1988 Mack R688T Owners Manual

The 1988 Mack R688T is a heavy-duty conventional truck featuring the renowned Maxidyne® engine with Maxitorque® transmission, air brake system, heavy-duty chassis, and durable construction for demanding vocational applications. Below are key sections covering operation, maintenance, specifications, and troubleshooting for this classic workhorse truck.

1. Vehicle Overview 2. Instrument Panel & Controls 3. Starting & Operating Procedures 4. Technical Specifications 5. Routine Maintenance 6. Engine Systems 7. Transmission & Drivetrain 8. Brake System 9. Electrical System 10. Troubleshooting Guide

Vehicle Overview

The Mack R688T features a conventional cab design with steel frame construction, designed for durability in construction, logging, and heavy hauling applications.

Component	Description
Engine	Mack ENDT 675 Maxidyne® diesel
Transmission	Mack T-2050 5-speed Maxitorque®
Brake System	Full air brakes with S-cam
Front Axle	FAW 537 12,000 lb capacity
Rear Axle	SWD 58 38,000 lb capacity
Wheelbase	Standard 178 inches
Fuel Tank	100 gallon aluminum
GVWR	Up to 64,000 lbs
Cab	Steel construction with air ride seat
Electrical	12-volt system with circuit breakers

Instrument Panel & Controls

Main instrument cluster includes speedometer, tachometer, air pressure gauges, coolant temperature, and fuel level.

Ignition switch: Start position with glow plug pre-heat
Air pressure gauges: Primary and secondary system pressure
Engine gauges: Coolant temp, oil pressure, voltmeter
Light switches: Headlights, marker lights, clearance lights
Wiper controls: Electric windshield wipers with washer
Heater controls: Fresh air heater/defroster system

WARNING! Do not operate vehicle if air pressure is below 60 PSI. Wait for system to build proper pressure.

Starting & Operating Procedures

Proper starting sequence ensures long engine life and reliable operation.

Pre-trip inspection: Check fluid levels, tires, air system
Ignition on: Allow glow plug indicator to extinguish
Start engine: Crank until oil pressure builds
Warm-up: Allow engine to reach operating temperature
Air pressure: Wait for system to reach 100-120 PSI
Transmission operation: Use progressive shifting technique

CAUTION! Do not exceed 2100 RPM during operation. Maxidyne engine operates best in torque curve range.

Technical Specifications

Engine: Mack ENDT 675, 6-cylinder diesel, 285 HP @ 2100 RPM
Displacement: 672 cubic inches (11.0 liters)
Torque: 1060 lb-ft @ 1200 RPM
Transmission: T-2050 5-speed manual with deep reduction
Clutch: 15.5" 2-plate ceramic
Front Axle: FAW 537, 12,000 lb capacity
Rear Axle: SWD 58, 38,000 lb capacity, 4.17 ratio
Brakes: Air-actuated S-cam, 15" x 4" front, 16.5" x 7" rear
Electrical: 12-volt, 75 amp alternator, maintenance-free batteries
Fuel Capacity: 100 gallons

Routine Maintenance

Regular maintenance ensures optimal performance and longevity.

Daily: Check engine oil, coolant level, air pressure, tires, lights

Weekly: Check battery terminals, brake adjustment, U-joints

Monthly: Change engine oil (15W-40), check transmission fluid, inspect brakes

Quarterly: Replace fuel filters, check wheel bearings, inspect exhaust system

Annually: Coolant analysis, brake system inspection, chassis lubrication

Engine Systems

Maxidyne® engine features constant horsepower design with governed RPM.

Fuel system: Robert Bosch inline injection pump
Cooling system: Pressurized with 50/50 coolant mix
Lubrication: Full-flow filtration with by-pass
Air intake: Dry type air cleaner with restriction indicator
Exhaust: Vertical stack with rain cap
Starting: 12-volt starter with solenoid engagement

Tip: Monitor exhaust smoke - black smoke indicates over-fueling, white smoke suggests coolant issues.

Transmission & Drivetrain

Maxitorque® transmission designed for specific torque curve matching.

Gear ratios: 8.66, 4.95, 2.79, 1.60, 1.00, Reverse 8.05
Shift pattern: 5-speed with deep reduction
Clutch adjustment: 1.5" free play at pedal
U-joints: Regular lubrication required
Rear axle: Double reduction with inter-axle differential
Progressive shifting: Shift at governed RPM for best performance

WARNING! Do not coast with transmission in neutral. Engine brake effectiveness requires transmission engagement.

Brake System

Full air brake system with dual circuit protection.

Air compressor: Single cylinder, gear-driven

Governor: Cuts in at 100 PSI, cuts out at 120 PSI

Air dryer: Removes moisture from system

Chambers: Type 30 front, Type 24/30 rear

Slack adjusters: Manual adjustment required

Parking brake: Spring brake application on rear axle

Electrical System

12-volt system with circuit breaker protection.

Batteries: Two Group 31 maintenance-free
Alternator: 75 amp output with internal regulator
Starter: 12-volt with solenoid shift
Lighting: Sealed beam headlights, incandescent markers
Fuses: Circuit breakers replace fuses for most circuits
Gauges: Electric sending units with mechanical backups

CAUTION! Disconnect batteries before performing electrical work. Observe polarity when connecting.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Hard starting	Weak batteries, glow plugs	Check battery voltage, test glow plug operation
Low power	Fuel restriction, air filter	Replace fuel filters, check air restriction gauge
Air pressure slow to build	Compressor, leaks	Check compressor output, inspect for air leaks
Transmission grinding	Clutch adjustment, synchronizers	Adjust clutch free play, double-clutch shifting
Overheating	Coolant level, thermostat	Check coolant level, test thermostat operation
Electrical issues	Ground connections, alternator	Clean battery connections, test alternator output

Emergency Procedures: Know parking brake application for brake failure situations.

Service: Contact authorized Mack dealer for major repairs and parts.