Closed-Loop Hydrostatic Drive Simulator

421A Updated October 2026 Free, no signup

Simulator

Closed-loop hydrostatic drive schematicRFmotor 1motor 2flushbrakecoolerABchargeservoflowcase flow
  • High pressure
  • Low pressure
  • Charge
  • Control (pilot)
  • Case drain
  • Suction
  • Idle

Operate the drive

Forward

This is a simplified, generic teaching model. This is not a service or repair procedure. Follow lockout and safety procedures.

Run a generic closed-loop hydrostatic drive in neutral, forward, reverse and stall, then find 5 drive faults from relative gauge and flow readings.

This free simulator runs a generic closed-loop hydrostatic drive and shows what each line and valve does. It is built for 421A Heavy Duty Equipment Technician apprentices preparing for Major Work Activity G, where Task 35 covers hydrostatic systems. Other trades that service mobile equipment hydraulics can use it too. Drive it, stall it, shift the two-speed motors and set the brake, then take a fault scenario and find the cause. It covers topics in the Ontario 421A Level 3 curriculum, including drive operation in neutral, forward and reverse, charge pump circuits and diagnosis of directional control problems. It is a simplified teaching model, not any real machine, so readings are relative and any value shown is an example.

How to read the drawing

Colour is backed up by width, pattern, arrows and labels: the high side is the thickest line, control lines are dashed and drain lines are dotted, and the charge line carries a text label.

  • High pressure. Thick solid red-orange line with moving arrows. The main line at high pressure; in this model, the one the pump is sending flow into.

  • Low pressure. Medium solid blue line with slower arrows. The main line at low pressure; in this model, the one carrying flow back to the pump, where charge oil enters.

  • Charge. Medium solid orange line with a dark outline, labelled charge. Charge pump flow on its way through the filter to the make-up checks and the control.

  • Control (pilot). Thin dashed purple line with a dark outline that pulses when pressurized. Control oil that moves the servo piston; the brake-release line is drawn the same way, but its pressure source is not modelled.

  • Case drain. Thin dotted light-blue line with a dark outline. Leakage oil from the pump and motor cases, plus charge relief flow into the pump case, on its way through the cooler to the reservoir.

  • Suction. Plain solid dark line. Oil drawn from the reservoir to the charge pump inlet.

  • Idle. Thin grey line with no motion. No flow or pressure shown for that line, for example the servo line not in use, or every line while the engine is off.

In fault scenarios the schematic is drawn without pressure colours or flow arrows, so only the readings you take show the evidence.

How a closed-loop hydrostatic drive works

Neutral

In neutral the swashplate is centred, so pump displacement is zero. No high-pressure oil goes to the motors, and they stay stopped. In this model the charge pump is driven from the main pump shaft, so it keeps running, and its flow passes through both make-up checks to fill both main lines. Once the loop is full, charge pressure opens the charge relief, and the extra flow returns to the reservoir through the pump case. Both main lines therefore read about the same, close to charge pressure. With no control input, the control returns the pump to neutral.

Forward and reverse

In this model the joystick energizes the forward or reverse solenoid, and pump displacement follows the solenoid current. The servo piston, positioned by the control, forces the swashplate to an angle. Moving the swashplate one way drives the motors forward; moving it the other way reverses them. A larger swashplate angle gives more displacement and more speed. In pumping mode, with the pump driving the motors, whichever main line the pump sends flow into is the high-pressure side; the other line returns flow from the motors at low pressure. This model shows pumping mode only. Charge oil enters the low side through its make-up check to replace leakage. Charge pressure reads slightly lower in forward or reverse than in neutral.

Changing direction

The pump is over-centre: the control can take it from full displacement in one direction to full displacement in the other. To reverse, the swashplate moves back through neutral and over to the other side of centre. As it passes through zero, the flow direction changes. In this simplified model momentum is not modelled; the pressure sides swap as the swashplate passes centre. Charge oil then enters through the make-up check on the new low side. Motor rotation reverses with the flow. In this model the swashplate moves at a limited rate, so the motors slow down, stop and then speed up the other way.

