Uncovering the CAT MM55SR Excavator: Troubleshooting, Maintenance, and Machine Identity

[MikePhua]

The Mystery Behind the CAT MM55SR
The CAT MM55SR is an uncommon mini excavator model that has sparked confusion among operators, parts dealers, and even experienced technicians. The “MM” designation, paired with the compact “55” tonnage class and the “SR” for Short Radius, indicates it is not a mainstream Caterpillar production. In reality, this machine is a rebadged unit manufactured by Mitsubishi Heavy Industries in collaboration with CAT Japan and sold under the CAT name in select international markets.
Understanding this origin is critical when sourcing parts, interpreting service manuals, or diagnosing technical issues. Many components are not interchangeable with standard CAT machines, and serial number prefixes often point toward Japanese domestic production.
Technical Characteristics of the MM55SR
While variations exist based on the year and region, typical specifications of the MM55SR include:

• Operating weight: Approx. 12,000–12,500 lbs
  
• Engine: Mitsubishi S4S or similar 4-cylinder diesel
  
• Hydraulic system: Open center with dual variable-displacement piston pumps
  
• Swing radius: Short tail design for confined spaces
  
• Boom type: Mono boom with auxiliary hydraulic lines
  
• Travel speed: 2-speed system with automatic shifting
  
• Cab: Enclosed with heater; optional A/C in select models
  

Many of these machines were imported as gray-market units, meaning documentation and part support from Caterpillar USA may be limited or nonexistent.
Common Technical Issues and Diagnostic Strategies
Owners and operators of the MM55SR often report challenges in diagnosis due to:

• Lack of English-language service manuals
  
• Unfamiliar part numbers that don’t align with CAT’s standard system
  
• Hydraulic schematics that differ from North American designs
  

Despite these difficulties, several key mechanical and operational issues have emerged consistently:
1. Weak Hydraulics
Symptoms include slow bucket movement, weak digging force, or hesitation when traveling uphill.
Possible causes include:

• Worn main hydraulic pump
  
• Internal bypassing in cylinders or valve block
  
• Dirty or clogged return filters
  
• Leaking O-rings on auxiliary circuit spools
  

Solution strategies involve:

• Installing pressure test gauges on main and pilot circuits
  
• Checking for heat buildup in hydraulic lines (indicates restriction)
  
• Cleaning or replacing the pilot filter (often overlooked)
  

2. Travel Motor Lag or Imbalance
Machines may veer off course when tracking or exhibit one side being slower.
Potential causes include:

• Uneven hydraulic flow from pump diverter valves
  
• Internal travel motor wear
  
• Debris in track line check valves
  

One user reported success after cleaning the case drain filter and swapping travel hoses to isolate the slow side. Another replaced the speed change solenoid, restoring balanced motion.
3. Engine Troubles and Fuel Delivery
Equipped with Mitsubishi diesels, MM55SR engines are generally reliable but sensitive to:

• Poor-quality fuel or water intrusion
  
• Dirty fuel filters
  
• Weak lift pump causing hard starts or stalling under load
  

Operators have addressed these issues by:

• Adding a transparent in-line fuel filter before the primary filter
  
• Replacing fuel lines with modern ethanol-resistant types
  
• Cleaning the tank thoroughly—many machines suffer from microbial buildup due to condensation
  

4. Electrical System Challenges
Common issues include:

• Unresponsive instrument panel
  
• Intermittent shutdowns
  
• Blown fuses linked to poorly grounded circuits
  

Given the age of many MM55SR units, wiring harnesses often suffer from cracked insulation and corrosion. A full rewiring or isolation with continuity tests may be necessary. Several mechanics noted replacing the original fuse box with a waterproof automotive fuse/relay block helped reduce failures dramatically.
Locating Parts and Cross-Referencing
Sourcing replacement components is a challenge due to the hybrid heritage of the MM55SR. However, several strategies have proven effective:

• Identify the Mitsubishi base model, often MM55 or MS120, and search Japanese parts suppliers or local gray-market vendors.
  
• Use serial number prefixes to identify the country and manufacturing year. Many begin with “3ZK” or similar.
  
• Cross-reference components such as the Mitsubishi S4S engine with those used in forklifts or agricultural equipment.
  
• Hydraulic components (like pumps or cylinders) are often sourced from Kayaba (KYB) or Hitachi, so third-party rebuild kits may exist even if CAT part numbers don’t apply.
  

Anecdotes from the Field
One landscaping contractor in Oregon purchased an MM55SR at auction for half the price of a comparable Bobcat. It ran well initially but began losing hydraulic performance. After struggling with parts identification, he discovered that the pump was a KYB unit used in older Yanmar excavators. With a $60 rebuild kit and some homemade gaskets, he restored full digging power.
In Australia, a farm operator imported two MM55SRs directly from Japan for a vineyard project. To keep them running, he created his own service book, tracking cross-referenced parts from Isuzu forklifts, CAT compact machines, and even Komatsu boom cylinders. After five years, both machines were still operational with over 6,000 hours each.
Preventative Maintenance Practices
Because of the parts complexity, regular inspection and proactive maintenance are essential. Owners are advised to:

• Replace all fluids and filters immediately upon acquisition
  
• Flush and clean the cooling system yearly
  
• Inspect all boom and arm pivot points for play
  
• Keep hydraulic quick-connects covered and cleaned
  
• Apply dielectric grease to all electrical terminals
  

Creating a personal logbook or digital spreadsheet for part numbers and sources can save hours of frustration during future repairs.
Conclusion: A Capable but Enigmatic Machine
The CAT MM55SR is a testament to Caterpillar’s global sourcing strategies in the 1990s and 2000s. Though not part of their mainstream North American product line, it offers excellent value and performance—if you know how to manage its unique identity.
Success with the MM55SR requires mechanical savvy, research skills, and a willingness to think beyond traditional parts channels. For those willing to learn its quirks, this short-radius mini excavator can deliver thousands of productive hours, even in tough environments. As one operator put it: “It may not be a real CAT, but it sure works like one.”