Drivetrain and Powertrain Systems

Drivetrain and Powertrain

Introduction to the Drivetrain System

The powertrain is the assembly of components that transmits power from the engine to the drive wheels. For a truck and transport technician, mastering this system is essential, as it represents approximately 30% of the Red Seal exam questions in this field. This chapter covers the fundamental principles, transmission types, diagnostic procedures, and calculations required to pass the interprovincial exam.

The drivetrain system includes, in order of power transfer: the clutch (or torque converter), the transmission, the driveshaft (propeller shaft), the differential, and the axles. Each component has a specific function and distinct failure modes that you must know.

Clutch and Torque Converter

Mechanical Clutch (Friction)

The mechanical clutch is used primarily with manual transmissions. It temporarily interrupts the power transfer between the engine and the transmission. The main components are: the flywheel, clutch disc, pressure plate, release bearing (throwout bearing), and the control mechanism.

The clutch pedal free play must be checked regularly. For a mechanically actuated clutch, typical free play is 25 to 40 mm (1 to 1.5 inches) at the pedal. For a hydraulic clutch, the free play is generally 6 to 13 mm (0.25 to 0.5 inch) at the release bearing check point.

Wear rate calculation: The total pedal travel divided by the free play gives a wear indicator. If the free play decreases by more than 50% from specification, the disc is likely worn and must be replaced.

Torque Converter

The torque converter is used with automatic transmissions. It consists of three main elements: the pump (impeller), the turbine, and the stator (reactor). The converter multiplies torque through the stator, which redirects oil flow back to the pump.

The maximum torque multiplication ratio of a typical truck converter is 2.0:1 to 2.5:1. This ratio decreases as turbine speed increases, until reaching 1:1 (coupling point). Slip at the coupling point is typically 2 to 5%.

Slip calculation: Slip (%) = [(Engine speed − Turbine speed) / Engine speed] × 100. Excessive slip (> 8%) at cruising speed indicates a converter or transmission problem.

Power Take-Off (PTO) Clutch

The power take-off (PTO) is a device mounted on the transmission, transfer case, or engine that supplies power to auxiliary equipment (dump body, crane, hydraulic pump). PTOs can be gear-driven, chain-driven, or belt-driven. The rated capacity of a PTO is expressed in maximum torque (N·m) and power (kW).

Manual Transmissions

Operating Principles

A manual transmission uses gears to change the ratio between engine speed and wheel speed. The gear reduction ratio is the ratio between the number of teeth on the driven gear and the number of teeth on the driving gear.

Ratio calculation: Ratio = Number of teeth on driven gear / Number of teeth on driving gear. For example, if the driving gear has 20 teeth and the driven gear has 60 teeth, the ratio is 3.0:1.

Constant Mesh Transmissions

Modern truck transmissions use constant mesh gears with synchronizers. Synchronizers equalize gear speeds before engagement, reducing wear and noise. The components of a synchronizer include: the hub, sliding sleeve, synchronizer rings (cones), and springs with detent balls.

Double-Clutch (Non-Synchronized) Transmissions

Non-synchronized transmissions, such as the Eaton Fuller Roadranger, require a double-clutching technique to equalize speeds. This technique involves: clutch in, shift to neutral, clutch out, accelerate or decelerate the engine, then clutch in to engage the gear. The technician must understand this procedure to diagnose shifting problems.

Typical Transmission Ratios

Note: Exact ratios vary by manufacturer and model. Always consult the manufacturer's specifications.

Vehicle Speed Calculation

Formula: Speed (km/h) = (Engine speed × 60 × Tire circumference) / (Transmission ratio × Axle ratio × 1000)

Where:

Example: A truck is running at 1800 rpm, with a transmission ratio of 1.00 (10th gear), an axle ratio of 3.55, and tires with a 0.5 m radius (circumference = 2 × π × 0.5 = 3.14 m).

Speed = (1800 × 60 × 3.14) / (1.00 × 3.55 × 1000) = 339,120 / 3550 = 95.5 km/h

Automatic Transmissions

Types of Automatic Transmissions

Automatic transmissions for heavy trucks include:

Hydraulic Components

The valve body directs hydraulic fluid to the various clutches and brakes. The main components include: pressure regulating valves, shift valves, accumulators, and solenoid valves.

Typical line pressure is 100 to 200 psi (690 to 1380 kPa) depending on the gear engaged and torque applied. Pressure must be tested with a gauge at the manufacturer-specified test ports.

Transmission Control Module (TCM)

The transmission control module (TCM) controls shifts based on: accelerator position, vehicle speed, engine speed, fluid temperature, and driver demands. Diagnostic trouble codes (DTCs) are accessed via the electronic diagnostic tool and must be interpreted according to SAE J1939 or J1587 standards.

Driveshaft and Universal Joints

Components and Functions

The driveshaft transmits power from the transmission to the differential. It consists of: the tube, yokes, universal joints (U-joints), slip joint, and carrier bearing supports.

Universal joints (U-joints) allow power transmission through a variable angle. The maximum recommended operating angle is 3 to 5 degrees for high-speed applications. Excessive angles cause vibrations and premature wear.

