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Car's Classroom on the Web
The Starting
System
by Fred Bordoff
 The "starting system", the heart of the electrical system in your car, begins
with the Battery. The key is inserted into the Ignition Switch and then turned to the start
position. A small amount of current then passes through the
Neutral Safety Switch to a Starter Relay or Starter
Selenoid which allows high current to flow through the Battery Cables to the Starter
Motor. The starter motor then cranks the engine so that the piston, moving
downward, can create a suction that will draw a Fuel/Air mixture into the
cylinder, where a spark created by the Ignition System
will ignite this mixture. If the Compression in the engine is high enough
and all this happens at the right Time, the engine will start.
 The automotive battery, also
known as a lead-acid storage battery, is an electrochemical device that produces voltage
and delivers current. In an automotive battery we can reverse the electrochemical action,
thereby recharging the battery, which will then give us many years of service. The purpose
of the battery is to supply current to the starter motor, provide current to the ignition
system while cranking, to supply additional current when the demand is higher than the
alternator can supply and to act as an electrical reservoir.
The automotive battery requires special handling. The electrolyte (water) inside the
battery is a mixture of sulfuric acid and water. Sulfuric acid is very corrosive; if it
gets on your skin it should be flushed with water immediately; if it gets in your eyes it
should be flushed with a mild solution of baking soda and water immediately and you should
see a doctor as soon as possible. Sulfuric acid will eat through clothing, so it is
advisable to wear old clothing when handling batteries. It is also advisable to wear
goggles and gloves while servicing the battery. When charging, the battery will emit
hydrogen gas; it is therefore extremely important to keep flames and sparks away from the
battery.
Because batteries emit hydrogen gas while charging, the battery case cannot be
completely sealed. Years ago there was a vent cap for each cell and we had to replenish
the cells when the electrolyte evaporated. Today's batteries (maintenance free) have small
vents on the side of the battery; the gases emitted have to go through baffles to escape.
During this process the liquid condenses and drops back to the bottom of the battery.
There's need to replenish or add water to the battery.
Today's batteries are rated in cold cranking amps. This represents the current that the
battery can produce for 30 seconds at 0 degrees before the battery voltage drops below 7.2
volts. An average battery today will have a CCA (Cold Cranking Amps) of 500. With the many
different makes and models of cars available today, batteries will come in many different
sizes, but all sizes come in many CCAs. Make sure you get a battery strong enough to
operate properly in your car. The length of the warranty is not indicative of the strength
of the battery.
Battery cables are large diameter, multistranded wire which carry the high current
(250+ amps) necessary to operate the starter motor. Some battery cables will have a
smaller wire, soldered to the terminal, which is used to either operate a smaller device
or to provide an additional ground. When the smaller cable burns it indicates a high
resistance in the heavy cable.
Even maintenance free batteries need periodic inspection and cleaning to insure they
stay in good working order. Inspect the battery to see that it is clean and that it is
held securely in its carrier. Some corrosion naturally collects around the battery.
Electrolyte condensation contains corrosive sulfuric acid, which eats away the metal of
battery terminals, cable ends and battery holddown parts. To clean away the corrosion, use
a mixture of baking soda and water, and wash all the metal parts around the battery, being
careful not to allow any of the mixture to get into the battery (batteries with top cell
caps and vents). Rinse with water. Remove the battery cables from the battery (negative
cable first), wire brush the inside of the cable end and the battery post. Reinstall the
cables (negative end last). Coat all exposed metal parts( paint or grease can be used) so
that the sulfuric acid cannot get on the metal.
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The ignition switch allows the driver to distribute electrical current to where it is
needed. There are generally 5 key switch positions that are used:
- Lock- All circuits are open ( no current supplied) and the steering
wheel is in the lock position. In some cars, the transmission lever cannot be moved in
this position. If the steering wheel is applying pressure to the locking mechanism, the
key might be hard to turn. If you do experience this type of condition, try moving the
steering wheel to remove the pressure as you turn the key.
- Off- All circuits are open, but the steering wheel can be turned and
the key cannot be extracted.
- Run- All circuits, except the starter circuit, are closed (current is
allowed to pass through). Current is supplied to all but the starter circuit.
- Start- Power is supplied to the ignition circuit and the starter motor
only. That is why the radio stops playing in the start position. This position of the
ignition switch is spring loaded so that the starter is not engaged while the engine is
running. This position is used momentarily, just to activate the starter.
- Accessory- Power is supplied to all but the ignition and starter
circuit. This allows you to play the radio, work the power windows, etc. while the engine
is not running.
Most ignition switches are mounted on the steering column. Some switches are actually
two separate parts;
- The lock into which you insert the key. This component also contains
the mechanism to lock the steering wheel and shifter.
- The switch which contains the actual electrical circuits. It is usually
mounted on top of the steering column just behind the dash and is connected to the lock by
a linkage or rod.
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This switch opens (denies current to) the starter circuit when the transmission is in
any gear but Neutral or Park on automatic transmissions. This switch is normally connected
to the transmission linkage or directly on the transmission. Most cars utilize this same
switch to apply current to the back up lights when the transmission is put in reverse.
Standard transmission cars will connect this switch to the clutch pedal so that the
starter will not engage unless the clutch pedal is depressed. If you find that you have to
move the shifter away from park or neutral to get the car to start, it usually means that
this switch needs adjustment. If your car has an automatic parking brake release, the
neutral safety switch will control that function also.
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A relay is a device that allows a small amount of electrical current to control a large
amount of current. An automobile starter uses a large amount of current (250+ amps) to
start an engine. If we were to allow that much current to go through the ignition switch,
we would not only need a very large switch, but all the wires would have to be the size of
battery cables (not very practical). A starter relay is installed in series between the
battery and the starter. Some cars use a starter solenoid to accomplish the same purpose
of allowing a small amount of current from the ignition switch to control a high current
flow from the battery to the starter. The starter solenoid in some cases also mechanically
engages the starter gear with the engine.
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Battery cables are large diameter, multistranded wire which carry the high current
(250+ amps) necessary to operate the starter motor. Some have a smaller wire soldered to
the terminal which is used to either operate a smaller device or to provide an additional
ground. When the smaller cable burns, this indicates a high resistance in the heavy cable.
Care must be taken to keep the battery cable ends (terminals) clean and tight. Battery
cables can be replaced with ones that are slightly larger but never smaller.
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The starter motor is a powerful electric motor, with a small gear (pinion) attached to
the end. When activated, the gear is meshed with a larger gear (ring), which is attached
to the engine. The starter motor then spins the engine over so that the piston can draw in
a fuel/ air mixture, which is then ignited to start the engine. When the engine starts to
spin faster than the starter, a device called an overrunning clutch (bendix drive)
automatically disengages the starter gear from the engine gear.
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Computer Systems, Inc.
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