🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Transportation › Diesel & Heavy Truck › Hydraulics for Heavy Trucks and Diesel Equipment › Lesson 10
TRN-DIES20-04 · Lesson 10 · 20 clock hours · lab

Pressure and Flow Testing: Diagnosis Procedure

Objective. Students will follow a structured diagnostic procedure to isolate a seeded hydraulic fault on the bench trainer, correctly identifying the faulty component and citing supporting test data in a written service diagnosis.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Pressure and Flow Testing: Diagnosis Procedure

Why Systematic Diagnosis Matters

A hydraulic system can fail in dozens of ways, and many failures produce similar symptoms — slow cylinder movement, excessive heat, or low force output. Guessing which component is bad wastes time and money. The professional approach is a structured test sequence that generates objective data at each point in the circuit. That data, not a hunch, drives the diagnosis.

The governing principle is start upstream, test downstream. Hydraulic energy originates at the pump. If pump output is low, every component downstream will appear faulty. By testing from the pump outward, you avoid condemning good parts.

---

Safety First

Before connecting or disconnecting any test instrument:

---

The Five-Step Pressure-Flow Test Sequence

Use this sequence every time — on a healthy trainer to establish a baseline, and again when diagnosing a fault.

Step 1 — Pump Output (Pressure and Flow)

Connect a calibrated pressure gauge and in-line flow meter at the pump outlet, upstream of the relief valve. Start the prime mover at the manufacturer's specified speed. Record pressure (psi) and flow (gpm). Compare to the technical service manual spec. Low pressure and low flow together point directly to the pump. Low pressure with normal or high flow suggests the relief valve is opening too early.

Step 2 — Relief Valve Setting

With the actuator stalled (cylinder fully extended or blocked), read the pressure gauge. This is the relief valve cracking pressure. It should match the specification. A reading significantly below spec means the relief valve is set too low or is stuck open; a reading significantly above spec means it is set too high or stuck partially closed.

Step 3 — Pressure Drop Across the Filter

Many filters have a differential pressure port or a bypass indicator. Record inlet and outlet pressure across the filter. A large differential (check the manual for the specific limit) indicates restriction. Restricted flow causes heat buildup and starves the circuit downstream.

Step 4 — Actuator Inlet Pressure and Cylinder Drift

Measure pressure at the cylinder port. Compare to pump outlet pressure. A large pressure drop between pump outlet and cylinder inlet suggests a restriction in the line or a partially closed valve. Then, load the cylinder and observe drift over 60 seconds with a stopwatch. Excessive drift with no external load indicates internal cylinder bypassing (worn seals or a scored bore).

Step 5 — System Temperature

Use a temperature gun to check reservoir and return-line temperatures after a work cycle. Abnormally high temperature is a symptom, not a root cause, but it narrows the field: heat always results from excessive pressure drop (energy converted to heat across a restriction or bypassing component).

---

The Four Seeded Faults and Their Signatures

| Fault | Pump Pressure | Pump Flow | Stall Pressure | Cylinder Drift | Temperature |

|---|---|---|---|---|---|

| Worn pump | Low | Low | Low | Normal | High |

| Stuck relief valve (open) | Low | Normal–High | Low | Normal | High |

| Bypassing cylinder | Normal | Normal | Normal | High | Moderate–High |

| Restricted filter | Normal–Low | Low | Normal | Normal | High |

Study this table. Each fault has a unique pattern across multiple measurements. A single low reading is not a diagnosis — the full pattern is.

---

Recording Data and Writing the Diagnosis

Record every reading in the diagnostic log as you go. Include test point, measured value, specification, and deviation. When you write your diagnosis:

1. State the symptom the system presents.

2. List the key test readings that are out of specification.

3. Identify the faulty component and explain why the data supports that conclusion.

4. Recommend the repair action.

A diagnosis without supporting data is an opinion. Data without a conclusion is just numbers. Tie them together clearly.

