The Essential Guide to Checking Hydraulic Oil Level in Excavators


What is hydraulic oil? Hydraulic oil is the fluid by which power is transferred within a hydraulic circuit. The oil is pressurized by a hydraulic pump and directed to different areas of the circuit by the use of spool bank/valve block. Typically a mineral oil.


Risks: If a hydraulic pump operates without an adequate amount of oil, cavitation will occur (also referred to as running dry). This will damage the internals of the pump. In a ‘Gear’ style pump; the gears, body, thrusts and shaft may become damaged due to excess friction. In a ‘Piston’ style pump; the rotating piston group and valve plate will become damaged due to excess friction.

Costs: ‘Gear’ style pumps will typically cost £300-£1500 whilst ‘Piston’ style pumps could cost £800-£20k+, this simply depends on the make / model / size of the machine.


Too much

    - Hydraulic oil will be forced out of the breather pipe or filler neck when tank pressure increases.

Too little

    - A loud ‘scringe’ or ‘whining’ noise may be heard as the pump struggles to ingest oil - this is a sign of cavitation.


  • Servicing; oil will be lost during filter replacement
  • Attaching and removing attachments; oil will be lost every time and auxiliary attached is connected and removed
  • Deteriorated hoses; cracked/rotten hydraulic hoses and pipe can leak under pressure as well as hose ferrules
  • Leaking rams due to damaged gland seals or scored chrome (piston)
  • Motors and pumps may be the cause of oil loss due to damaged / blown shaft seals which are leaking under pressure. For a pump you may notice oil around input shaft seal, whereas a motor the same may be observed at the output shaft. A common symptom of a damaged (PERISHED) final drive motor seal is excess pressure in final drive gearbox, thus blowing out the duo cone seal.
  • The rotary distributor - also referred to as a swivel joint - can be the source of hydraulic oil leakage in center of a 360 excavator (you might be seeing oil around the machines slew bearing...). Deterioration of sectional deals within the rotary distributor are likely the culprit here.
  • Another common cause that may be the parent to each of the above: over pressurization the hydraulic service or excess return pressure.


The design of each excavator is different, and you should also refer to the manufacturer specific guidance / maintenance manual. Below we provide a general guide on checking the hydraulic oil level on the average excavator.

Inspect the machine for leakages.

Ensuring your machine has an adequate amount of hydraulic oil will prove frivolous if it does not remain at such a level after some operation. It is essential to fix any leaks before going any further; see above for things to look out for and what might be causing them.

Level Ground

Your machine must be stationed on level ground in order to gather an accurate reading of hydraulic oil level.

Machine Position

The machine should have all rams extended half way. That is to say: ~50% of the chrome on each hydraulic ram should be visible. This is a general principle and some machines may vary. By doing this we know that all of the machines rams are using half of their potential oil capacity. If the rams were full extended, we would then observe a lower tank level as more of the oil would have been transferred to the rams. And the inverse is true, where if all rams were fully retracted, we would see more oil in the tank.

Stop the Engine

The engine should be stopped when checking the hyd oil level, this ensures that the turning of the hydraulic pump is not having an effect on the oil level reading.

Locate Level Indicator

The hydraulic oil sight gauge is usually located on the side of the hydraulic oil tank / reservoir. The sight gauge may be a perspex tube / rectangle with high and low markings. The image below shows the sight gauge on an Airman mini digger, but other manufacturers often have a similar design.

Add Oil if Needed

Hydraulic oil can be added through the reservoir filler neck or top inspection plate, again, different manufacturers have different designs, though the fill location might be identified by the hydraulic level symbol in the image below...


The vast majority of modern hydraulic excavators use ISO46 as the hydraulic fluid. This is an appropriate replacement for ISO32 that was often used in older plant and agricultural machinery with gear pumps. In some applications, hydro-static systems, 10W is recommended over ISO46, although their properties are similar. These oils (10W and ISO46) can be mixed, though it is not recommended.


In conclusion, ensuring the proper level of hydraulic oil in excavators is a routine maintenance task and a crucial aspect of preserving the integrity and functionality of your machine. Neglecting to check hydraulic oil levels can lead to a cascade of issues, from costly repairs to significant downtime. By understanding the importance of maintaining adequate oil levels, identifying symptoms of oil loss, and following a systematic approach to checking levels, operators can safeguard their equipment and avoid unnecessary expenses.
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