Stall and relief

A stall is when the pump is stroked, with its swashplate off centre, but the motors cannot turn. High-side pressure climbs to the setting of the high-pressure relief valve (HPRV) for that direction. The HPRV then opens and connects the high side to the low side. On a gauge, high-side pressure rises and then holds at about the relief setting. HPRVs are meant for short pressure spikes, not as working valves, and running over relief for long periods causes severe heat. Outside the fault scenarios, the simulator shows a heat warning while an HPRV is open at stall. Some pumps also have a pressure limiter that destrokes the pump before the HPRV opens, so pressure holds at the limiter setting; this model leaves it off by default.

Charge circuit

In this model the charge pump is a fixed-displacement pump driven from the main pump shaft. It supplies cool oil, keeps the loop under positive pressure, supplies servo control pressure and makes up internal leakage. Its flow passes through a pressure-side filter, then enters the loop through the make-up check on the low-pressure side. The charge relief holds charge pressure at its setting and sends any flow that is not needed to the pump case. Leakage oil from the pump and motor cases returns to the reservoir through the case drain, usually through a cooler. If charge pressure is low, the pump goes toward neutral, and cavitation noise and overheating can follow.

Two-speed motors

This model has two motors in parallel, and each has two displacement positions. A two-position motor normally starts at maximum displacement, which gives maximum starting torque. Shifting to minimum displacement makes the motor turn faster on the same flow, but with less torque. Two motor sizing relations explain it: speed is proportional to input flow divided by displacement, and torque is proportional to displacement times the pressure difference. Minimum displacement gives the higher travel speed. A motor stuck at minimum displacement gives low output torque.

Brake

The brake in this model is spring-applied and hydraulically released (SAHR). Springs apply it, and pressure at the brake port releases it. A solenoid valve controls the release: de-energized, it drains the brake and the brake applies; energized, it pressurizes the brake port and the brake releases. So the brake is released only while the solenoid is energized and the brake port is pressurized. Component manufacturer literature calls for a brake system that does not depend on the hydrostatic drive, because braking through the drive itself can be lost.

Parts of the circuit

Main pump and swashplate

Symbol: A circle with two filled triangles pointing outward at opposite ports, a diagonal arrow through it for variable displacement, a drive shaft with a rotation arrow and a case drain line.

What it does: A variable-displacement swashplate pump: the swashplate angle sets its displacement. It is over-centre, so it can go from full displacement in one direction to full displacement in the other; in pumping mode, with the pump driving the motors, the swashplate position decides which main line is high pressure.

If it fails: Internal leakage or wear in the pump lowers system pressure and charge pressure in stroke, so the drive is slow.

Servo piston

Symbol: A double-acting cylinder with a rod out of both ends, centring springs and a mechanical link to the arrow on the pump symbol.

What it does: The servo piston, positioned by the control, forces the swashplate to an angle. Its pressure comes from the charge pump.

If it fails: If charge pressure is lost, the control returns the pump to neutral, so the drive weakens or stops. A fault in the control input or the control module can make neutral hard to find, which can show as creeping in neutral.

Forward and reverse solenoids

Symbol: Each solenoid is a small box with a diagonal line on the end of the control valve; a proportional solenoid adds a diagonal arrow through the box.

What it does: Energizing one solenoid strokes the pump toward one direction, and in a proportional control the displacement follows the current in that solenoid. With neither solenoid energized, the control returns the pump to neutral.

If it fails: If one direction will not drive while the other is normal and charge pressure is normal, manufacturer troubleshooting lists several places to check for that direction: the control input signal, the control module, servo pressure, and the check and relief valves on that side.

Charge pump

Symbol: A smaller circle with one filled triangle pointing outward and no diagonal arrow, on the same shaft as the main pump.

What it does: A fixed-displacement pump, driven in this model from the main pump shaft. It supplies cool oil, keeps the loop under positive pressure, supplies control pressure and makes up internal leakage.

If it fails: A worn or malfunctioning charge pump gives too little charge flow, so charge pressure reads low and the drive is sluggish or will not move. Low charge can also bring cavitation noise and overheating.

Charge filter

Symbol: A diamond with a dashed line across it, drawn in the line after the charge pump, with a bypass check beside it.

What it does: Filters the oil that the charge pump sends into the circuit. Filtration can be on the suction side, before the charge pump, or on the pressure side after it; this model uses full-flow pressure filtration, with the charge relief after the filter and a bypass valve beside the filter.

