Chapter 24 Mechanical Repairs

Objectives Explain the basics of front, rear, and computer- controlled suspension systems Describe the design and operation of steering systems Understand how various brake systems work, and describe procedures for manual and pressure bleeding

Objectives (continued) Perform key cooling and air-conditioning system repairs and maintenance Inspect an exhaust system, and describe the guidelines for working on an emission control system

Introduction After a collision it is often necessary to remove mechanical parts With a front hit, parts behind the grille are often damaged If a vehicle hits a stationary object, force is transmitted through the wheels and into the steering and suspension systems Other parts on the bottom of the vehicle may also be damaged from the impact

Powertrain Powertrain/drivetrain is all of the parts that produce and transfer power to the drive wheels Engine provides energy to move the vehicle and power all accessories Cylinder is a hole bored into the block to guide pistons, which transfer the energy to the connecting rod that links to the crankshaft Crankshaft changes the reciprocating motion of the piston and rod into rotary motion Power to turn the driving wheel comes from the rear of the crank and accessories from the front

Powertrain (continued) Cylinder head seals the top of the cylinder Combustion chamber is a small area between the top of the piston and the bottom of the cylinder where the air-fuel mixture is burned Valves allow the air-fuel mixture into and exhaust out of the combustion chamber, and are controlled by the camshaft Flywheel is a metal disc that keeps crankshaft turning smoothing and connects the engine power to the transmission

Engine Support Systems Lubrication system forces oil into friction points in the engine Oil pan holds a supply of motor oil and bolts to the bottom of the engine block Oil filter traps debris and prevents it from circulating through the engine oil galleries Oil filter, oil pan, and related parts can be made of thin metal and area easily damaged Check the oil level with the oil dipstick, and repair any leaks

Cooling System Cooling system maintains the correct operating temperature Antifreeze prevents freeze-up in cold weather –Radiator transfers coolant heat to the outside air Water pump circulates coolant through engine –Thermostat regulates the coolant Heater system uses coolant heat and a heater core to warm the passenger compartment

Cooling System (continued) Antifreeze should not be reused –Contains compounds that break down over time Hydrometer determines the freeze-up protection of the coolant mixture Check fan belts for cracks, tears, glazing and tension; check electrical connections for corrosion; and radiator caps for calcium deposits Perform cooling system leak test by installing a pressure tester on the radiator neck Radiator cap pressure test is done using a cooling system pressure tester

Figure If the cooling system has been drained for major repairs, refill it with a 50/50 mix of water and antifreeze. Run and warm the engine before checking the reservoir level. Do not remove the radiator cap with a warm system or steam can cause severe burns.

Figure A cooling system pressure tester is commonly used to check the condition of the system. With the tester mounted on the radiator, pump in the cap-rated pressure. Then check for leakage under the engine compartment. The system should hold the pressure without leakage.

Exhaust System Exhaust system collects and discharges exhaust gases caused by combustion Header pipe carries exhaust gases from exhaust manifold to the catalytic converter Catalytic converter is a thermal reactor for burning and chemically changing exhaust products into harmless gases Muffler is a metal chamber for dampening pressure pulsations to reduce exhaust noise Faulty exhaust system parts must be replaced

Fuel System Fuel system must feed the correct amount of fuel into the engine or it will not run efficiently Fuel injectors are electrically operated valves located in the intake manifold When the intake valve opens, fuel is sprayed into the intake port and pulled into the combustion chamber by airflow Fuel pump forces fuel into the engine, and a fuel filter strains debris out of the fuel Fuel pressure regulator controls the amount of pressure at the fuel injectors

Drivetrain Drivetrain uses engine power to turn drive wheels Transmission has a series of gears for increasing torque to the drive wheels In an automatic transmission, a torque converter is used in place of a clutch A transaxle is a transmission and differential combined into a single housing Front-wheel drive vehicles use a transaxle to transfer engine torque to the front drive wheels

Drivetrain (continued) CV-axles transfer torque from the transaxle to the wheel hubs Drive shaft is a long tube that transfers power from the transmission to the rear axle assembly Rear axle assembly contains the ring gear, pinion gear, differential assembly and axles Differential assembly uses gears to allow different amounts of torque to be applied to each drive wheel when the vehicle is turning

Drivetrain/Powertrain Service Check the condition of the CV-joint boots A vibration that increases with speed is rarely due to the CV-joints Drive shaft should be checked for signs of contact against the chassis or rubbing –Could be a sign of a weak or broken spring or engine mount, or chassis misalignment You may need to remove the drivetrain to make structural repairs

Figure The automatic transmission level should normally be checked with the engine idling. If needed, use a long funnel to add the correct amount and type of transmission fluid.

