Electrical system



Battery Wiring
for other battery wiring information

Testing Battery and Charging

Slow Cranking System Test



For a full Ford OEM 1989 wiring diagram, you can open this file   
1989 Ford Mustang Wiring Schematic

This wiring diagram works by grid and page locators on the index pages
(bottom of list).   Locate the page with the letter and number shown.
The letter and number maps the schematic location of the component you found in
the index.



Safe Battery Installation

is just a quick
general overview of
the system.

A standard automotive or marine electrical system contains two major electrical power sources:

  1. A battery to supply energy to
    start the car, and to run automotive
    electronics, while
    the alternator is
    putting out little or no
  2. An
    alternator that
    charges the battery
    and supplies energy
    to run electrical
    devices while the
    alternator pulley is
    spinning fast

In any electrical system, current must have a complete path between the
negative and positive of the power source. Most people
consider only
the positive lead, but the negative lead or “ground path” must also carry the same
current level as positive lead currents. Every ampere leaving or entering the positive
terminal must also enter or leave the negative terminal! This means if you have
300 amperes running a starter, the battery negative lead, through the engine
block to the starter’s metal case, must also handle 300 amperes. 

Resistance in either
the hot lead or the ground path results in a voltage drop across that resistance. Resistance, if excessive
for the required current level,
can compromise operation of electronic systems. The more current something draws,
or the more sensitive it is to voltage, the less total
resistance we can have in a path.  Resistance is a function of
conductor cross section (like the inside area of a pipe), conductor type (such
as copper or steel), and overall conductor length. 

We often choose conductor size or type based on current or amperage, but proper sizing
actually depends on two things:

  1. How much voltage drop can be tolerated
  2. How hot the conductor gets

Voltage drop depends exclusively on the amount of resistance and the current
through that resistance:

  • The longer the wire, the higher resistance becomes for a given conductor
    cross section
    and material. Longer wires may need to be oversized to larger cross section.
  • The smaller the cross section (if you sliced it and looked at it from
    the cut end), the higher resistance becomes for a given length
    and material. Thin narrow materials, or thin narrow connections, increase resistance.
  • The poorer material conductivity, the higher resistance
    becomes for a given length, cross section, and material. Poor materials may need to be
    oversized, and poor conductivity materials can be just as good as excellent
    materials when the poor materials have enough cross
    section. Even a poorer conductor, like steel, can be very “low resistance” if the
    cross sectional area is large. This is why car bodies almost always make excellent
    ground returns, and why car bodies are usually better ground conductors than the thickest of
    copper cables. 

A small push-on or crimped connection can have more resistance than several
feet of wire. This resistance often makes a high current push-on or crimp
connection run hot, and is why high-current push-on connectors can be problems
over time. Overloaded connectors or lugs become discolored by heat, and lose
their gripping tension. The best idea for long life is to either oversize the
lug or connector, or
use a properly installed screw compression connectors (terminal blocks or bolted
terminal lugs). Properly
sized and installed push-on plugs can be alright up to 30-40 amperes, but beyond
that they often become unreliable. One of the most unreliable areas in a
connector is the crimp area, careful soldering is often much more reliable.

In any connector used in wet or damp
environments, a coating of clear silicon dielectric compound is advisable. A
“grease” or paste helps seal moisture out of the connection, and
increases connection life. Silicon compound is commonly available at automotive
stores, such as Permatex “Dielectric Tuneup Grease”. Buy it and use it! It works
to preserve quality of pressure connections! It’s fine for everything from large
battery posts to tiny sensor plugs. I would NOT use it on extreme voltages, like
spark plug wires. Plug wires are usually best installed dry.

