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Electrical System
General Electrical System Basics
STARTING SYSTEM
The starting system consists of four main parts: an ignition switch, a starting safety switch, a starter relay and solenoid and a starter motor.
The ignition switch has four positions: "Off" cuts power to the ignition system, the starter and all accessories; "On" passes current to the ignition and accessories, but not the starter; the "Start" position is spring loaded and passes current to the starter relay or solenoid while maintaining power to the ignition circuit; and "ACC" provides power only to the accessories when the engine is off.
A park-neutral safety switch within the transmission shift linkage and ignition switch circuit prevents the engine from starting if the transmission is in gear.
On most newer vehicles with manual transmissions, a switch on the clutch pedal prevents the engine from cranking unless the clutch is depressed.
Many so-called starter problems are actually problems with the ignition switch or safety switches. A misadjusted or open safety switch can prevent the starter from cranking as can openings in the ignition switch or wiring. The ignition switch circuit should therefore be checked if the starter is dead.
Another component in the starting system is a solenoid or relay that shunts power directly from the battery to the starter motor when the engine is cranked. A magnetic coil pulls a plunger or armature to close a set of contact points when the ignition switch is turned to the "start" position.
A relay or solenoid is necessary because the starter requires too many amps for the ignition switch itself to handle. A starter can pull 100 to 200 or more amps when cranking, which means the current needs to flow through the battery cables.
The starter itself is a high torque direct current electric motor. Inside are a rotating armature, brushes and a pair of field coils or permanent magnets. The starter uses the opposing magnetic forces of the armature and field coils or permanent magnets to drive itself.
Though most starters are "direct drive" some have an extra set of reduction gears to increase cranking torque. Reduction gears also allow a smaller, higher speed motor to be used.
The starter shaft has a gear and drive mechanism which engages the flywheel to crank the engine. On starters that have a solenoid mounted on the housing, the solenoid operates a lever to push the starter drive out to engage the flywheel.
The drive mechanism only engages the flywheel when the starter is cranked. It disengages when the engine starts. An overrunning clutch in the starter's drive mechanism helps protect the starter against damage if the driver continues cranking after the engine has started. If the starter fails to disengage, the engine can over-rev it, causing damage.
A common cause of starter failure is prolonged cranking. If an engine is hard to start, the starter should not be cranked for more than 30 seconds at a time. It should then be allowed to cool down for a minute or two before the engine is cranked again.
Overboosting the battery in an attempt to coax a dead engine back to life can also overtax and damage the starter.
Starters also wear out. Brushes and bushings take a beating as does the starter drive, solenoid, armature and field coils. A starter may fail to crank an engine for any of a number of reasons so the cranking problem should always be thoroughly diagnosed before the starter is replaced. This helps prevent unnecessary warranty returns.
CHARGING SYSTEM
The main parts in the charging system are the alternator, a regulator to control the alternator's charging output, and a charge indicator (either a gauge or warning light).
The charging system replaces the amps pulled out of the battery to crank the engine.
The charging system keeps the battery at or near full charge. A lead-acid car battery will sulfate rather quickly if allowed to run down or if it is chronically undercharged. The charging system also supplies current for the engine's ignition system, fuel injectors, lights and all other electrical accessories on the vehicle. As soon as the engine starts, the charging system comes to life, supplying these needs as well as recharging the battery.
The charging system uses magnetism to make electricity.
When electric current flows through a wire, it creates a magnetic field around the wire. If the wire is wrapped around a piece of iron, the iron becomes a powerful electromagnet with a very strong magnetic field. Thus, when an alternator is charging, it becomes magnetic. Hold a screwdriver near it and it will be attracted by the magnet.
Current is produced by rotating the magnetic field inside a stationary conductor. The alternator's rotor carries this current which creates a rotating magnetic field as the rotor turns. As the magnetic poles pass beneath the three stationary stator windings, a three-phased alternating current (AC) is induced in the stator windings. The current is then rectified (converted) to direct current (DC) by the alternator's diodes to keep the battery charged and to meet the demands of the electrical system.
The alternator's output is controlled by switching its field current on and off. The voltage regulator does this by monitoring the battery and vehicle's voltage requirements.