Brake System Brake system uses hydraulic pressure to slow or stop wheel rotation Master cylinder develops hydraulic pressure Brake lines carry fluid to the wheel cylinders which push the brake pads or shoes outward Brake pads have friction lining for rubbing the brake rotor Rotor provides heavy metal friction surfaces bolted between the hub and the wheel

Figure With disc brakes, the caliper mounts over the disc. When fluid pressure enters the caliper, the piston is pushed outward. This applies the brake pads to the spinning rotor or disc to slow or stop the vehicle.

Brake System Service Brake lines must be replaced with the same material used by the factory Do not reuse fluid drained from the system Brake system bleeding removes air from the brake fluid Bleeding brake system is a ten-step process Most anti-lock brake systems have self- diagnosis –Computer prints out a trouble code in the case of an electrical malfunction

Figure Always make sure that the brake master cylinder is full before releasing the vehicle to the customer.

Steering Systems Steering system transfers steering wheel motion through gears and linkage rods to swivel the front wheels Steering shaft extends through the steering column and rotates the steering gearbox Parallelogram steering uses two tie-rod assemblies connected to the steering arms –Center link holds tie-rods into position, and an idler arm supports the center link on one end –Other end is attached to the pitman arm

Rack-and-Pinion Steering Rack-and-pinion steering uses a simplified steering system used on most unibody vehicles –Pinion gear at the end of the steering shaft meshes with the rack When the steering wheel is turned, the pinion rotates in a circle and moves the rack sideways Two inner tie-rod ends are threaded onto the end of the rack and covered by boots The inner tie-rods are threaded onto the outer tie-rod ends, which connect to the steering arms

Figure Note the parts of common rack-and-pinion steering. If damaged, the whole rack assembly is normally replaced.

Power Steering In the conventional system, a pump forces pressurized oil through the system With a computer-assisted system, sensors provide feedback to control the power assist In an electronically controlled power steering, an electric motor is mounted to the rack assembly A DC motor armature with a hollow shaft allows passage of the rack through it Rotary motion of the armature is transferred to linear movement of the rack

Steering System Service Torque all steering system fasteners to specs with a torque wrench Always install new cotter pins, and never try to straighten steering system parts Protect a power steering system from dirt and moisture at all times Replace fluid lost with the recommended type Check power steering hose routing when reassembling the system

Suspension Systems Suspension system allows the tires and wheels to move with the road surface Control arms mount on the frame and ball joints mount on the outer end of the control arms Steering knuckles hold the wheel bearings and wheels Shock absorbers are dampening devices that absorb bouncing, and may be gas, oil or air filled Air spring suspension uses air springs to support the vehicle load

Suspension System Service Always inspect the suspension system for signs of damage from the collision Replace damaged or warn suspension parts Loose, worn, or broken suspension parts will allow the wheels to shift out of alignment A metallic thumping or knocking sound when driving over bumps or unusual tracking indicates a need for service Do not try to straighten any suspension parts

Air-conditioning Systems Receiver-driers and accumulators use a desiccant bag to remove moisture A/C systems are divided into two sides, high and low, by the compressor and expansion device –High-side contains high-pressure/high- temperature refrigerant; hoses feel hot to touch –Low-side contains low-pressure/low-temperature refrigerant; hoses feel cool to the touch

Air-Conditioning System Service Discharge A/C system before removing parts Recovery system captures the used refrigerant to keep it from contaminated the atmosphere Evacuating removes air and moisture Purging uses refrigerant to push air and dirt out of the hoses – always purge before charging R-134a is the refrigerant used after 1992 Mixing R-12 and R-134a even in trace amounts can be fatal to a system

Figure A recovery machine is needed to capture used refrigerant, which can then be recycled. This protects our atmosphere from damage.

Emission Control Systems Emission control system prevent potentially toxic chemicals from entering the atmosphere The exhaust gas recirculation valve opens to allow the engine vacuum to siphon exhaust into the intake manifold Catalytic converter speeds up chemical reaction between exhaust gas and oxygen Fuel evaporative system pulls fumes from the gas tank into a charcoal canister so they can be drawn into the engine and burned PCV channels engine crankcase blowby gases into the engine intake manifold

Emission Control System Service Clean Air Act requires technicians to restore emission control systems and prescribes penalties for shops that fail to restore them Replace damaged parts with good parts Using parts that prevent the proper operation of the emission system is against the law Repairs must be made to manufacturer specs All replacement parts must satisfy the original design requirements

Summary Powertrain/drivetrain is all of the parts that produce and transfer power to the drive wheels –Engine, transmission or transaxle, drive axle, and other related parts Suspension system lets tires and wheels move up and down with road surface irregularities Suspension systems: front, rear, and computer controlled

Summary (continued) A/C system refrigerant leaks are found with: –Electronic leak detector –Refrigerant cans with dye –Soap and water solution in a spray bottle