Operating Angles and Vibrations

Golden rule: The operating angles of the universal joints must be equal and opposite to ensure constant angular velocity of the driven shaft. If the angles are not equal, periodic vibrations will occur.

Angle calculation: Use a digital inclinometer to measure the driveshaft angle relative to horizontal, then measure the differential pinion angle. The difference is the joint operating angle.

Vibration Diagnostic Procedure

Differential and Axles

Types of Differentials

Axle Ratio

The axle ratio is the ratio between the number of teeth on the ring gear and the pinion gear. For example, a 3.55:1 ratio means the ring gear has 3.55 times more teeth than the pinion.

Ratio calculation: Ratio = Number of ring gear teeth / Number of pinion gear teeth. To determine the ratio without disassembly, count the driveshaft revolutions for 10 wheel revolutions, then divide by 10.

Differential Bearing Preload

The preload on differential bearings is essential for component service life. Typical preload is 20 to 40 N·m (15 to 30 lb-ft) of rotational resistance. The backlash between the ring gear and pinion must be 0.13 to 0.23 mm (0.005 to 0.009 inch) for most applications.

Tooth Contact Pattern

The contact pattern is checked with marking compound (prussian blue). The correct pattern should be centered on the tooth, slightly toward the toe for heavy-duty applications. A pattern too high or too low indicates incorrect pinion height adjustment.

Lubrication and Fluids

Types of Lubricants

Service Intervals

Service intervals vary by manufacturer and operating conditions. In general:

Severe conditions (towing, mining operations, extreme temperatures) reduce these intervals by 50%.

Oil Analysis

Oil analysis is a preventive diagnostic tool. Key parameters include:

High iron levels indicate gear or bearing wear. High copper levels indicate bushing or synchronizer wear.

Canadian Standards and Regulations

Canadian Electrical Code

The Canadian Electrical Code, Part I (CE Code) (CSA C22.1) applies to electric and hybrid vehicles. Rule 8-200 requires that power circuits of electric vehicles be protected by compliant fuses or circuit breakers. Technicians must disconnect the high-voltage battery before any work on the powertrain.

CSA B149.1

The CSA B149.1 standard (Natural Gas and Propane Installation Code) applies to vehicles operating on compressed natural gas (CNG) or propane. Although this standard primarily concerns the fuel system, it has implications for the powertrain, particularly for engine mounts and thermal insulation.

SAE Standards

SAE (Society of Automotive Engineers) standards are widely used in the industry:

Advanced Diagnostic Procedures

Automatic Transmission Pressure Test

Typical pressure: In neutral, line pressure is 60 to 100 psi (414 to 690 kPa). In Drive at idle, it is 80 to 120 psi (552 to 827 kPa). At full stall, it can reach 200 psi (1379 kPa).

Torque Converter Stall Test

The stall test verifies the condition of the converter and transmission:

Stall speed should be 80 to 90% of the engine's maximum speed. A speed that is too low indicates an engine or converter problem. A speed that is too high indicates internal transmission slip.

Vibration Diagnostics

Powertrain vibrations can originate from:

Procedure: Use a vibration analyzer to identify the dominant frequency. The frequency in Hz divided by the shaft rotational speed (in rpm) gives the vibration order. An order of 1 indicates imbalance, an order of 2 indicates an incorrect angle.

Advanced Calculations

Output Torque Calculation

Formula: Output torque = Engine torque × Transmission ratio × Axle ratio × Efficiency

Typical mechanical efficiency is 85 to 92% for a manual transmission and 80 to 88% for an automatic transmission.

Example: An engine produces 1500 N·m. The transmission is in 1st gear (ratio 9.96) and the axle ratio is 3.55. Efficiency = 0.88.

Output torque = 1500 × 9.96 × 3.55 × 0.88 = 46,700 N·m

Tractive Force Calculation

Formula: Tractive force (N) = Output torque / Tire rolling radius

Example: With an output torque of 46,700 N·m and a tire radius of 0.5 m:

Force = 46,700 / 0.5 = 93,400 N

Rolling Resistance Calculation

Formula: Rolling resistance (N) = Vehicle mass (kg) × 9.81 × Rolling resistance coefficient

The rolling resistance coefficient is 0.010 to 0.015 for radial tires on paved roads.

Maximum Gradeability Calculation

Formula: Grade (%) = (Tractive force − Rolling resistance) / (Mass × 9.81) × 100

This value is essential for determining whether a vehicle can climb a given grade with a specific load.

Preventive Maintenance

Visual Inspection

Visual inspection should include:

Fluid Level Checks

Fluid level checks should be performed:

Bolt Torque

Transmission and differential mounting bolts must be torqued to specification. Driveshaft bolts should be checked for looseness. Center carrier bearings should be inspected for rubber bushing wear.

Common Pitfalls to Avoid

Summary

The powertrain is a complex system that transmits power from the engine to the wheels. For the Red Seal exam, you must master:

Key points to remember:

Practicing calculations and memorizing typical specifications are essential for passing the exam. Always consult the manufacturer's specifications for exact values, as they vary by model and application.