Terex 640B Transmission Service and What You Should Know

[MikePhua]

History and Positioning of the Terex 640B
The Terex 640B — originally branded under Fermec before company acquisitions — is a mid‑size wheel loader built for heavy loading, construction, and material‑handling applications. As part of Terex’s product line during the late 1990s and early 2000s, the 640B was aimed at operators needing robust performance but with mobility and versatility superior to larger loaders. Many units found their way into municipal fleets, road maintenance crews, quarries, and construction contractors around the world.
Because of its solid frame, adequate hydraulic and transmission design, and reasonable footprint, the 640B enjoyed broad adoption. Although exact sales numbers are not publicly published, industry sources estimate that several thousand units were sold globally during its production run, and many remain in service today thanks to diligent maintenance or retrofits.
The transmission (drive train) of the 640B is especially critical — it translates engine power to wheels and enables steering, braking and load hauling. Proper service and maintenance of the transmission directly affect the loader’s performance, safety, and longevity.

Transmission Design Overview
The 640B's transmission system combines multi‑speed gearbox components with a torque converter and final drives linked to the loader’s axles. Key design aspects include:

• Torque converter: multiply engine torque during start, maintain smooth acceleration, and absorb shock loads.
  
• Multi‑speed gearbox: allows shifting for different travel speeds when loaded or hauling.
  
• Hydrostatic components: some loader functions rely on hydraulic pressure, but driving torque is mechanical/hydraulic hybrid — requiring clean fluid and correct pressures.
  
• Final drives and axles: transfer torque from transmission to wheels, with reduction gears and differentials built for heavy loads and mixed‑terrain usage.
  

Because of the interdependence of torque converter, gearbox, hydraulic clutch packs (if any), and final drives, failure in one part can cascade to others. Hence, regular service is vital to prevent catastrophic breakdown.

Common Issues Observed in Aging 640B Transmissions
Operators servicing older 640B units typically face several recurring problems, especially if maintenance has been deferred or operating conditions are harsh (e.g. heavy loads, abrasive materials, frequent starts/stops, steep terrain):

• Contaminated transmission fluid — dirt, dust, water or metal particles can degrade clutch packs, bearings, and gear surfaces.
  
• Worn clutch packs or bands — leading to slipping, inability to transmit torque, overheating or failure to shift properly.
  
• Torque converter problems — damaged turbine/stator vanes or worn bearings cause shuddering, slipping under load, or inability to deliver full torque.
  
• Gearbox wear — teeth pitting, bearing failure, or mis-alignment can lead to noisy operation, limited speed, or loss of drive.
  
• Final drive seal leaks or bearing fatigue — loss of fluid, increased friction, overheating of axles or wheel hubs.
  

In reported fleet maintenance data, transmission-related failures (fluid contamination, clutch slippage, final‑drive leaks) comprised nearly 45–60% of all unscheduled loader downtime.

Essential Service and Rebuild Procedures for Transmission
To keep a 640B transmission healthy or restore one properly, it’s recommended to follow a full-service protocol:

• Drain and inspect transmission oil — look for metal shavings, water, dark discoloration, or burnt smell.
  
• Replace filter elements, magnets, strainers — remove all possible debris and ensure clean oil delivery.
  
• Measure clutch pack wear — check thickness, friction surfaces, and engage test at idle. Replace worn friction disks or bands rather than risk slippage under load.
  
• Inspect torque converter — spin turbine manually (if possible), check for irregular play, vane damage or bearing wear; replace or rebuild converter if needed.
  
• Inspect gearbox gears and bearings — check for pitting, chipping, bearing noise, or backlash beyond specification; machine or replace parts when out of spec.
  
• Seal final drives and axle bearings — renew seals, pack bearings, and ensure proper oil levels in final drives.
  
• Perform pressure and load tests — with full hydraulic and engine load, test shift behavior, torque converter lockup, and drive response before returning machine to service.
  

Owners who follow such comprehensive maintenance often extend the transmission’s service life by thousands of hours compared to units maintained only partially.

Upgrades and Modern Maintenance Enhancements
Given the age of many 640B loaders, informed operators often adopt upgrades or improved maintenance practices to enhance reliability:

• Use high‑quality synthetic transmission fluid with better oxidation and heat resistance. This reduces breakdown under high temperature and heavy load cycles.
  
• Install aftermarket magnetic or fine‑mesh filters/pan magnets to capture small metal particles before they circulate — improves clutch pack and gear life.
  
• Schedule fluid and filter changes more frequently than original intervals, especially in dusty or abrasive working environments.
  
• Add temperature and pressure sensors (if not present) — helps detect overheating or pressure drop early, enabling preventive shutdown before damage.
  

Such enhancements often reduce unscheduled breakdowns by 30–50%, according to field reports from contractors with mixed-use fleets.

Real‑World Case Study: Rebuilding a 640B Transmission
A quarry operator purchased a used 640B loader with a slipping transmission under moderate load. After diagnosis, the maintenance crew discovered black, metallic‑flecked fluid, a worn clutch pack, and a partially seized torque converter. They proceeded to:

• Drain and clean hydraulic/transmission system thoroughly
  
• Replace clutch pack friction disks and bands with quality aftermarket replacements
  
• Rebuild the torque converter, replacing bearings and sealing turbine housing
  
• Replace all seals and oil in final drives and axles
  
• Install improved filter and magnetic pan inserts
  

Post‑repair performance restored full drive torque, smooth shifting, and even improved fuel efficiency (about 8% lower fuel burn per ton of material moved) because the transmission no longer slipped under load. The loader went on to perform well in heavy material handling for 18 months before any minor follow-up service — demonstrating that with correct rebuild and maintenance, even older 640B units can have extended useful life.

Recommendations for Owners and Operators
For those operating a 640B (or similar wheel loaders) today — especially aged units — here are practical recommendations:

• Prioritize full transmission service if fluid appears contaminated, especially before heavy-duty seasons (winter, quarry work, demolition seasons).
  
• Replace wear-prone parts (clutch disks, seals, bands) pre‑emptively when service intervals are long or operating conditions are harsh.
  
• Use high‑quality transmission fluids and filters; avoid generic or low‑grade oils that degrade quickly under load and heat.
  
• Monitor temperatures and listen for abnormal noises — the transmission often gives early warnings before catastrophic failure.
  
• Keep maintenance records: tracking hours, fluid changes, part replacements helps project expected maintenance cycles and reduce unexpected downtime.
  

Conclusion — Service Matters for Longevity and Productivity
The Terex 640B remains a capable and versatile loader when properly maintained. The transmission system — though robust — depends heavily on fluid quality, part integrity, and timely service to deliver reliable performance.
Owners who commit to thorough transmission maintenance or rebuild — especially when acquiring used machines — find that the 640B continues to deliver strong operational value. Neglecting transmission care, by contrast, often leads to downtimes, expensive rebuilds, or premature machine retirement — losses that easily outweigh initial savings from buying used or avoiding maintenance.
For contractors, municipal fleets, or rental operators, investing in proper transmission care is one of the most cost‑effective ways to secure productivity, safety, and long–term asset value.