While ignition systems of today look dramatically different from the ones we used to tune with a matchbook (to set the gap on the points), the principles that control the delivery of the spark to the air/fuel mixture remain the same.
Leaving behind the very basic, old style systems, the challenge for counterpeople is to understand what is required to go right for the systems to work. Any ignition system, old or new, consists of two separate circuits. The Primary Circuit consists of the battery, ignition switch, a primary switch (points or ignition module), and the primary (low voltage) winding of the ignition coils. It operates at low voltage from 10 to 12 volts up to 250 volts at the primary coil windings. On a Distributorless Ignition System (DIS), the Secondary Circuit is consists of the secondary windings of multiple ignition coils, and secondary high voltage spark plug wires and spark plugs. The secondary circuit, regardless of type, supplies high voltage electricity (10,000 to 50,000 volts) to the spark plugs to ignite the air/fuel mixture.
It is helpful to recognize that it is the collapse of the magnetic field which induces the very high voltage in the Secondary Circuit, regardless of the ignition system. Old style systems had points that opened and closed. Systems without points use sensors and electronic switches to accomplish the same task. The time that the circuit remains closed is still referred to as dwell, a remnant of the mechanical ignitions.
DIS systems are notable for the Direct Ignition Coil Packs most often employed. Commonly, the number of ignition coils is one for every two cylinders. In such a system, spark is delivered once each revolution. This results in one spark plug firing to ignite a fresh air/fuel charge while the spark plug firing off the same coil at the same time would be firing into a cylinder with exhaust gasses only.
Some systems now in use have gone a step further and have a separate coil for each cylinder. These are sometimes referred to as Direct Ignition Systems, often with the coil situated directly over the spark plug. One advantage of this approach is that the high voltage pulse has very little distance to travel.
The ability to fire cylinders in pairs or separately enables the engine computer to very accurately control ignition parameters. This is particularly important as compression ratios continue to rise leading to the increasing possibility of detonation or knock.
In DIS systems cylinder position and rpm are indicated by sensors on the camshaft, crankshaft or both. An engine speed (rpm) signal is generated by the Crankshaft Sensor. Cylinder position can be either indicated by a second, or combination, crankshaft sensor or alternatively by a separate Camshaft Sensor. In each case a signal is generated by the rotation of a disc or wheel with notches or slots. These may be cut into an existing engine part such as a harmonic balancer, or the camshaft sprocket.
The Combination Crank Position Sensor provides the crank top-dead-center signal and the synchronization signal, or sync pulse, that indicates which cylinder’s ignition should be fired. Some systems use a crank and cam sensor for ignition timing and a separate cam sensor for timing the fuel injection.
Knock Sensors are tuned to detect the vibration of a cylinder charge undergoing detonation. The signal it sends to the computer or to the ignition module causes the ignition timing to be retarded on all cylinders at once, pairs of cylinders, or even individual cylinders. It may also cause the air/fuel ratio to be adjusted. These sensors, however, can be fooled by other transient vibrations, such as a banging exhaust shield, and unnecessarily retard ignition timing. In fact, one method of checking the function of a knock sensor is to rap the engine with a hammer.
Spark Plugs are the business end of the ignition system. While you are surely aware of the fact that spark plugs are designed for different heat ranges, did you know that they are responsible for conducting the majority of heat from the combustion process to the cylinder head? Understanding this makes it clear why it is so important to have the right heat range installed in a vehicle to remove enough of the heat to keep the combustion temperatures at a manageable level and yet retain enough to allow for complete, deposit free combustion.
FUEL AND EMISSIONS SYSTEMS
Early emissions systems sapped power, worked erratically and just plain hid the engine in a maze of hoses. For the most part, the Canadian market was spared the electronic feedback carburettor, but those have fallen deep into the category of historical curiosity with few tears shed to mark their passing.
Today, systems are far more complex, even if they don’t look that way. But they work much better, too. In fact modern cars emit only a fraction of the harmful exhaust gasses of those only a decade ago.
A large part of the push behind the increasing complexity of the fuel system is the increasing demands to reduce emissions and improve fuel mileage. This is to the point that the fuel system as a separate entity is no-longer, being as it is so closely tied in to the ignition and emissions systems and powertrain controls. Without tightly controlling spark advance, combustion temperature and fuel delivery, meeting the government mandated emissions limits would be impossible.
In general, all fuel systems seek to maintain a fuel-air ratio of 14.7:1. This is referred to as stoichiometric. While some engine systems sold in other markets work can burn ratios as lean as 30:1 and up, employing gasoline direct injection, these will not be the norm for some time.
For the more conventional systems, however, there is a fairly standard set of components. It should be noted that not all systems have all components and that newer, OBD II equipped vehicles may have more than one, particularly oxygen sensors.