If it fails: In this model, a blocked filter opens its bypass, so unfiltered oil passes on. In a suction-side layout, a clogged inlet filter raises charge inlet vacuum and lowers charge pressure, giving sluggish drive, noise and heat.

Charge relief valve

Symbol: A single square with the flow arrow offset from the ports (normally closed), a spring with an adjustment arrow, a dashed sensing line from its inlet and its outlet to the pump case.

What it does: Holds charge pressure at its setting and sends charge flow that is not needed to the pump case. Its setting is referenced to neutral; in forward or reverse, charge pressure reads slightly lower.

If it fails: A charge relief that leaks, for example one stuck open, or is set too low depressurizes the system: charge pressure reads low in neutral, and the drive is sluggish or will not move.

Make-up check valves

Symbol: A small circle (the ball) against a V-shaped seat, one in each line from the charge circuit into the loop, so oil can only flow into the loop.

What it does: Let charge oil into whichever main line is at low pressure, with one check for each side. In neutral, charge flow passes through both checks and fills both main lines.

If it fails: A leaking check or relief on one side makes that direction weak; where both sit in one cartridge per side, swapping the two cartridges moves the weak direction to the other side. On some pumps a leaking multi-function valve can also make neutral hard to find.

High-pressure relief valves (HPRVs)

Symbol: The relief valve symbol drawn twice across the loop, each one able to open from one main line into the other.

What it does: Protect the pump and motors from overload by connecting the high side to the low side when pressure gets too high, with one HPRV for each direction. In this model each HPRV shares a cartridge with the make-up check on its side, as in a multi-function valve: one cartridge per side that holds the check, the relief, a bypass and, on some versions, a pressure limiter.

If it fails: A leaking HPRV is an internal bypass on its side: that direction is weak, and its high side does not reach the relief setting at stall. Where the two cartridges are the same part and setting, swapping them is the test, because a cartridge fault moves to the other direction with the cartridge.

Drive motors

Symbol: A circle with two filled triangles pointing inward and a diagonal arrow, for a variable motor that turns both ways.

What it does: Turn loop flow into shaft rotation: speed is proportional to input flow divided by displacement, and torque is proportional to displacement times the pressure difference. This model has two motors in parallel.

If it fails: Internal leakage or wear in a motor raises motor case drain flow, lowers charge pressure in stroke and lowers system pressure, so the drive is slow in both directions and can run hot.

Two-speed displacement control

Symbol: Drawn here as a control valve and a servo piston that move the motor’s displacement arrow between its two positions.

What it does: Shifts each motor between maximum displacement, for maximum starting torque, and minimum displacement, for more speed with less torque.

If it fails: A motor stuck at minimum displacement gives low output torque.

Brake and brake-release valve

Symbol: Drawn here as a single-acting cylinder whose spring applies the brake, linked to the motor shaft, with its release port fed through a solenoid valve.

What it does: A spring-applied, hydraulically released (SAHR) brake: springs apply it, and pressure at the brake port releases it. The release valve drains the brake when de-energized and pressurizes the brake port when energized.

If it fails: If the release solenoid is not energized or no pressure reaches the brake port, the brake stays applied. Braking through the hydrostatic drive itself can be lost, which is why component manufacturers call for a brake system independent of the hydrostatic drive.

Case drain, cooler and reservoir

Symbol: Case drain lines run from each case port through the cooler to the reservoir, which is drawn as a rectangle open at the top. This page draws drain lines dotted and control lines dashed, so the two are easy to tell apart.

What it does: Case drain lines carry hot leakage oil away from the pump and motor cases to the reservoir, usually through a cooler (heat exchanger). In this model the charge relief also empties into the pump case, so motor wear is read on the motor case drain, where the pump charge relief flow does not mix in.

If it fails: High motor case drain flow, read with loop flushing defeated, points to internal leakage or wear in a drive motor.

Test ports

Symbol: Each test port is a short cross line at the end of a branch line, labelled with its name; a gauge is a circle with an arrow.

What it does: Gauges read loop A, loop B, charge pressure and servo pressure on the commanded side, and flow meters read charge pump flow at the pump outlet and motor case drain flow from both motors combined. Reading A and B shows which side is high, and reading charge pressure in neutral and in stroke helps separate charge faults from loop faults.

