Batteries and Alternators
A car’s headlights, the ignition coils, engine cooling fans and other non-essential things as the radio (my son would argue that one), air conditioning fans (my wife would argue that one), and other things all need to somehow be powered up by an electrical system. All of that power has to come from somewhere!! A lot of people might think that power comes from the battery, and that is true to some extent. But the real answer is that the power really comes from gasoline in your gas tank, and the link from the gas tank to the battery is that mysterious thing called the alternator. It takes mechanical power from the crankshaft, transmits it via a “fan” belt or serpentine belt and turns the alternator. Simply put, the main function of the alternator is to convert power from the gasoline engine that drives you along the road, to electrical energy to keep the battery in tip-top condition.
Questions & Answers
1. What happens when the alternator goes bad?
2. How does an alternator work?
3. What causes an alternator to go bad?
4. How is a bad alternator detected with a device?
5. Is there a quicker and easier way to detect a failing alternator?
6. I think my alternator is bad. Do I need to make an appointment?
1. What happens when the alternator goes bad?
At first, nothing. That is because the battery has some reserve power in it, enough to keep the engine running for quite some time and for many miles. A bad alternator doesn’t necessarily mean a tow truck should be called right away. As long as energy is conserved elsewhere, like turning off the blower motor, the rear window de-fogger, the stereo and the headlights (if possible), you could make it for some distance on just the battery reserve alone. Once the battery loses its charge, there will not be sufficient voltage to keep the engine running.
2. How does an alternator work?
In general, alternators fail in stages. An alternator only job is to generate alternating current (AC). Since the battery can’t use alternating current, the alternator output is fed into what are called diodes, which convert the AC into DC. The alternator has a unique feature in that it is able to generate a relatively high voltage while the engine is at idle. The old generators needed to be running at a fast pace before they got up to 13 or 14 volts. The alternator can do this since it is really three alternators in one body. Each of the three sections of the alternator generates its voltage out of phase with the other two sections. Since the complete cycle (one revolution) of the alternator is 360 degrees, each phase is shifted by 120 degrees from the next phase. So in one revolution of the alternator it puts out three separate voltages.
3. What causes an alternator to go bad?
Since each of the three phases in an alternator has its own windings and each of the windings has its own pair of diodes, each of these windings and/or diodes can fail, one set at a time. If this happens the alternator can still charge the battery, but only with a limited current, approximately 2/3 of its original capacity if one system fails. If two systems fail, then it puts out only 1/3 of its rated capacity. What that means to you is that you can go a long time on a limping alternator. Chances are if you don’t need headlights or air conditioning or other high current using accessories, you would never know that the alternator was in the process of failing! The time you will find out is when it is 10 below zero and you wear down the battery by cranking the starter, then put the fan on high for heat, and then drive in the dark.
4. How is a bad alternator detected with a device?
You will need a simple voltmeter. You can get one at Radio Shack for under ten dollars. Here’s what you do – start the car, make sure all the accessories are off and rev up the motor to a fast idle. Set the Voltmeter to the DC scale (not AC or Ohms). Measure the voltage across the battery terminals – red lead of the voltmeter on the positive terminal, black on the negative (ground in most cars). The voltage should, and probably will, read around 14 volts. If it reads less than 12 volts you may indeed have a failed alternator and you can skip the next step. Next, turn on the heater, the rear window de-fogger, the radio, the headlights and anything else that draws power. Now rev up the motor and watch the voltmeter. It should still be reading around 14 volts. If it reads lower than 13 volts the chances are that your alternator is not up to snuff.
5. Is there a quicker and easier way to detect a failing alternator?
Yes and it is summarized in one word: Noise. The rotor inside the alternator rotates on bearings, normally very high precision needle bearings, and these can fail. When they do you will hear a loud grinding noise associated with the alternator. To isolate the noise take a length of tubing and put one end to your ear and move the other around in the vicinity of the alternator. To further isolate the noise, disconnect the drive belt and spin the alternator by hand. If you hear a rumble or grinding noise then the bearings are shot.
6. I think my alternator is bad. Do I need to make an appointment?
Yes, feel free to call us at 425.774.4646 or click here to request a callback. Arios Auto Repair prides itself in offering the best mechanic service at the most affordable price in the Lynnwood Seattle area