The Electronic Control Module’s (ECM) job includes the fuel system controls but also stretches to ignition and other systems, depending on the vehicle. It receives signals from the sensors and issues commands. It is also too often misdiagnosed as the culprit when driveability and starting problems arise.
The Air Charge Temperature Sensor converts air temperature to a voltage signal and operates similar to the engine coolant sensor. It helps the ECM properly meter the air-fuel ratio.
The Engine Coolant Temperature Sensor (ECT) also converts temperature into a voltage signal. The signal is processed by the ECM to control fuel mixture, spark advance, and cold start idle as well as other parameters.
The Cold Start Valve provides an engine with additional fuel for better cold starting. It’s operation is controlled by the Thermal (or Thermo) Time Switch. Mounted on the block, it reads the coolant temperature and closes the Cold Start Valve when the temperature is above a predetermined point.
The Crankshaft Position Sensor/Camshaft Position Sensor reads the position of the crankshaft or camshaft using a magnetic field and sends a signal to the computer. There are three types: magnetic, Hall Effect and photo-optical. The principles of the first two are based on fluctuating magnetic fields, the third relies on a light source (an LED). The computer uses the resulting signal to time ignition spark, injectors etc.
The Exhaust Gas Recirculation Valve and the EGR Valve Position Sensor work together to control NOx (nitrous oxide) emissions. Feeding exhaust gasses back through the combustion chamber has the effect of cooling combustion, curbing the production of the NOx by-products. The EGR Valve Position Sensor reads how open or closed the EGR valve is and sends a signal to the computer which, depending on the signals from a number of sensors, sends a signal to a solenoid to either open or close the EGR’s vacuum contr ol or directly open or close the EGR valve for those systems so equipped. The EGR Valve Position Sensor may also be referred to as an EGR Pintle Position Sensor.
A Manifold Absolute Pressure (MAP) Sensor uses a pressure sensitive disc to convert manifold air pressure to a voltage or frequency signal for the ECM. Its function is to allow the ECM to monitor engine load to accurately control ignition timing and the air-fuel ratio.
A Mass Air Flow (MAF) Sensor or Meter performs essentially the same function as a MAP sensor, but uses a vane which is forced open by engine vacuum/air flow rather than reading pressure.
The Oxygen (O2) Sensor is, as its name would imply, a device for measuring the oxygen content in the exhaust manifold or exhaust pipe. It supplies a varying voltage signal to the ECM to control the air-fuel ratio. There are a variety of types, with OBD II sensors being both more sophisticated and more expensive than their predecessors.
A Throttle Position Sensor can be found on either fuel injected or some of the later carburettor-equipped vehicles. This sensor is sends a variable signal depending on throttle position. The computer uses this signal to set air-fuel mixture, spark timing, torque converter lockup, air conditioning operation, EGR flow rate and idle.
The Fuel Injection System delivers fuel to the engine. Fuel injection systems can vary greatly but are generally divided into Throttle Body Injection (TBI), and Multi-port. TBI systems locate the injectors in essentially the same place as a carburettor, with the air-fuel mixture following a similar route to the cylinders, through an intake manifold or plenum. Multi-port systems put the injector as near to the cylinder as possible, leading to better balanced and timed fuel delivery. On these systems, the intake manifold carries only air for most of its length with fuel being added only at the lower intake manifold. Direct injection systems, not currently offered in North America, place the injector directly in the cylinder, as in diesel systems.
The Fuel Pump is an obviously necessary part of a fuel system. Old style fuel pumps for carburettor-equipped vehicles were most often mechanical and operated at about 5 to 15 pounds per square inch (psi) pressure. Mechanical pumps were driven by the camshaft and mounted on the engine. Low pressure electric fuel pumps can also be found on carburetted vehicles. Fuel pumps for fuel injected vehicles operate at much higher pressures, usually 30 to 50 psi but some pumps deliver more than 100 psi. High pressure electric pumps are very often located in the gas tank, although many systems use a low pressure pump in the tank and a high pressure in-line pump attached to the vehicle frame.
The Fuel Pressure Regulator’s job is to maintain the correct pressure to the fuel injectors. While the pump’s outlet pressure may vary somewhat in normal operation, keeping the pressure supplied to the fuel injectors within tight limits is necessary for accurate air-fuel mixture. In a Throttle Body Injection system, it is located at the throttle body housing. In a multi-port injection system, it is commonly located at the outlet end of the fuel rail, allowing excess fuel to return to the tank. There are some systems which incorporate it into the fuel pump assembly inside the fuel tank and do away with the fuel return line.
Fuel Filters are an important part of the fuel system. There are types that fit into the fuel line, usually with a paper element, as well as filters that are fitted to the bottom of in-tank fuel pumps, often referred to as strainers. Clogged or dirty fuel filters or strainers are a common cause of poor fuel pressure and volume.