When voltage is low, the regulator switches the field current on causing the alternator to go to full output. When the voltage becomes too high, the regulator shuts off the field current causing the alternator's output to go to zero.
By constantly cycling the field current on and off many times a second, an average output that is just right to meet the demands of the battery and electrical system is achieved. In other words, as the voltage regulator increases field current dwell time -the ratio of on to off time- alternator output goes up. As it decreases the average dwell time, alternator output drops.
On many alternators, the voltage regulator is mounted inside or on the alternator itself. These are called integral alternators. If the regulator is mounted elsewhere in the engine compartment, the alternator is said to be externally regulated. On 1985 and up Chryslers as well as other newer vehicles, the regulator function has been integrated into the powertrain control module (PCM).
ELECTRONIC VOLTAGE REGULATION
The regulator uses a large power transistor as the main switch for turning the alternator's field current on and off. The transistor will complete the power circuit to the field coils until it is toggled off by the second transistor which gets its clue from a Zener diode.
A Zener diode is an electronic component that suddenly becomes conductive when a certain threshold voltage is achieved. In other words, it doesn't switch on until a certain voltage is reached. The voltage threshold of the Zener diode, therefore, determines the reference voltage for the charging system.
It is the functional equivalent of the magnetic coil and spring loaded armature in an old fashioned electromechanical voltage regulator. The only difference is there are no moving contacts in the solid state version, and no way to mechanically adjust the regulator's built-in setting to increase or decrease charging output.
When alternator output voltage reaches the threshold limit of the Zener diode, the diode passes voltage to the number two transistor which in turn toggles the main power transistor off. The field current is broken and the alternator's output drops to zero.
As the charging voltage drops back under the Zener diode's threshold level, the Zener diode turns off, tripping the number two transistor back to its original position, allowing the main power transistor to switch back on. Thus, the on and off cycling that regulates alternator output is achieved with no moving contacts, springs or armatures.
Temperature compensation in the electronic regulator is accomplished electrically by using a thermistor, a special kind of resistor that changes conductivity according to temperature in the Zener diode's supply circuit. As the thermistor changes its resistance in response to underhood temperature, the threshold voltage of the Zener diode is manipulated to alter charging system output. Thus, a GM Delco alternator that is normally set to produce a maximum voltage output of 14.8 to 14.9 volts can actually range from as low as 13.8 to 14.9 depending on ambient temperatures.
The main enemies of the electronics inside the voltage regulator are heat, excessive current and excessive voltage. A sudden current surge or a voltage spike can literally burn right through a semiconductor and render it useless.
If the regulator in an internally regulated alternator fails, the whole assembly must be replaced. But if the regulator on an externally regulated alternator fails, it isn't necessary to replace the alternator because the regulator can be replaced separately.
GENERAL MOTORS "CS" SERIES ALTERNATORS
In 1986, the first of a new series of "CS" alternators from Delco Remy began appearing on certain General Motors cars. The three most common CS Delcotron units are the CS-121, the CS-130 and the CS-144.
"CS" stands for "Charging System" and the number denotes the outside diameter of the stator. The CS series alternators are high output units and differ from the earlier SI series alternators in a number of unique ways.
One difference is that CS series alternators have a different type of voltage regulator that uses a digital rather than analog switching pattern.
The analog switching; pattern used by all earlier Delco regulators switched the frequency of the field current as load and rpm changed. The CS series digital switching pattern, by comparison, stays constant at about 400 cycles per second throughout the rpm and load range of the alternator.
By varying the on-off time, the average field current is regulated for the correct charging voltage. At high speeds, the on-time may only be 10 percent and the off-time 90 percent. At low speeds with high electrical loads, the on-time may be as high as 90 percent.
One reason for this change was to make the regulator interactive with the onboard computer so the computer could customize the charging rate.
When an engine is running at low speed and a heavy electrical load is applied, it can lug down the engine if it reacts too quickly, to the load. In a small displacement engine, this can cause an objectionable idle shake.
The CS series voltage regulator is a pulse-width modulated unit. It gradually increases the field current as an electrical load is applied to gradually increase the alternator's output. This reduces the kind of idle shake problems that can result when a sudden load is applied.
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