Battery Voltage

The minimum charged battery
voltage in common use is around 12.6
volts. This is without an alternator running. We call these systems “12-volt

Because of resistance in the battery and resistance in battery wiring, that voltage
may come down a
little under heavy
loads, but
12.6 volts is a
reasonable battery voltage estimate
for modest loads
when a fully-charged 12-volt lead-acid battery is
supplying all

To charge a 12-volt car battery, even at a
slow rate, an
alternator has to
produce at least 13.8 volts. 
The ideal alternator charging voltage is
over 14 volts, but
less than 14.7
volts. Below 14 volts battery charging is far too slow, and above 14.7 volts
the battery will
start to “boil” or
gas excessively.
This releases acid
and harmful vapors.
You can read how to
test your charging

this link.
Notice the voltage is critical, not the alternator
current. A 160-ampere alternator that runs at 13.9 volts when charging will
actually charge slower and less reliably than a 40-ampere alternator running at 14.3
volts when charging. Measuring battery terminal voltage while charging, with
lights and accessories running,  gives you the best idea of the health of
your entire charging system. Low charging voltage with high load, below the
lower 14-volt range at battery and alternator terminals, could mean you have a
defective alternator or need a larger alternator. If the voltage is well over 14
volts at the alternator, but the battery around 14 volts or less, indicates the
alternator to battery wire is too small or a poor connection in that lead.

Slowing Alternator Speed

Ever hear this
claim? “Slowing the
alternator speed by
changing pulleys
frees up

This idea is basically
wrong! Slowing the
alternator too much
will actually

high RPM
horsepower. Slowing the alternator reduces horsepower demand at very low speeds
and idle, but only if it also stops the battery from charging or if the
alternator stops running the vehicle electrical load. Do you really care if you
have more available horsepower at idle, or would you rather have the battery
charging? Why would anyone want to not charge a battery, and actually discharge
the battery at slower engine speeds, and then take horsepower away at high
engine speeds to charge the battery? As illogical as this is, this is exactly
what larger alternator pulleys or smaller crank pulleys do. Changing the crank
to alternator pulley ratio to a lower numerical ratio, so the alternator turns
slower, is usually a very bad idea.

Here’s how an alternator works….

An alternator has
a field winding in
the rotating part.
The field winding
produces a magnetic
field. The strength
of this magnetic
field depends on the
electrical current
flowing through the
field winding and
the number of turns
in that field
winding. Current
gets to the spinning
field winding through very
hard carbon brushes.
The field current
comes through a
voltage regulator
that controls the
current in the
winding based on the
alternator output

As the rotor
spins magnetic force
lines cut across a
stationary winding
called the stator
The stator consists
of  several
windings, but they
are usually grouped
so they act like
three windings. 
Each stator winding
has a coil of heavy
wire. These are very
heavy high current

the magnetic flux
in the stator varies
with the passing
flux from the
rotating armature, each
stator winding
produces voltage.
That voltage is
alternating current,
very much like the
electricity in your
house. There are
several rectifier
diodes that act like
a “gate”. They convert
the alternating
polarity to a single
polarity (DC) that
pulses. Since there
are at least three
windings spaced
strategically around the
stator, there are
several pulses that
are very slightly
time-delayed from
each other. When
the pulses are added
together the output
is an almost steady
direct current,
very similar to a
battery. (If you
have ever heard a
faint whining tone
in the middle pitch
range of musical
tones in a car radio
or stereo, this is
the very small
ripple or
imperfection caused
by summing all the
positive voltage
cycles together with
the diodes.)

The stationary
windings, when the
alternator drives a
load, has current. This
net current is
the same as the load
current. If you draw
100 amperes from the
alternator, the
averaged sum
of currents in all
the stator windings
must be 100 amperes!
This current just like
the current in the
rotor generates a
magnetic field. As a
matter of fact
anytime we have
current flowing a
magnetic field
is created. This magnetic
field “bucks” or
pushes back against
the magnetic field
of the rotating
armature. This
“bucking” or
dragging from the
rotor and stator’s
mixing fields is
how energy gets
transferred from the
pulley to the electrical load
on the alternator.
The mechanical load
on the rotating
shaft is directly
proportional to the
load power on the
alternator. The more
electrical stuff you run from
the alternator, the
more you load the
alternator shaft.
Since the energy
conversion is not
100% efficient a
portion of the
horsepower supplied
at the pulley is
wasted as heat.