---

Preparing Your Two-Minute Verbal Report

Organize your report: symptom → data → conclusion → repair. Quote specific gauge readings. If the class cannot follow your numbers back to your conclusion, the logic is not clear enough. Practice aloud before the presentation.

Key terms8

Pump outlet pressureThe hydraulic pressure measured immediately downstream of the pump, before any other component; the starting reference point for circuit diagnosis.
Relief valve cracking pressureThe pressure at which the relief valve first begins to open and divert flow back to the reservoir; measured by stalling the actuator.
Differential pressureThe difference in pressure measured across a component such as a filter; a high differential indicates restriction to flow.
Flow meter (in-line)A test instrument inserted into a hydraulic line to measure the volumetric flow rate, typically in gallons per minute (gpm).
Cylinder driftUnintended movement of a hydraulic cylinder under load when the control valve is in the hold position; indicates internal bypassing past the piston seals.
Internal bypassLeakage of hydraulic fluid past worn seals or a damaged bore inside a cylinder or motor, reducing force output and causing drift without external leakage.
Diagnostic logA structured written record of each test point, measured value, specification, and deviation used to document and support a hydraulic diagnosis.
Start upstream, test downstreamA diagnostic strategy that begins measurement at the pump (the energy source) and proceeds toward the actuators, preventing good components from being misidentified as faulty.

Worked examples4

1. A hydraulic trainer shows slow cylinder extension. Pump outlet pressure measures 1,650 psi; specification is 2,500 psi. Pump flow measures 4.2 gpm; specification is 4.0–4.5 gpm. Relief valve stall pressure is 1,650 psi. What does this data indicate?
  1. Step 1 — Compare pump pressure to spec: 1,650 psi measured vs. 2,500 psi spec. Pressure is low.
  2. Step 2 — Compare pump flow to spec: 4.2 gpm measured vs. 4.0–4.5 gpm spec. Flow is within spec.
  3. Step 3 — Note that stall pressure equals pump outlet pressure (1,650 psi). This means the relief valve is opening at 1,650 psi instead of holding the circuit at 2,500 psi.
  4. Step 4 — Apply the fault table: Low pressure with normal flow points to the relief valve opening too early (stuck open or set too low), not a worn pump (which would show low pressure AND low flow).
Answer: The relief valve is stuck open or set too low at approximately 1,650 psi instead of the specified 2,500 psi. The pump is functioning correctly. Recommended repair: inspect, adjust, or replace the relief valve.
2. A cylinder inlet pressure reads 2,450 psi and pump outlet pressure reads 2,500 psi. The cylinder drifts 3 inches in 60 seconds under a 2,000 lb load with the control valve in the hold position. There is no external leakage. What is the likely fault?
  1. Step 1 — Calculate pressure drop across the line: 2,500 psi − 2,450 psi = 50 psi. A 50 psi drop across the supply line is within normal range; no significant restriction is present.
  2. Step 2 — Observe drift: 3 inches of cylinder movement in 60 seconds under load with no external leakage indicates fluid is moving past the piston seals internally.
  3. Step 3 — Apply the fault table: Normal pump pressure, normal flow, but high cylinder drift with no external leakage = internal cylinder bypass.
Answer: The cylinder is bypassing internally due to worn piston seals or a damaged bore. Pump and relief valve are functioning correctly. Recommended repair: disassemble and rebuild the cylinder, replacing piston seals and inspecting the bore.
3. Filter inlet pressure reads 2,480 psi; filter outlet pressure reads 2,100 psi. The specification states that differential pressure across the filter should not exceed 25 psi during normal operation. Calculate the actual differential and determine what action is needed.
  1. Step 1 — Calculate differential pressure: 2,480 psi − 2,100 psi = 380 psi.
  2. Step 2 — Compare to specification: 380 psi >> 25 psi allowable. The differential is far above specification.
  3. Step 3 — Interpret: A high differential across the filter means the filter element is severely restricted, reducing flow to downstream components and generating heat.
  4. Step 4 — Determine action: Replace the filter element immediately. Inspect fluid for contamination and check for a failed bypass indicator.
Answer: The actual differential pressure is 380 psi, which exceeds the 25 psi specification by a large margin. The filter element is severely restricted. Action: replace the filter element and investigate the source of contamination.
4. A pump's specified flow rate is 5.0 gpm at 1,800 rpm. During testing, the in-line flow meter reads 3.1 gpm at 1,800 rpm. Pump outlet pressure is 2,450 psi (spec 2,500 psi — within tolerance). Calculate the percentage of specified flow actually being delivered.
  1. Step 1 — Set up the percentage formula: (Measured flow ÷ Specified flow) × 100.
  2. Step 2 — Substitute values: (3.1 gpm ÷ 5.0 gpm) × 100 = 62%.
  3. Step 3 — Interpret: The pump is delivering only 62% of specified flow. Combined with nearly normal pressure, this pattern matches a worn pump with significant internal slippage — flow is bypassing internally across worn pump gears or vanes back to the inlet side.
Answer: The pump is delivering 62% of specified flow. Combined with near-normal pressure, this indicates significant internal pump wear. Recommended repair: replace or rebuild the pump.

