Mechanic's Corner - Oct 11
“Understanding Three KEY Electrical Parts”
By Mike Godwin
Many people view the electrical system in their beloved Mustang as some mystical creature that runs around with shorts, just looking for the opportunity to cause grief. In fact, that idea couldn’t be further from the truth once the owner understands a few of the basics of how the electrical system works. Let’s dive under the hood and take a look at three of the key elements that make up the better part of your car’s electrical system.
First is the battery, and to call it a stowage battery is probably the most correct and descriptive name that can be used. The battery stores right at 12 volts of Direct Current (DC) power at high amperes in a safe and confined space. View the volts as the amplitude that the battery can deliver, and the amperes as the force that it can deliver that voltage at. When you turn the ignition key in your car to the start position, you are actually engaging the battery. A path to the starter motor is created as a relay closes and allows the voltage to flow from the battery to the starter. Your starter motor only needs 12 volts, but plenty of amperes, to perform the function of turning the engine over quickly so that the car will start and run. Now remember the battery is a stowage device, so we just used up some of the stored energy when we started the car. The 12 volts stored in the battery, is right at the limit of the battery, no matter what type of battery is being used. Each battery for automotive use has six (6) individual cells and each cell is capable of producing 2 volts. So at six times two, we end up with a twelve volt battery.
To replace that expended energy, two components will work in unison to bring the battery back to a fully-charged condition. Remember now, these two items are working together to accomplish this job, even though we will explore them one at a time.
Next the Alternator as it is commonly called, is spinning at a specific speed, but will increase as the RPM of the engine increases. This increase of speed will result in an increase of output voltages until the maximum output voltage capacity of Alternator is achieved. All Alternators in every year of Mustang are belt-driven, and increase and decrease in rotational speed with the speed of the engine. This is important, as the alternator depends on this spinning action to create electricity.
In fact, the Alternator would be more properly described if us automotive enthusiasts would call it by the most correct name, “Alternator Rectifier”. That is precisely what it does. It creates Alternating Current (AC) voltage that is passed through a set of four diodes. Those four diodes are arranged in what is called, a Full Wave Rectifier, and each one “clips” a cycle of the AC voltage, and in the final output, is a DC voltage that our Mustang likes. The problem is that the DC output of the Alternator is not in a very pure or steady state, but rather jumps around, and the Amplitude varies with the engine speed.
To regulate this, a little bit the output of the Alternator is sent to a device called the Voltage Regulator. This little gem started out in the older Mustangs as three relays stuffed in an unvented metal case and mounted on the radiator support. Not the best location for a heat sensitive item. Being relays that opened and closed quite often, the contact areas wear out over time. Unfortunately, they usually fail without any warning, but with great fanfare as the car either overcharges the battery or ceases to provide charging voltage to the battery. Later model cars have electronic Voltage Regulators which are typically good for the life of the car. If you read the description of several reproduction Voltage Regulators, you will discover that on the inside, they are made of the newer electronic type components while the exterior cover a stamped ink labeling that looks like the original product.
So now our voltage is ready to return to the battery. Remember that the battery is a stowage device that holds 12 Volts DC for the car. Like a jar full of any material, you can’t add more items without using some force. So the Voltage Regulator not only makes the output of the Alternator a cleaner and less distorted DC signal, it controls how much amplitude is sent to the battery. The battery requires an input of about 14.2 volts in order to replenish the energy that was used to start the car. So like the full jar example, the higher voltage allows the battery to take in more energy and store it until it is required.
Those are the basics of how the electrical system works. So as you can see, nothing really mystical here, but some old automotive practices need to be debunked. One of the more common approaches to determining if it is the battery or the alternator that is defective, is to start the car and remove the positive lead of the battery. Rule is that if the car continues to run, the alternator is fine and working perfectly.
Warning!!! Warning!!! Danger here Mustang owner. ,This action on any Mustang can completely blow the diodes wide open on any year of Mustang. Worse yet, on the newer cars, you can wipe out the on board computer. How do you spell a $2,500 plus repair bill? Defective computer. So let’s skip that type of test, and either drive the car to a local parts house that can perform a couple of tests for free, and determine where the problem lies. Or, take the car to a repair shop that is equipped to perform the appropriate test. It must be noted that this particular test does not account for a defective voltage regulator.
An interesting experience that happened to me last year demonstrated the need to read test results and know what is going on with tested parts. I was helping a fellow club member with a very ill 1968 Mustang. As we had to remove all the accessories from the engine, I requested that she have the Alternator tested at one of the local chain store type auto parts houses. The Alternator failed, and I told her that rather than buy one to replace the defective unit, that I would rebuild it for free. I brought the defective Alternator home and replaced all four of the diodes without any problems. I cleaned the interior and exterior of the unit and reassembled it. After doing this, it is necessary to perform a short test that is called Flash Field testing. It is not so much a test, as it is a process to align all the magnets so that all north poles are located on one end of the Alternator. Having done as much as I could at home, I took it to my friend at Auto DC Electrical to have it tested at speed and under load on his test stand. He noted that it ran fine and tested good under all conditions. He inquired about what the original problem was, and I produced the failed test report from the auto parts store. It took no time at all for him to start laughing loud and strong at my situation. The alternator failed the Voltage Regulator test at the auto parts store. “Of course it would!”, my friend exclaimed from behind the counter as he gasped for air between words.
Ford uses an external voltage regulator, not the GM style that has a Voltage Regulator built in. So know what your test results really mean before buying a replacement part or letting some knucklehead like me rebuild a part for you.