The voltage
regulator looks at
the alternator
output voltage and
adjusts the field
current supplied to
the rotating field
windings on the
rotor. The regulator
attempts to hold the
alternator output
voltage at a
predetermined value
that is suitable for
charging the
battery, generally
around 14.5 volts. The regulator
might supply 3 or
more amperes under
heavy electrical
loads or when the
alternator is not
spinning fast enough
to keep up with the
load. More current
from the regulator
increases the
magnetic flux in the
rotor, and that
increases the drag
on the alternator
shaft. This is how
the alternator draws
the right amount of
horsepower to make
the correct amount
of electricity!

Think a little
bit about how this
works. Nothing is
free. If we load
the alternator with
a great big electric
fan and/or an
electric water pump
the alternator has
to draw MORE
mechanical energy
than it would take
to drive the fan
blades or pump
directly. We will
always lose
significant energy through
conversions from
mechanical to
electrical and back
to mechanical. The
alternator, at best,
is about 60% or so
efficient. The
electric motors are,
at best 85%
efficient and are
more likely around
70% efficient. This
means the overall
efficiency to
convert crankshaft
load to electrical
energy and back to
mechanical energy is
40-50%. If a water
pump and fan took 1
horsepower to drive
directly, it would
take about 2
horsepower to drive
it fully from an

efficiency is low,
why do automobile
manufacturers use
electric fans?

This is a good
logical question;
and it has a logical
answer. If we watch
the fan operate we
see it often does
not run at all.
Unlike a mechanical
fan, the
manufacturer can
turn the electric
fan completely off
when not needed.
Over a long period
of time, even with
greatly reduced
efficiency, the much
shorter operating
time consumes less
fuel. This is a
major advantage.

A second much
less important
factor is the
electric fan can be
operated at the
optimum speed for
the blade and motor
design. The
mechanical fan is
not always operating
at a speed where it
optimized, the
electric fan can be
operated at peak
efficiency all the
time, and this
offsets a little bit
of the efficiency deficit.

A third factor is
ease of design. Can
you imagine mounting
a belt driving a
mechanical fan with
a sideways-mounted
engine? Even with a
rear-wheel drive, it
is physically easier
and cleaner to use
an electric fan.

Finally, even if
we doubled the power
loss, a fan only
requires about 2
horsepower maximum.

For the racecar
driver concerned
with every last
, we
probably don’t want
to use an electric
fan driven from an
Fortunately, the
power used to turn a
clutch or flex fan
is so low that
doubling the power
loss makes very little
difference. The
cleanliness of the
might mean more
than the one

we can expect from
driving an electric
fan from an
alternator. If the
fans run from the
battery, and if we
do not recharge the
battery from the
engine, an electric
fan makes excellent
sense because we can
get that energy from
an external charger.
We can cool the car
in the pits while
the engine is off.
That’s an advantage!

Now let’s do a
sniff test for
“BS”. I’ve
seen claims where an
electric fan can
free up 10-20
horsepower. Let’s
see if that makes

horsepower equals
746 watts.

746 watts
is 746/13 = 57
amperes of current
at 13 volts.

An 80%
efficient fan (which is far more efficient than real fans) would
use 71 amperes of
current draw to run the 1 horsepower fan.

the mechanical fan needed 10
horsepower from the crank, we would
need 12.5
horsepower’s worth
of electrical energy
with an 80%
efficient motor and
a 100% efficient
That would be
746*12.5 = 9,325
watts, or 720 amperes
at 13 volts, from a perfect

Do you have a 720
ampere fan? Do you
really think the fan
blade requires 10
horsepower? If you
do, you must have listened to
advertising people selling fans.
10 HP average power
is enough to run
many homes!

