1986–1994 Hitachi EX120-2 Excavator Technical Manual

This technical manual covers the 1986–1994 Hitachi EX120-2 excavator with the Isuzu 4BD1T engine, including operating principles, performance tests, and repair diagnosis for the 12N series from serial number 12N-30001 onward. References to the earlier EX120 concern component interchangeability.

PDF navigation and diagram detail: Embedded bookmarks reach system chapters and individual test procedures. Technical chapters and diagrams do not support text search; small labels in the main wiring and engine harness diagrams require zooming.

• Hydraulic controls: Main pump and pilot pump operation, control valve analysis, relief-pressure testing, and auxiliary spool speed adjustment.

• Electrical diagnosis: EC/PVC fault codes, Dr.EX monitoring, connector views, wiring diagrams, and circuit-specific harness tests for sensors and solenoid valves.

• Engine performance: Engine speed and compression testing, EC motor installation, engine-control sensor adjustment, and automatic engine-speed programming.

• Swing and travel: Parking brakes, travel reduction devices, center joint operation, swing bearing play checks, and motor drainage tests.

• Attachment performance: Cylinder cycle-time and drift checks, combined boom/arm/swing tests, working-range drawings, lifting capacities, and attachment interchangeability.

• Service reference: Hydraulic fluid, engine oil, coolant, and gear oil maintenance specifications, with refill capacities and component-specific adjustment data.

This manual is suited to excavator technicians familiar with hydraulic testing and electrical fault isolation, including Dr.EX and the specified diagnostic harnesses.

Main Pump, Pilot Circuits, and Control Valves

System drawings identify components across the front-end attachment, upperstructure, and undercarriage. Hydraulic chapters explain how the HPV091DS main pump, pilot device, and variable pressure compensated valve (VPCV) regulate motor and cylinder circuits.

Main pump section views and the Combined Boom/Swing hydraulic diagram show the relationship between pump regulation, shared flow, and machine movement. Separate coverage follows the shockless valve, unloading valve, surge relief valve, and pilot piping, including warm-up circuit and shuttle valve logic.

Accumulator nitrogen-gas checks extend the coverage beyond component operation. PC-pressure adjustment notes explain how backing the proportional solenoid valve adjusting screw out too far can unseat its O-ring and cause oil leaks.

The hydraulic safety instructions address level parking, a grounded bucket, and pressure relief before opening lines to prevent fluid-injection injuries.

Performance Tests and PVC Versions

Performance standards distinguish PVC versions through 0800, 1000/1001/1002, and 1200 onward, with corresponding pump P-Q curves. The test section separates standard values from service limits and provides checks for confirming performance after repair.

Pump flow testing is documented with Dr.EX or portable hydraulic tester ST6299. Dr.EX flow readings are calculated from pump angle and engine speed, an important distinction when comparing monitor information with instrumented tests.

Diagnostic charts connect DP-sensor readings, load-sensing shuttle faults, and unloading valve behavior to slow operation. These checks help distinguish control-system faults from actual pump-output loss before planning pump replacement.

Swing-device coverage explains make-up valves, anti-cavitation, and swing dampener operation. Symptom charts trace weak travel, mistracking, swing drift, and parking-brake problems to their hydraulic or mechanical causes.

EC/PVC Diagnosis and Wiring Tests

Troubleshooting A addresses controller fault codes; B follows operating symptoms, including faults without codes; C isolates gauge and indicator problems. N-, P-, A-, and DP-sensor coverage links sensing signals to engine-speed control and pump regulation.

The Electric Wiring Diagram, connector terminal-position tables, and engine harness drawings support circuit tracing. Harness checks specify Hitachi lamp harness test kit 4283595 and cover sensor circuits, solenoid valves, control motors, and relays.

Electrical repair notes explain how removing powered harnesses can cause short circuits that damage wiring, fuses, and fusible links.

Attachment Compatibility and Service Scope

The interchangeability tables explain that EX120 buckets fit the EX120-2 with reduced crowd and dump angles, while EX120-2 buckets do not fit the EX120.

Fluid capacities cover the hydraulic tank, engine crankcase, and swing/travel reduction gears. Maintenance data include hydraulic suction-filter cleaning at hydraulic oil changes; torque specifications accompany relief and pilot-pressure adjustments.

Engine coverage centers on performance tests and control-system diagnosis; internal engine repair is referred to a separate Engine Shop Manual.

Printable PDF and Included Sections

One printable 451-page PDF contains 221 pages of specifications and system operation, 72 pages of performance testing, and 144 pages of troubleshooting. The total also includes two introduction pages, 11 safety pages, and one title divider.