Mack MR688S and E7-400 Engine Service Insights and Operational Reliability

[MikePhua]

The MR688S and Its Role in Vocational Trucking
The Mack MR688S is a cab-over-engine vocational truck designed for refuse hauling, construction, and municipal service. Introduced in the 1990s, it features a rugged steel frame, low-profile cab, and exceptional maneuverability in urban environments. Its popularity stems from its ability to handle heavy loads while maintaining visibility and control in tight spaces. The MR688S is often paired with the Mack E7-400 engine, a turbocharged inline-six diesel known for its torque and longevity.
Mack Trucks, founded in 1900 and now part of Volvo Group, has built a reputation for durable, purpose-built vehicles. The MR series was a staple in North American fleets for over two decades, with thousands of units still in operation.
Terminology Note

• E7-400: A 12-liter inline-six diesel engine producing 400 horsepower and up to 1,460 lb-ft of torque.
  
• Jake Brake: An engine compression brake that slows the vehicle by releasing compressed air from the cylinders.
  
• Air-to-Air Intercooler: A system that cools turbocharged air before it enters the engine, improving efficiency.
  
• Service Manual: A technical document detailing maintenance procedures, torque specs, wiring diagrams, and troubleshooting steps.
  

Engine Configuration and Maintenance Essentials
The E7-400 engine is built with a cast iron block, wet sleeve cylinders, and mechanical fuel injection. Key service intervals include:

• Oil and filter change every 15,000 miles
  
• Valve lash adjustment every 100,000 miles
  
• Fuel filter replacement every 20,000 miles
  
• Coolant flush every 2 years or 100,000 miles
  

Common wear points include:

• Turbocharger bearings and seals
  
• Injector tips and fuel lines
  
• Rocker arm bushings and valve springs
  
• Front and rear crankshaft seals
  

Using Mack’s recommended 15W-40 oil and OEM filters helps maintain pressure and reduce sludge. Technicians should monitor boost pressure and exhaust temperature to detect early signs of turbo wear.
Transmission and Driveline Integration
The MR688S typically uses a Mack T2180 18-speed manual transmission or Allison automatic depending on application. The driveline includes:

• Meritor tandem rear axles with inter-axle lock
  
• Air suspension or camelback leaf springs
  
• Driveshaft U-joints requiring greasing every 10,000 miles
  

Clutch inspection and adjustment are critical, especially in stop-and-go refuse work. Worn clutch plates can lead to hard shifting and driveline vibration.
Electrical System and Diagnostic Strategy
The MR688S uses a 12V electrical system with dual batteries and a belt-driven alternator. Common issues include:

• Corroded ground straps causing intermittent faults
  
• Failed relays in the cab fuse panel
  
• Worn connectors at the ECM and sensor harnesses
  

Technicians should use a multimeter to verify voltage drops and continuity. Replacing connectors with weather-sealed units improves reliability in wet environments.
Cooling and Air Systems
The E7-400 uses a belt-driven water pump and thermostatically controlled fan clutch. The air system includes:

• Bendix air dryer with desiccant cartridge
  
• Dual air tanks with drain valves
  
• Brake chambers and slack adjusters
  

Service tips:

• Replace air dryer cartridge annually
  
• Drain tanks weekly to prevent moisture buildup
  
• Inspect fan clutch for engagement delay or bearing noise
  

Overheating is often caused by clogged radiators or failed thermostats. Using a pressure tester can identify leaks in the cooling system before failure.
Anecdote from the Field
In 2022, a sanitation fleet in New Jersey rebuilt an MR688S with over 600,000 miles. After replacing the turbo, injectors, and clutch, the truck returned to service with improved fuel economy and smoother shifting. The foreman noted that using the original service manual helped avoid wiring mistakes and ensured proper torque specs on the cylinder head bolts.
Manufacturer History and Fleet Impact
Mack’s MR series was produced from the early 1980s through the mid-2000s. The E7 engine family powered tens of thousands of trucks across North America. Though replaced by the MP series engines and Granite chassis, the MR688S remains a favorite in refuse and construction fleets due to its visibility, durability, and ease of service.
Sales of the MR688S peaked in the late 1990s, with many units still operating thanks to rebuildable components and strong aftermarket support.
Recommendations for Fleet Managers and Technicians

• Keep service manuals and wiring diagrams accessible in the shop
  
• Use OEM parts for engine and transmission rebuilds
  
• Document service intervals and component replacements
  
• Train drivers on proper shifting and brake use to extend component life
  
• Monitor oil pressure, coolant temperature, and boost levels during operation
  

Conclusion
The Mack MR688S paired with the E7-400 engine represents a durable and serviceable platform for vocational trucking. With disciplined maintenance, access to technical documentation, and informed diagnostics, these trucks can deliver reliable performance well beyond their original service life. Whether hauling refuse or supporting construction, the MR688S continues to prove its value in demanding environments.