CAT 320 / 320GC / 323 Next Gen Travel Hydraulic System: Complete Troubleshooting Guide

The travel hydraulic system of the Caterpillar 320, 320GC, and 323 Next Generation Excavators is engineered for strong torque, smooth control, and accurate straight‑line travel. This guide explains the system using real technician logic: component identification, system operation, testing, calibration, and troubleshooting.

This article is structured like a professional Caterpillar training module, rewritten in clear, original language for technicians and students.

1. Travel Hydraulic System Components (Identification & Functions)

Before diagnosing travel issues, technicians must identify each component and understand its function. Below is a complete list rewritten from real CAT training logic.

Component Functions (Rewritten for Clarity)

Travel Motor Assemblies – Convert hydraulic pressure into rotational torque to drive the tracks.
Final Drives – Reduce high‑speed motor rotation into slow, high‑torque output using planetary gears.
Swivel (Center Joint) – Transfers hydraulic oil between upper and lower frames while allowing 360° rotation.
Travel Speed Switch – Lets the operator select low‑speed/high‑torque or high‑speed/low‑torque travel modes.
Counterbalance Valves – Prevent uncontrolled movement, hold the machine on slopes, protect the motor from over‑pressure, and ensure smooth deceleration.
Pump Pressure Sensors – Send real‑time pump pressure data to the ECM for pump control, straight‑travel correction, and system protection.
Straight Travel Spool – Balances hydraulic flow between left and right travel motors to maintain straight travel.

Technician Worksheet: Component Identification

Use this worksheet-style table during diagnostics or training.

No.	Component	Function (Rewritten)
1	Pump Supply Hose	Delivers high‑pressure oil from the pump to the travel circuit.
2	Case Drain Line	Returns leakage oil from the travel motor to the tank.
3	Case Drain Filter	Filters leakage oil to protect the hydraulic tank and pump.
4	High‑Pressure Line (Forward)	Supplies forward‑direction pressure to the travel motor.
5	Line Check Valve	Prevents reverse flow and stabilizes pressure.
6	High‑Pressure Line (Reverse)	Supplies reverse‑direction pressure to the travel motor.
7	Control Valve Section	Directs oil to left or right travel motor based on joystick input.
8	Final Drive	Reduces speed and multiplies torque to rotate the sprocket.
9	Left Travel Motor	Converts hydraulic pressure into rotation for the left track.
10	Right Travel Motor	Converts hydraulic pressure into rotation for the right track.

2. How the Travel Hydraulic System Works (Operation Logic)

The travel system uses two pumps working together:

Drive Pump → Right Travel Motor
Idler Pump → Left Travel Motor

Pilot pressure shifts the travel spool, sending oil to the motors. The system automatically adjusts flow based on load, slope, and operator input.

Straight Travel Valve Operation

Balances flow to both motors.
Prevents machine from pulling left or right.
Activates automatically during straight travel.

2‑Speed Travel Operation

Controlled by a solenoid.
High speed = low torque.
Low speed = high torque.
Automatically shifts under heavy load.

3. SAHR Travel Brake (Spring‑Applied, Hydraulic‑Released)

Each travel motor contains a SAHR brake that locks the tracks when the joystick is neutral. The brake releases only when pilot pressure is applied.

Condition	Pilot Pressure	Brake Status
Joystick Neutral	0 psi	LOCKED
Travel Command	~595 psi (4100 kPa)	RELEASED

4. Travel Motor (Sectional View & Failure Symptoms)

The travel motor converts hydraulic pressure into mechanical rotation. Inside the motor:

Piston Group – Generates torque.
Brake Piston – Releases SAHR brake.
2‑Speed Valve – Changes displacement.
Case Drain Circuit – Removes leakage oil.

Travel Motor Failure Symptoms

Slow travel when hot
Weak travel on slopes
High case drain flow
Metal in case drain filter
One track weaker than the other

5. Testing & Adjusting the Travel Hydraulic System

Key Tests

Main Pump Pressure Test
Pilot Pressure Test
Travel Motor Case Drain Test
Straight Travel Valve Function Test
2‑Speed Solenoid Test

Calibration Steps

Calibrate travel speed control
Calibrate pilot pressure sensors
Verify ECM travel parameters

6. Troubleshooting: Symptoms, Causes & Solutions

Symptom	Possible Cause	Corrective Action
Machine Pulls to One Side	Straight travel spool sticking	Clean/replace spool; check pilot pressure
Slow Travel When Hot	Worn travel motor (high case drain)	Perform case drain test; rebuild motor
No Travel / Engine Bogging	SAHR brake not releasing	Check pilot pressure & brake piston
Weak Travel on Slope	Low pump pressure or relief valve issue	Test pump pressure; inspect relief valves
High Speed Not Working	2‑speed solenoid failure	Check solenoid signal & valve movement

Frequently Asked Questions

Q1: Why does my CAT 320 travel slow when hot? A worn travel motor with high case drain flow is the most common cause.

Q2: Why is my excavator pulling to one side? A sticking straight travel spool or pump imbalance is usually the cause.

Q3: What pressure releases the CAT 320 travel brake? The SAHR brake releases at ~595 psi (4100 kPa) pilot pressure.

Q4: How do I test a travel motor? Perform a case drain test and compare flow to specifications.

Q5: Why is high speed not working? A failed 2‑speed solenoid or blocked valve is the usual cause.

Disclaimer: This guide is for educational purposes. Always follow the official Caterpillar Testing & Adjusting Manual for your machine’s serial number.

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