The Evaporative Emissions Control System is a method of recapturing fuel vapour that would otherwise end up in the atmosphere. Generally this is in the form of a canister with a charcoal filter. Vapours collect there and are, where they condense. the canister is purged at normal engine operation and the fuel routed to fuel system.
The Idle Air Control Valve , also known as an Air Bypass Valve, is a motor/solenoid which varies the amount of air passing around the throttle plates on injected vehicles. It is controlled by the ECM which uses this valve to control engine idle speed.
The Idle Speed Control (ISC) controls the idle speed during periods of closed throttle. It is an electric motor-operated plunger located adjacent to the throttle body. A motor in the ISC extends and retracts a plunger which limits the closing position of the throttle lever.
The Catalytic Converter uses precious metals in a honeycomb-type structure or as beads to convert exhaust gases to less harmful gases and water. There are two types: single bed and dual-bed, also referred to as three-way. In both, a chemical reaction, these precious metals work to cause oxygen (O2 ) to be combined with unburned hydrocarbons (HC, essentially unburned fuel) and carbon monoxide (CO) to produce water (H2O) and carbon dioxide (CO2) as well as Nitrogen (N2). The dual-bed type adds a stage ahead of this to convert nitrogen oxides (NOx ) and oxygen. These gases then travel along with the HC and CO to the oxidation catalyst where the reaction mentioned first takes place.
The Air Pump is designed to feed air into the exhaust gases to cause unburned fuel to burn. Some systems inject air into the exhaust stream at the manifold, others do inject air at the catalytic converter to aid in the conversion of exhaust gases. Air pumps are protected from hot exhaust gases by an Air Pump CheckValve, which only allows air to flow from the pump.
Air Diverter Valves (or Air Management Valves) re-route the compressed air from an Air Pump under certain conditions. This air may be vented to the outside or, on some vehicles, it may direct air upstream of the O2 sensor on cold starts, to clean up HC and help heat the O2 sensor.
Pulse Air Injection Valves perform the same function as air pumps but use the natural pressure variations in the exhaust stream to draw in fresh air.
The PCV Valve is the oldest emissions control item. It replaced the old dump tubes that vented crankcase vapours to the atmosphere. The PCV Valve is a one-way, check valve that vents these vapours (mostly HC from unburned fuel) back through the induction system to the combustion chamber for burning.
STEERING, SUSPENSION AND DRIVELINE
Together the steering, suspension and driveline categories make up one of the most important parts categories that counterpeople have to deal with. These three separate systems are nonetheless related through the commonality of their general location, under the car, as well as the fact that problems with one area can be caused by those in another.
Steering Systems can be divided into two types in common use today: conventional and rack and pinion. Conventional steering systems, also known as Parallelogram, use two tie rod assemblies connected to the steering arms and a long centre link. An idler arm supports the centre link on one end and the other end is attached to the pitman arm. Steering action is transmitted from the steering wheel via a steering (or gear) box.
Power steering versions that are the most common add an hydraulic pump which applies hydraulic pressure. A system includes a pump, a flow control valve, a spool valve and a power piston. The spool valve directs the hydraulic pressure to one side or other of the power piston for left and right turns. There are three mail types of pumps: roller type, vane type; and slipper type. The flow control valve regulates pressure from the pump to provide proper steering assist through all rpm ranges.
There are Integral systems that have the spool valve and power piston integrated with the gear box and there are Linkage types which have an external power piston and spool valve and use a manual gear box.
Some systems are equipped w ith a Steering Damper which is really a specially designed shock absorber mounted horizontally. It reduces kickback through the steering mechanism from road bumps and imperfections that would otherwise end up affecting the car’s stability and the driver’s ability to maintain control over the steering wheel.
Power Rack and Pinion steering systems are integrated in the sense that most of the components are contained within the housing. The rack functions as the power piston and the spool valve is connected to the pinion gear. It is these parts, or rather the seals within them, which are often the cause of failures due to leakage which may or may not be detectable from the outside.
Suspension Systems can use coil springs, leaf springs or torsion bars to support the weight of the vehicle. There are a variety of arrangements in use. Here we will deal only with the common front suspension arrangements.
Each corner of a common Front Coil Spring Suspension is made up of an upper control arm, a lower control arm, a steering knuckles, a spindle, an upper and a lower ball joint, bushings, a coil spring and a shock absorber. The most common style is the Short-Long Arm (SLA) Suspension.
On this type, the Control Arms are attached to the vehicle frame with bushings (metal or rubber) which prevent the wheel assembly from moving side to side but do allow up and down movement. Upper and lower control arms are of different lengths so that a slight camber change occurs as the wheel moves through jounce and rebound. This minimizes track change which would cause the tires to scrub sideways.