The math in this lesson2

Learners compute cylinder drift rate in inches per minute and filter differential pressure.
6.RP.37.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor introduces the 'start upstream, test downstream' diagnostic strategy; students practice a five-step pressure-flow test sequence on a healthy trainer to establish baseline readings, recording results in a diagnostic log. · Instructor seeds one of four hidden faults (worn pump, stuck relief valve, bypassing cylinder, restricted filter) on the trainer without telling the student; student conducts the full diagnostic sequence, makes a written diagnosis, and recommends a repair. · Students present their diagnostic findings to the class in a two-minute verbal report, citing specific gauge readings and flow meter data; class votes on whether the diagnosis is supported by the data before the instructor reveals the actual fault.
Materials: Bench hydraulic trainer with four pre-seeded fault sets, calibrated pressure gauges (two), in-line flow meter, temperature gun, diagnostic log forms, stopwatch, technical service manual excerpts
Check: Correct identification of the seeded fault with supporting data cited; diagnostic log complete with all test-point readings; verbal report clearly links data to conclusion (instructor evaluates using a 10-point rubric, 8/10 to pass).

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Hydraulic Systems Diagnosis
1. When using the 'start upstream, test downstream' diagnostic strategy, where should the technician take the first pressure and flow measurements?
Hydraulic Systems Diagnosis
2. A hydraulic system shows low pump outlet pressure AND low pump flow at the specified engine speed. According to the fault pattern table, which component is most likely faulty?
Hydraulic Systems Diagnosis
3. A technician stalls a hydraulic cylinder and reads pump outlet pressure at 1,800 psi. The system specification calls for a relief valve setting of 2,750 psi. The pump flow is normal. What is the most likely diagnosis?
Hydraulic Systems Diagnosis
4. A cylinder drifts steadily under load with the control valve in the hold position. There is no external fluid leakage, and pump pressure and flow are both within specification. What is the most likely cause?
Safety and Shop Practices
5. Before connecting a pressure gauge to a pressurized hydraulic test port, the technician must first:
Hydraulic Systems Diagnosis
6. During testing, filter inlet pressure is 2,500 psi and filter outlet pressure is 2,060 psi. The maximum allowable differential pressure specification is 25 psi. What is the differential pressure, and what condition does it indicate?
Hydraulic Systems Diagnosis
7. A technician records pump flow at 2.8 gpm; the specification is 5.0 gpm. What percentage of specified flow is being delivered?
Safety and Shop Practices
8. Which PPE item is specifically required when working with petroleum-based hydraulic fluid on the bench trainer?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →