Battery Wiring for other battery wiring information

Testing Battery and Charging System

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 Guidelines

This 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 power.
  2. An alternator that charges the battery and supplies energy to run electrical devices while the alternator pulley is spinning fast enough.

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:

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 proper "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 systems".

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 energy.

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 system at 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 horsepower." This idea is basically wrong! Slowing the alternator too much will actually cost 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 voltage.

As the rotor spins magnetic force lines cut across a stationary winding called the stator winding.  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 windings.

As 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 alternator's 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 alternator.

If 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 horsepower, we probably don't want to use an electric fan driven from an alternator! 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 installation might mean more than the one horsepower loss 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 advertising "BS". I've seen claims where an electric fan can free up 10-20 horsepower. Let's see if that makes sense:

One 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.

If 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 alternator. That would be 746*12.5 = 9,325 watts, or 720 amperes at 13 volts, from a perfect alternator. 

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 LOSE 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 are:
  1. Mechanical. A cleaner engine compartment with more room, or easier fan mounting
  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 Alternator

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. The 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 alternator 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 alternator