1988 R688T Owners Manual

The 1988 R688T is a heavy-duty conventional truck manufactured by Mack Trucks, featuring a robust chassis, powerful diesel engine, durable transmission system, and advanced braking capabilities for its time. This manual covers operation, maintenance, specifications, and troubleshooting for this classic workhorse vehicle. Below are key sections for safety, features, operation, maintenance, and technical specifications.

Features at a Glance

Key components: Mack E6 diesel engine, Fuller transmission, Rockwell axles, air brake system, heavy-duty chassis, and durable cab design.

Feature	Description
Engine	Mack E6 series diesel engine
Transmission	Fuller Roadranger transmission
Brake System	Air-over-hydraulic braking system
Cab Design	Conventional steel cab with sleeper option
Axle Configuration	Rockwell single or tandem rear axles
GVWR	Up to 66,000 lbs depending on configuration
Electrical System	12-volt system with heavy-duty alternator
Fuel System	Dual fuel tanks with combined capacity
Suspension	Heavy-duty leaf spring suspension
Steering	Power steering with manual backup

Vehicle Operation

Proper operation procedures for daily use and optimal performance.

1. Pre-start inspection: Check fluid levels, tire pressure, and brake system.
2. Starting procedure: Depress clutch, turn ignition key, allow glow plugs to warm.
3. Shifting: Use proper double-clutch technique for smooth gear changes.
4. Braking: Allow adequate stopping distance for air brake system.

WARNING! Always perform pre-trip inspection. Never operate with air pressure below minimum specification.

First-Time Setup

Initial vehicle preparation and break-in procedures.

1. Fluid checks: Verify engine oil, coolant, transmission fluid, and differential oil.
2. Break-in period: Follow recommended break-in procedure for first 500 miles.
3. System checks: Test all electrical, braking, and steering systems.
4. Documentation: Record initial mileage and service dates.

CAUTION! Follow break-in procedures carefully to ensure long engine life.

Specifications

• Engine: Mack E6-275 (275 hp) diesel
• Transmission: Fuller RT-9509A 9-speed
• Wheelbase: 180-250 inches (depending on configuration)
• Brakes: Air-over-hydraulic with S-cam design
• Fuel Capacity: 100-150 gallons (dual tanks)
• Cab: Steel conventional with optional sleeper
• Electrical: 12-volt system with 90-amp alternator
• GVWR: 50,000-66,000 lbs
• GCWR: Up to 80,000 lbs

Controls and Instrumentation

Dashboard layout and control functions.

Instrument Panel: Speedometer, tachometer, air pressure gauges, coolant temperature, oil pressure, fuel level, voltmeter.

Primary Controls: Ignition switch, headlight controls, wiper controls, heater/defroster, clutch pedal, brake pedal, accelerator, gear shift lever. Secondary Controls: Air brake controls, trailer brake hand valve, differential lock, engine brake control, hazard lights.

Maintenance Procedures

Scheduled maintenance intervals and procedures.

1. Daily: Check fluid levels, tire pressure, air system, lights.
2. Weekly: Lubricate chassis, check brake adjustment, inspect belts.
3. Monthly: Change engine oil and filters, inspect brakes, check wheel bearings.
4. Quarterly: Service transmission and differentials, inspect suspension.

Tip: Keep detailed maintenance records for warranty and resale value.

Engine Operation

Mack E6 diesel engine operation and care.

1. Starting: Allow glow plug cycle in cold weather before cranking.
2. Warm-up: Allow engine to reach operating temperature before heavy loads.
3. Operating: Maintain proper RPM range for fuel efficiency.
4. Shutdown: Allow engine to idle briefly before shutting down after heavy use.
5. Maintenance: Regular oil changes at 10,000 mile intervals.
6. Cooling: Maintain proper coolant mixture and level.
7. Fuel: Use clean, dry diesel fuel with proper additives in cold weather.
8. Air System: Regular air filter inspection and replacement.

WARNING! Never operate engine with low oil pressure or overheating indicated.

Safety Features

Built-in safety systems and emergency procedures.

Air Brake System: Dual circuit design with emergency backup. Lighting: Standard marker lights, turn signals, brake lights, and emergency flashers. Mirrors: West Coast style mirrors with convex spot mirrors. Emergency Equipment: Fire extinguisher, warning triangles, first aid kit recommended.

Care & Maintenance

Cleaning and preservation procedures.

Exterior: Wash regularly with mild soap, wax annually. Interior: Vacuum regularly, clean vinyl surfaces with appropriate cleaners. Underhood: Keep engine compartment clean, check for leaks regularly.

CAUTION! Avoid high-pressure water near electrical components and air intake.

Regulatory Information

Compliance and certification details.

FMVSS: Complies with Federal Motor Vehicle Safety Standards applicable in 1988. EPA: Meets 1988 emissions standards for heavy-duty diesel engines. DOT: Certified for interstate commerce. Copyright © 1988 Mack Trucks, Inc. For support, contact authorized Mack dealerships or Mack Customer Service.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Hard starting	Weak batteries/glow plugs	Check battery voltage; test glow plug operation; verify fuel delivery.
Low power	Fuel/air restrictions	Check air filter; inspect fuel filters; verify turbocharger operation.
Air pressure low	Compressor/leaks	Check compressor output; inspect for air leaks in system.
Transmission noise	Low fluid/wear	Check transmission fluid level; inspect for internal damage.
Brake fade	Adjustment/contamination	Adjust brake shoes; inspect for oil contamination; check linings.

Service: Contact authorized Mack service centers for major repairs.

Emergency: Mack Roadside Assistance available for qualified owners.