If it fails: An absolute reading only means something against the specification in the machine manufacturer’s service manual; comparisons such as A against B, or neutral against stroke, still point the way.

Fault-finding table

FaultWhat the operator reportsWhat to testWhat you would seeCause
Worn charge pumpThe machine is sluggish in both directions and may run hot.Charge pressure with the pump in neutral, then charge pump flow at the pump outlet, before the charge relief.Charge pressure low in neutral. Charge pump flow low.Worn charge pump: too little charge flow.
Charge relief valve stuck openThe machine is sluggish in both directions and may run hot.Charge pressure with the pump in neutral, then charge pump flow at the pump outlet, before the charge relief.Charge pressure low in neutral. Charge pump flow normal.Charge relief valve stuck open: charge flow leaks to the case.
Leaking relief / make-up valve, side AThe machine drives normally in reverse but is weak in forward.Loop A pressure at stall in forward and loop B pressure at stall in reverse; then, since both cartridges are the same part in this model, swap them and test again.Forward: A rises but stays below the relief setting. Reverse: B reaches the relief setting and holds. After the swap, the weak direction moves to reverse.Leaking relief / make-up valve on side A: an internal bypass on that side only.
Bypass valve left openThe machine will not drive in either direction.Charge pressure in neutral, then loop A pressure, loop B pressure and servo pressure with the pump stroked.Charge pressure normal in neutral. With the pump stroked, servo pressure on the commanded side is normal. Neither loop A nor loop B rises much above charge pressure, because the open bypass passes the oil across the loop.Bypass valve left open: the loop cannot build pressure.
Worn drive motor (internal leakage)The machine is sluggish in both directions and may run hot.Motor case drain flow with the pump stroked, the motors loaded and any loop flushing defeated, charge pressure in neutral and in stroke, loop A pressure at stall in forward and loop B pressure at stall in reverse.Motor case drain flow high. Charge pressure normal in neutral but low in stroke, more than the usual slight drop. In this model, the high side stays below relief level at stall in both directions.Worn drive motor: internal leakage into the motor case.
Worn charge pump: why, and how to tell it apart

Why

A worn charge pump cannot deliver enough flow to make up loop leakage and feed the control, so charge pressure falls. With low charge pressure the pump goes toward neutral, which weakens or stops the drive. Low charge also brings cavitation noise and heat.

How to tell it apart

A charge relief stuck open also reads low charge pressure in neutral, so charge pressure alone does not separate the two. Charge pump flow at the pump outlet, before the relief, does: in this model it is low with a worn pump but normal with a stuck-open relief, because that pump is healthy and its flow escapes over the open relief to the case. On a real machine, use the charge flow test in the service manual. A clogged suction filter or a restricted inlet line also lowers charge pressure, with high charge inlet vacuum, so check inlet vacuum before condemning the pump.

Charge relief valve stuck open: why, and how to tell it apart

Why

The charge relief sets charge pressure and sends unused charge flow to the pump case. Stuck open, it leaks charge flow to the case and depressurizes the system. The effects are those of any low-charge fault: the pump goes toward neutral, with cavitation noise and heat.

How to tell it apart

A worn charge pump gives the same low charge pressure in neutral. Here charge pump flow, read at the pump outlet before the relief, is normal: the pump is fine and its flow is escaping over the open relief to the case. On a real machine, use the charge flow test in the service manual.

Leaking relief / make-up valve, side A: why, and how to tell it apart

Why

In this model side A is the high-pressure side in forward, so a leak in the side A relief or make-up valve is an internal bypass that shows only when A is high. The leak keeps A from reaching the relief setting at stall, so forward is weak. Swapping the cartridges moves the leak to side B, and the weak direction follows it.

How to tell it apart

Wear spread through a motor slows both directions and raises motor case drain flow; weakness in one direction only points to a cause for that direction, such as an internal bypass on its high-pressure side, and the swap test shows whether that bypass is in the valve cartridge. If charge pressure drops in one direction only, a loop-flushing shuttle sticking to one side is another cause given in manufacturer troubleshooting; loop flushing is off by default in this model.

Bypass valve left open: why, and how to tell it apart

Why

Where the pump has multi-function valves, the bypass is built into each cartridge, one per side, and both are opened to engage it. With the bypass valves open, the loop cannot build pressure in either direction. Closing the bypass valves is the fix.

How to tell it apart

A worn charge pump or a stuck-open charge relief also weakens or stops the drive, but those read low charge pressure in neutral. A control fault can give similar loop readings, because a pump that is not being stroked builds no loop pressure either; servo pressure on the commanded side helps tell them apart, since it is normal with the bypass open. With only one bypass valve open, only the direction in which that side is high may be lost, so the complaint can look like a one-side valve fault.

Worn drive motor (internal leakage): why, and how to tell it apart

Why

Worn parts inside the motor leak high-pressure oil into the motor case, so motor case drain flow rises. Charge flow has to make up that leakage, so charge pressure falls in stroke; the same leakage also lowers system pressure under load. The drive is slow both ways and the oil can run hot.

How to tell it apart

Charge faults read low charge pressure already in neutral, while motor wear shows up in stroke, together with high motor case drain flow. A misset relief or a weak charge pump can also lower charge pressure in stroke, so high motor case drain flow is the reading that points to the motor. Loop flushing also sends oil to the motor case, so motor case drain flow is read with flushing defeated; it is off by default in this model.

About this simulator

It does not represent any specific machine, manufacturer or product. Pressures, flows and behaviour are illustrative only. Symbols are drawn independently following common ISO 1219-1 conventions, except that this page dots drain lines to set them apart from control lines; no part of the standard is reproduced. Values shown are examples; real values come from the machine manufacturer’s service manual.

Sources: component manufacturers’ service manuals and technical information, the Red Seal Occupational Standard for Heavy Duty Equipment Technician (Task G-35), red-seal.ca exam information, and the Skilled Trades Ontario 421A Level 3 curriculum standard. Checked October 2026.

Frequently asked questions

Why does a hydrostatic drive need a charge pump?

A closed-loop drive sends oil from the pump to the motors and back, and some of it leaks into the pump and motor cases. The charge pump makes up that leakage, keeps the loop under positive pressure, supplies cool oil and provides the control pressure that strokes the pump. If charge pressure falls, the pump goes toward neutral, and cavitation noise and overheating can follow.

What happens when a hydrostatic drive stalls?

Without a pressure limiter, a stroked pump with stalled motors raises high-side pressure until the relief valve for that direction opens. The relief connects the high side to the low side, and the gauge holds near the relief setting. Some pumps have a pressure limiter that destrokes the pump first, so the gauge holds at the limiter setting instead. Holding a stall over relief for long periods causes severe heat.

What should the gauges read in neutral?

In neutral the swashplate is centred, so the pump sends no flow to the motors. The charge pump keeps running and fills both main lines through the make-up checks, and extra charge flow goes over the relief to the pump case. Both main-line gauges should read about the same, close to charge pressure. Neutral is where charge pressure is checked against the charge relief setting in the service manual; where that setting is given as a pressure above case pressure, subtract the case pressure reading from the charge gauge reading.

Why would a hydrostatic drive be weak in one direction only?

One cause in manufacturer troubleshooting is an internal bypass on the weak side, such as a leaking relief or check valve that keeps that side below the relief setting. Where the two cartridges are interchangeable, swapping them is the test: if the weak direction moves, the cartridge is at fault. If one direction will not drive at all, check that direction’s control input and control module first, then its servo pressure and valves.

What does high motor case drain flow tell you?

Case drain lines carry leakage oil from the pump and motor cases to the reservoir, usually through a cooler. Manufacturer troubleshooting measures the flow from a motor case with loop flushing defeated; high flow there points to internal leakage or wear in that motor. The drive can then be slow both ways and run hot, with charge pressure dropping in stroke. In this model the charge relief empties into the pump case, so the reading is taken on the motor case drain, which carries the case flow of both motors.

What is the bypass valve for?

Bypass valves let the motors turn so the machine can be moved a very short distance, very slowly, when the pump cannot run. They are not for longer or faster moves; follow the machine service manual for those. Turning the pump or motors with no charge flow to lubricate them can damage them. Close the bypass valves after the move; while they are open, the loop cannot build pressure and the machine will not drive. Where each cartridge has its own bypass, one open valve may stop only one direction. A control fault can give similar readings; servo pressure on the commanded side helps tell them apart.