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Trouble Shooting DC Hydraulic Power Unit

Q: How do I prime my pump?

A: When filling a new system with oil it is important to fill all components with oil eliminating air pockets.  To “prime” a DC power unit loosen or remove the high-pressure line at the cylinder.  With the reservoir full, briefly (1-second or less) engage the power unit until clear oil is flowing.  Tighten or reattach high pressure line.

Q: Why does my pump lose prime?

A: The most common culprits of a pump losing prime is as follows:

1.      Suction tube not staying submerged due to pump mounted improperly

2.      Partially clogged suction filter

3.      Loose or poorly install suction line or pick-up tube

4.      Reservoir not being properly vented

5.      Oil has too low of viscosity for the operating temperature (too thick)

6.      Front pump seal failure; this is a fairly rare occurrence and all other possibilities should be eliminated first.

Q: What type of oil does my pump take?

A: DC power units typically require Dextron automatic transmission fluid.  There are other compatible alternatives but they must meet the following requirements:

1.      Fluid must be compatible with Buna-N sealing compounds

2.      The pour point must be below lowest expected temperature

3.      Viscosity range should be as near to 200 SUS as possible

Q: What is the Volt/Amp requirements?

A: The Voltage requirement should be labeled on your system.  DC motors use high amperage and low volts to make the torque required to create flow and pressure.  Amperage requirements would be determined by the amount of load on the motor.  The load changes depending on the amount of pressure that your hydraulic system is required to perform.  How hard the motor has to work is also dependent on the displacement of the pump within the power unit.  When setting up a system it is a good rule of thumb is to limit the system to the 150-225 amp range.  This will allow a longer life expectancy to the motor.  It is necessary to size components of the electrical system to accommodate your maximum potential current draw to avoid shorts and potential fire dangers.  Please see the 12vdc charts below for reference.

Q: What pressure is my pump producing?

A: The only way to conclusively tell is to incorporate a pressure gauge in to the hydraulic circuit.  Some double-acting power units may have separate relief settings for the power up / power down circuits.

Q: Why is my pump so slow?

A: DC power units are usually fairly limited in the amount of GPM they are able to produce.  However, if your power unit is slower than it has been in the past with the same amount of load, there are usually only a couple of causes.  Make sure that your power unit is receiving the full voltage required to operate.  Check to make sure the unit is properly grounded. 

A)      If the unit is receiving full electrical power and sounds like the motor is being “labored” your motor will need to be re-built or replaced.

B)      If the unit is receiving full electrical power and does not sound like the motor is being “labored” there is most likely fluid bypassing within the system.  This can be caused by relief valve not being fully seated, a worn pump, or even a faulty piston seal on a cylinder.  First eliminate the possibility of the piston seal bypassing by using standard cylinder trouble shooting procedures.  Once that is done, attach a gauge to the pressure port on the power unit.  By “Dead-heading” the pump you will we able read the relief valve setting. 

1.      If the pump slowly builds up to a steady pressure, either your relief valve is not seating properly or the Cartridge valve on the return circuit (if so equipped) is malfunctioning.

2.      If the pump quickly builds to a set pressure, try adjusting the relief pressure higher.  If it fails to raise the pressure regardless of adjustment, your pump is wearing out.

Q: Why does the cylinder go up but won’t come back down.

A: The most likely cause is the failure of the cartridge valve on the return circuit.

Q: Why is oil shooting out of the breather cap on my reservoir?

A: Clear oil coming out of the reservoir might indicate that the system is over-filled due to a cylinder not being fully retracted when the system was filled.  Foamy oil coming out might indicate air in the system.  Common causes to this can be a loose suction tube, or no down spout.  Other potential causes can be excessive return oil velocity.  This can be typically solved by slowing the return oil through a unidirectional flow control valve, or increasing reservoir capacity size.

Q: How do I turn up/down the pressure on my power unit?

A: Many DC power units have adjustable reliefs built in to the unit.  You will likely need a model specific description to determine location.  Most have a jamb nut that will need to be loosened before adjusting.  Clockwise rotation raises pressure, counter-clockwise lowers pressure.  Use a pressure gauge while making adjustments.  Reset the jamb nut to secure new setting once adjusted.  Do not exceed max pressure setting of the unit.

Q: Do I need to replace the motor on my power unit?

A: If the motor fails to engage, the first step is to ensure that it is receiving sufficient volts/amperage.  Inspect the condition of all wires and connections for breaks and or corrosion.  Check to make sure that the trigger wire is engaging the start solenoid.  If the solenoid is engaging you should here an audible “clicking” coming from the solenoid.  No clicking, replace solenoid and continue diagnostics.  If you hear the solenoid engaging, check that there is power coming across the solenoid to the motor side.  Once you are positive that you have sufficient amperage reaching the motor and it still fails to engage the motor will need to be re-built or replaced.

DC motors do not always “catastrophically fail” rather most get weaker and slowly burn out over time.  It can be difficult to determine if it is your motor or the pump that is wearing out.  If the motor stalls out before making target pressure, the motor will need to be re-built or replaced.  If the motor continues to run, but the unit fails to make pressure, the failure is usually in a different component.

Q: How do I check if my cartridge valves are working?

A: The first step is to ensure that your electro-magnetic coil is working.  You will need to make sure that the coil is receiving the proper Volts and wattage to operate.  Broken wires and bad grounds are likely culprits.  If the coil is receiving proper power it should produce a magnetic field that is strong enough to pull a small wrench or screwdriver towards it from less than a ¼inch away.  Once you have determined that the coil is working and properly installed onto the cartridge valve, you should be able to feel a small “clicking” the moment it is engaged.  The internal tolerances on the valves are extremely tight and deposits and contamination in the oil can gum up the moving components.  If you do not feel or hear the clicking, you may want to soak the cartridge in a brake cleaner type solvent.  After cleaning the cartridge and reassembling, test it once again.  If there is still no clicking, discard and replace.

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