4 hours ago
The JD 790 and Its Hitachi Heritage
The John Deere 790 excavator is a product of a transitional era in construction equipment manufacturing. Built during the late 1980s and early 1990s, the 790 was essentially a rebadged Hitachi UH-103, outfitted with a John Deere engine and sold under the Deere brand in North America. This collaboration between Deere and Hitachi marked a significant shift in the excavator market, blending Japanese hydraulic precision with American powertrain familiarity.
Hitachi’s UH series was widely respected for its mechanical simplicity and reliability. The UH-103, in particular, was known for its robust undercarriage, straightforward valve control system, and long service life. Deere’s decision to pair it with their own engine and branding helped expand its reach, especially among contractors loyal to the JD name.
Terminology Clarification
While the JD 790 has a solid reputation, age and design quirks mean buyers should inspect several key areas:
Hydraulic System Complexity and Air-Assisted Design
Unlike modern excavators that rely solely on pilot hydraulics, the JD 790 features a hybrid system. Mechanical linkages actuate valve spools, and an air compressor pressurizes the hydraulic tank to improve fluid delivery. This design, while effective in its time, can confuse technicians unfamiliar with air-assisted hydraulics.
Operators should ensure:
Engine Performance and Fuel Economy
The JD 790 uses a John Deere diesel engine, which some operators found to be mounted higher than in the original Hitachi design. This may affect fuel economy slightly due to altered airflow and cooling dynamics. While not a deal-breaker, it’s worth noting for those comparing fuel consumption across similar models.
In contrast, the Hitachi version often used Hino engines, which were praised for their longevity and efficiency. One owner reported over 10,000 hours on a Hino-powered UH143 with minimal engine work beyond routine maintenance.
Is It Still a Viable Machine Today
The JD 790 is undeniably dated, but its mechanical simplicity and Hitachi DNA make it a viable option for budget-conscious operators or those with strong mechanical skills. It’s not a plug-and-play machine—expect to spend time tuning linkages, bleeding pumps, and maintaining the air system.
However, when properly maintained, it can still perform essential excavation tasks reliably. For land clearing, trenching, and general utility work, the 790 holds its own. Just don’t expect the finesse or comfort of newer pilot-controlled machines.
Recommendations for Ownership
If considering a JD 790:
The John Deere 790 excavator is a relic of a transitional era—mechanical, rugged, and built on the bones of one of Hitachi’s most respected models. While it requires more hands-on attention than modern machines, it rewards that effort with dependable performance. For those willing to learn its quirks and maintain its systems, the 790 remains a capable workhorse decades after its debut.
The John Deere 790 excavator is a product of a transitional era in construction equipment manufacturing. Built during the late 1980s and early 1990s, the 790 was essentially a rebadged Hitachi UH-103, outfitted with a John Deere engine and sold under the Deere brand in North America. This collaboration between Deere and Hitachi marked a significant shift in the excavator market, blending Japanese hydraulic precision with American powertrain familiarity.
Hitachi’s UH series was widely respected for its mechanical simplicity and reliability. The UH-103, in particular, was known for its robust undercarriage, straightforward valve control system, and long service life. Deere’s decision to pair it with their own engine and branding helped expand its reach, especially among contractors loyal to the JD name.
Terminology Clarification
- Pilot Controls: Hydraulic-assisted joystick controls that became standard in later excavators, replacing mechanical linkages.
- Mechanical Linkage: Direct physical connections between control levers and valve spools, common in older machines.
- Hydraulic Tank Pressurization: A system using air pressure to supercharge hydraulic fluid delivery, improving pump response.
- Relief Valve: A safety valve that limits hydraulic pressure to prevent system damage.
- Drive Coupling: A mechanical connection between the engine flywheel and hydraulic pump input shaft.
While the JD 790 has a solid reputation, age and design quirks mean buyers should inspect several key areas:
- Hydraulic system responsiveness: Machines with sluggish boom or travel functions may have air leaks, clogged filters, or worn pump linkages.
- Linkage wear: Mechanical control systems rely on tight tolerances; worn pins or bushings can lead to erratic movement.
- Air compressor function: Some units use compressed air to pressurize the hydraulic tank. If the system isn’t building pressure, performance will suffer.
- Filter removal issues: Hydraulic filters may be difficult to extract if internal retaining mechanisms are corroded or misaligned.
- Pump bleeding: After hose failures, trapped air must be bled from the pump using hex plugs on top of the pump housing.
Hydraulic System Complexity and Air-Assisted Design
Unlike modern excavators that rely solely on pilot hydraulics, the JD 790 features a hybrid system. Mechanical linkages actuate valve spools, and an air compressor pressurizes the hydraulic tank to improve fluid delivery. This design, while effective in its time, can confuse technicians unfamiliar with air-assisted hydraulics.
Operators should ensure:
- The air pressure gauge is in the green zone during operation.
- The relief valve on top of the hydraulic tank is functional and not stuck open.
- The small air tank is drained periodically to remove moisture and oil buildup.
- Linkages to the pumps are properly adjusted and not bent or seized.
Engine Performance and Fuel Economy
The JD 790 uses a John Deere diesel engine, which some operators found to be mounted higher than in the original Hitachi design. This may affect fuel economy slightly due to altered airflow and cooling dynamics. While not a deal-breaker, it’s worth noting for those comparing fuel consumption across similar models.
In contrast, the Hitachi version often used Hino engines, which were praised for their longevity and efficiency. One owner reported over 10,000 hours on a Hino-powered UH143 with minimal engine work beyond routine maintenance.
Is It Still a Viable Machine Today
The JD 790 is undeniably dated, but its mechanical simplicity and Hitachi DNA make it a viable option for budget-conscious operators or those with strong mechanical skills. It’s not a plug-and-play machine—expect to spend time tuning linkages, bleeding pumps, and maintaining the air system.
However, when properly maintained, it can still perform essential excavation tasks reliably. For land clearing, trenching, and general utility work, the 790 holds its own. Just don’t expect the finesse or comfort of newer pilot-controlled machines.
Recommendations for Ownership
If considering a JD 790:
- Perform a full hydraulic system inspection, including air pressure checks and pump bleeding.
- Replace filters and verify fluid cleanliness before operation.
- Adjust mechanical linkages and inspect for wear or misalignment.
- Keep spare air system components on hand, including relief valves and drain plugs.
- Consider retrofitting pilot controls only if the machine will be used extensively.
The John Deere 790 excavator is a relic of a transitional era—mechanical, rugged, and built on the bones of one of Hitachi’s most respected models. While it requires more hands-on attention than modern machines, it rewards that effort with dependable performance. For those willing to learn its quirks and maintain its systems, the 790 remains a capable workhorse decades after its debut.
