamped on the valve, three digits times ten in psi, and Eaton's relief subassemblies carry the same suffix (101048-022 is the 220 psi valve). The ID tag tells you variable pump (21), fixed motor (31) or variable motor (41). Kit 990090 is the Models 54 and 64 pump kit and 990094 the matching motor kit, not interchangeable, and the shaft seal is never in the main kit. See our Eaton heavy-duty hydrostatic page, Models 54 and 64 parts and Models 33, 39 and 46 parts.
Denison Gold Cup hydrostatic
Gold Cup says replenish where Danfoss says charge. Minimum servo pressure is roughly twice replenish, rising about 40 psi per 1000 psi of system pressure (62 psi on the 30 series), so low replenish drags the servo down with it. On a new system, set the compensator or reliefs to 500 psi or less and shut down if replenish has not reached 200 psi in 30 seconds. Parts: Gold Cup piston pumps and P24 and P30 parts.
Sundstrand and Sauer-Danfoss parts
- Sundstrand brand hub and Sundstrand piston pumps
- Sundstrand parts catalog and rebuild guide, Series 15 through 90
- Sundstrand seal kits, seal kit collection, Series 90 parts, Series 42 parts
- Series 90 overhaul kits: 55 cc, 80020S30, 75 cc, 81020S30, 100 cc, 11035669
Frequently asked questions
Why won't my hydrostatic drive move?
Check oil level, the control input, and whether the bypass valves are open, then put a gauge on the charge pressure port. Low charge in neutral points at the charge relief, charge pump or inlet filter. Good charge in neutral that falls away in stroke points at internal leakage in the pump or motor.
What should Series 90 charge pressure be?
The nominal setting is the last two digits of the model code, in bar. Danfoss gives measured ranges at 1500 rpm in neutral with 50°C oil: code 20 reads 18.1-21.7 bar (262-315 psi), code 24 reads 22.0-26.9 bar (319-390 psi), and code 28 reads 25.8-30.7 bar (374-445 psi), all with case pressure subtracted.
Why does my hydrostatic transmission lose power when hot?
Usually because worn running surfaces leak more as the oil thins, which steals charge oil and lets loop pressure sag. Rule out a dirty cooler, low oil, low relief settings and high inlet vacuum first, then measure charge pressure and motor case flow at full operating temperature.
Why does my hydrostatic creep in neutral?
The pump is not at true zero displacement. Either mechanical neutral or control neutral is off, the linkage is binding and holding the pump slightly on stroke, or a multi-function valve is leaking. The factory fix is a neutral adjustment using gauges on the servo or system ports.
Why does my hydrostatic only go in one direction?
A fault on one side of the loop: a blocked control or servo orifice, a faulty relief or multi-function valve on that side, or a loop flushing valve sticking. On a Series 90, swapping the two multi-function valves shows whether a valve is to blame, because the fault moves to the other direction.
Can I tow a hydrostatic machine with the bypass open?
Only a very short distance at very slow speed. Danfoss states that Series 90 bypass valves are not tow valves, because with the engine off there is no charge flow lubricating the pump and motor.
Do I need to replace the motor if the hydrostatic pump failed?
In most cases, yes. The pump and motor share the same oil in a closed loop, so pump debris goes straight through the motor. Danfoss calls for replacing all filters, draining and flushing the system and the lines when contamination is found. Rebuild both units and clean or replace the cooler.
About the technical information on this page. Figures, procedures and part numbers here come from manufacturer service literature and are provided for reference only. They are only as accurate as the factory data, and the revision we have on file may not be the latest for your unit. Always confirm against the current manufacturer documentation for your exact model and follow safe work practices. RestoPower is not responsible for damage, injury or loss resulting from the use or misuse of this information, or from incorrect assembly, adjustment, installation or operation of any unit.
Send us the readings and the nameplate
Tell us the series and full model code, the charge and case readings in neutral and in stroke, and whether the problem gets worse as it warms up. Those numbers usually tell us whether you need a valve, a seal kit or a rebuild. We reply with price, availability and lead time.
Request a quote · (810) 882-1171 · Repair and rebuild service
Related reading
- How to identify a Sundstrand or Sauer-Danfoss pump
- Sundstrand Series 20 and 90 guide
- Why is my hydraulic pump overheating?
- Hydraulic pump not building pressure
- Hydraulic pump making noise
- What hydraulic pump repair actually costs
- Sundstrand document reference index
- Sauer-Danfoss Series 90 Model Code Decoder
- Hydraulic Pump Case Drain Flow Test: How Much Is Too Much?
- Hydraulic Fluid Cleanliness: ISO 4406 Codes and Flushing After a Pump Failure
- Hydraulic Pump Start-Up Procedure After a Rebuild or Replacement
- Hydraulic Motor Repair & Rebuild