Ball Joints are used to connect the steering knuckle to the control arms and form the steering axis. One ball joint is call the load carrier, the other is call the follower. If the spring’s upper mount is on the frame and the lower mount is on the lower control arm, vehicle weight would be transmitted through the spring to the lower control arm, and then through the control arm to the lower ball joint, making it the load carrier ball joint and the upper ball joint the follower. If the spring were mounted to the frame and the upper control arm, the opposite would be true.
Shock Absorbers act to control the up and down movement of the vehicle suspension. They do not support any vehicle weight. Shocks have a single job: to control bounce, roll or sway, brake dive and acceleration squat.
MacPherson Strut Suspensions operate using the same basic principles and have lower control arms and ball joints as well as steering knuckles and spindles. The coil spring encircles the strut, with the upper and lower spring seats as part of the strut assembly. However, in a Strut Suspension, the Struts perform all of the functions of a Shock Absorber but also provide structural support, taking the place of the upper control arm and ball joint. This is why a strut shaft is much larger than a shock’s.
Modified Strut Suspensions are arranged so that the spring is located separately from the strut, rather than encircling the strut.
Double Wishbone Suspensions combine the space saving of a strut suspension with the ability to ride low to the ground (a shortcoming of strut suspensions). This allows for a more aerodynamic hoodline. On a Double Wishbone system, the lower portion of the strut forms a wishbone shape where it attaches to the lower control arm. The wishbone does not rotate as the strut does in a strut suspension. Instead, the spindle rotates on upper and lower ball joints (similar to an SLA system).
A Strut Rod is used in a variety of suspensions and provides bracing to the (usually lower) control arm to limit front and rear movement. Some also allow for adjustment of caster during alignment.
Anti-Sway Bars, also known as Anti-Roll Bars, Stabilizer Bars or (incorrectly) “Sway Bars,” can be found at the front or rear suspensions. Each is constructed so that the unequal movement of the left and right suspensions causes them to twist, exerting torsional forces. Connecting left and right hand suspensions, they only resist suspension movement when under cornering forces or when one side only is under bump (or more bump than the other). They do not provide any resistance to movement when both wheels are under equal bump. They are attached to the chassis by brackets and rubber (or urethane) bushings which, because the Anti-Sway Bars rotate, are a wear point.
Driveline Systems can be generally divided into three categories: rear wheel drive; front wheel drive; and 4 wheel drive. Since we’re dealing with automotive systems here, we’ll leave out the 4WD area.
Rear Wheel Drive systems, with a few mid- and rear-engine exceptions, drive the rear wheels from the transmission via a drive shaft and a differential.
The typical Drive Shaft uses two Universal (or Cardan) Joints, a “driving” (front) U-joint and a “driven” (rear) one. Together, they act to allow for misalignment of the front and rear ends of the driveshaft during normal rear suspension travel and transmission movement (albeit small). The shortcoming of U-joints is that they can only operate acceptably when angle of misalignment is quite small as the actual velocity of the shaft changes with each rotation, causing vibration. Some systems were introduced to combat this using a Double Cardan arrangement which puts two U-joints very close together. However, even on these systems vibration was a problem once the angle between the two shafts became more than a few degrees.
Constant Velocity (CV) Joints were introduced as a solution to this problem which became extremely acute on front wheel drive vehicles. There are two types of construction: Rzeppa, which uses ball bearings, a cage and inner and outer bearing races; and the Tripode-Tulip, which uses three roller bearings attached to the arms of a driving shaft. Both these constructions can be found in Fixed CV Joints as used at the outboard, or wheel end of CV driveshaft (or, more correctly, halfshaft) and in Plunging CV Joints as found at the transaxle, or inboard, location. The plunging capability allows for the change in shaft length that must be accommodated under suspension and steering movements.
Often, the same vehicle models can have regular and heavy duty variations as related to the type of brake system installed. These may be distinguished by shaft diameter (e.g. 3/4″ vs. 1″) as well as the number of splines on the shaft. On some models, too, the number of splines may change, seemingly without reason. It is important that items such as this be checked whenever there is a question.
Both types of CV joints require Special Lubricants that are formulated for the specific conditions. This grease is kept in the joint, and dirt out, by the CV Boot. These come in a variety of materials for the specific applications. The main ones are Hytrel, Neoprene, Silicone and Vamac. Boots are also sold in the aftermarket in “split” configuration which enables the installer to replace a torn boot without having to remove the CV shaft.
It is notable that much of the CV joint service has migrated to straight halfshaft replacement as costs of the assembly have come down to the point where it is often less expensive to remove and replace rather than rebuild in the shop once the cost of labour is considered. Plus there is the warranty issue which is to be considered as well.
Accordingly, as much of the business is for remanufactured parts, a professional counterperson will be fully apprised of core credits and policies, and how to account for them on a sale.