The fan spinning
around on the front
of the water pump
actually draws about
2 horsepower if we
spin it at very high
RPM. It draws much
less if it is a
clutch or flex fan,
probably in the
order of 1/2
horsepower or so. If
we got rid of a good
mechanical clutch or
flex fan and ran an
electric fan totally
off a battery
charged in the pits,
we would gain about
1 horsepower. If we
ran it off the
alternator we would
about 1/2 horsepower
more, or about 1
horsepower total. This is because the alternator is not near 100% efficiency.

There are several good
reasons to use an
electric fan. They

  1. Mechanical.
    A cleaner engine
    compartment with
    more room, or
    easier fan
  2. Operational.
    Cooling the
    radiator down
    while the engine
    is off
  3. Slowing the fan with high RPM engines
  4. Stopping the fan when it is not needed

 Freeing up
Power from the

Here is another
myth. If we
slow the alternator
down, we free up
racing power.

An alternator, when turning at normal operating speeds,
supplies all of the electrical system electrical power.
alternator also keeps the battery charged at a “float” charge. The battery just
goes along for the ride with a full charge, in case alternator voltage falls too
low to properly run things. Below around 13.8 alternator volts, the battery
starts to pick up a share of the electrical load. The battery’s share of current
increases rapidly as alternator voltage falls below 13 volts. 

When we slow an
alternator down too much, alternator
output voltage
drops. It no longer can maintain battery charge at slow engine speeds, so the
battery drains. At the same time, because the alternator voltage drops, the
voltage regulator turns alternator
field current up.
The extra magnetic field from the higher field current loads the
pulley more, and
drags on the belt
and crank more. By the time
the closed loop of
regulator feedback and
output voltage stabilizes, the alternator is loading the
pulley with more
horsepower loading than
if the alternator
was spinning faster all the time!
While this may seem
contradictory, it
occurs because the
alternator system is
often more efficient
operating at modest shaft speeds, and because a proper alternator shaft speed
keeps the battery fully charged.

The last thing racers want is a battery being charged, to make up for idle
battery power drain, when they run the engine up in a race. This is exactly what
happens when underdrive pulleys slow the alternator so much it does not charge
the battery at slow engine speeds. Charging a discharged battery is usually a
bigger horsepower drain than the rest of the electrical system! 

For a given load in
watts the alternator
has to consume a
certain amount of
horsepower. Because
the alternator is
around 60%
efficient, it
consumes about 1
horsepower for every
450 watts of load.
In a 13.8-volt
charging system this
means our
alternators draw one horsepower
for every 33 amperes
of load current. 
If we’ve slowed the
alternator down to
make more power in a race, we
really have wasted
money. The battery drains at slow speeds, and the alternator draws more power at
high RPM to recharge the battery! Worse yet, if we added an
electric fan, it will discharge the battery even more when the alternator is
spinning too slow.

Worse yet people slow the alternator and then, when the electrical system
can’t keep up, add a larger alternator. This just aggravates the problem,
because the engine turns an extra large alternator to make up for loss of charge
at slow speeds. The larger the alternator and the less alternator shaft speed at
idle or slow speeds, the more belt loading at high speeds.

If we want to reduce alternator
and fan drag, we should spin the alternator at normal idle speeds and turn alternator field
current off with wide open throttle. That’s
the only way to do
it. We can’t
possibly run lights,
charge the battery,
and/ or run fans off
a slow alternator
without loading the
engine down when we finally speed the engine up. If we
draw 20 amps at 13.8
volts, we have to
supply at least the
same amount of power
to the alternator if
we slow it 50% or
speed it 100%. It is
the load power it
delivers that
determines higher
RPM  horsepower
the alternator uses, not the RPM
of the pulley.





















The alternator is
a closed loop
system. If the
output terminal
voltage is below a
certain voltage, the