Hey, I'm glad you asked that. I taught Automotive Electrical with an emphasis on charging systems.
Three things are needed to generate electrical voltage / current mechanically. They are a magnet, piece of wire, and movement between them. The wire is the stationary coil of many loops of wires. Each loop adds to the capacity of the generator. The magnet is an electromagnet. That's the coil of wire that rotates and is driven by the belt and pulley. The movement is that fact that the electromagnet, (rotor or field winding) is rotating.
Current and voltage can get confusing and a lot of people mix them up. Think of current flow in a river or water pipe. That's the same as current flow in a wire. When current flows through a wire, it sets up a magnetic field around that wire. When a wire is moved through a magnetic field, it causes a voltage to be developed which causes a current to want to flow. So it works both ways. Voltage is electrical pressure. Just as pressure is needed to make water flow up into a water tower, voltage is needed to make electrical current want to flow.
You have a 12 volt battery in your car. That voltage, (pressure) exists even when everything is turned off. It just sits there waiting to do something. A fully charged battery will actually measure very near 12.6 volts. When anything is turned on, that pressure causes current to flow through that device and drain the battery. When you start the engine, the current from the generator supplies all of the things that are turned on and running, plus it recharges the battery. If you think of the battery as the water tower, if there's 100 pounds of pressure in the tower, you have to have MORE than 100 pounds of pressure from the pump to force more water up there. The same is true of the charging system. That's why they are designed to develop at least 13.75 volts. That's enough to force more current backwards into the battery to recharge it. It's the job of the voltage regulator to maintain the system voltage between at least 13.75 volts but no more than around 14.75 volts.
We haven't even discussed resistance, and it's not really important to this story, but with water, resistance is a byproduct of the size and length of the pipes and how heavy the person is who's standing on the garden hose!
The number of loops of wire in the generator's stationary coil, (stator), is the biggest factor in how much current the unit can produce. With a higher capacity comes the ability to generate a higher, perhaps unsafe voltage. The other things that determine output capacity are how fast the movement is, meaning how fast the engine is going, and how strong the magnetic field is. The number of loops of wire is fixed; you can't jump under the hood and change that while you're driving. If system voltage goes too high, you could slow the engine speed but the people behind you won't appreciate that. The only practical way to reduce or control output is to weaken the magnetic field. THAT's the job of the voltage regulator.
The name is a little misleading in one sense. It's actually a current regulator but its action regulates system voltage. To make the electromagnet, current flows from the battery, through the field winding, through part of the regulator, and back to the battery. That's called a "series circuit" meaning all current has to flow through all items one after the other. (Think of a river that flows through multiple cities, one at a time). That part of the regulator adds resistance to the circuit. As its resistance goes up, current flow goes down. As current flow goes down, the strength of the magnetic field goes down, and that causes a lower voltage to be developed in the stator winding. The regulator has a wire connected right to the battery for it to sense system voltage. A similar scenario would be too much water coming out of the garden hose and threatening to wash away your flowers. By adding resistance, you reduce the flow of water, (current), and you reduce pressure at the nozzle, (voltage). That added resistance could be in the form of turning the nozzle closed or standing on the hose. If you want more current, reduce the amount of resistance.
That's how the regulator does his thing. For most generators, 3 amps is the most you will get through the field winding to make the magnetic field, and it will be a very strong one. Three amps is about half of what it takes to run one high beam head lamp bulb. With that tiny input, you will get the maximum output the unit can produce which can easily be over 100 amps, but you don't get something for nothing. For the rotating magnetic field to push the electrons in the output circuit, it takes a huge amount of energy. That's where the belt and pulley come in. You can also hear it with a little emergency generator. The engine really struggles when you draw a lot of current.
All GM generators have the voltage regulator built into the assembly and they are extremely difficult to disassemble to replace them. Also, there is no practical way for us to test them. They are a real miserable pile. On their older units from 1986 back to the early 1970s they could be easily tested on the car and were easy to replace. That style was, in my opinion, the world's second best generator. The newer design has a lot of problems and is a huge design disappointment. It's very common to go through four to six of them in the life of the car.
Chrysler had, by far, the best system through the 1970s and into the late 1980s, depending on car model. They never put the voltage regulator inside the alternator. (Except for a few imports), so you only had to replace the part that was defective. Troubleshooting was so terribly easy, you could diagnose any problem within five minutes and that included enough time for a coffee break! On the newer cars, they stick the regulator in the Engine Computer. Luckily it is extremely rare for it to cause problems which would mean replacing an expensive computer, but by doing so, the desired system voltage can be adjusted if necessary, and the alternator can be turned off completely to reduce load on the engine. This is typically done when you hit wide-open-throttle to pass that freight train or pull a trailer up a steep hill. This could gain you ten horsepower.
Ford has kind of combined the best of both worlds. There older systems had the voltage regulator on the fender (external) like Chrysler did, and they were just as easy to diagnose. Their newer cars, from around the mid 1980s, use a voltage regulator built into the generator (internal), but unlike the GM idiots, the Ford engineers put it on the back where you can get to it and test it. GM's idea has always been unit replacement with no troubleshooting needed. That's fine if you're too stupid to learn how to work on them but it forces you to buy things that don't need to be replaced. Chrysler's idea was to just replace what is defective and leave the rest alone. With your Ford, you can do either. If you want it fixed now, replace the entire unit. This is especially useful for intermittent problems although there are wiring things that can cause intermittent problems too. If you want to take the time to troubleshoot the system, you might get away with replacing just the regulator. That would save a basketful of money.
While it might be hard to get your eyeballs back there, if you look at the regulator, it is bolted to the back of the generator with four screws in a rectangular pattern. Along the bottom are two more screws that connect to the two brushes that pass the three amps of current to the rotating field winding. One of them might have a round plastic cap covering it if it hasn't fallen off yet. The other one is a test point. Remember that series circuit story? One way the regulator can fail is to become open, meaning a break in the circuit that stops current flow. (That's what switches do when they turn a circuit off). To determine if the regulator is open, use a piece of wire to ground the test terminal. If you can see it, you will see it says "Ground here to test". By watching the voltmeter across the battery, or by watching the brightness of the headlights, if either go up when you ground the test point, you will know the regulator is defective. That is called the "full-field" test. It makes the generator produce its maximum output. If there is no change when you perform that test, further diagnosis is needed.
As a point of interest, as a former instructor, it was important to use correct terminology. You'll notice I referred to the GM and Ford generator and the Chrysler alternator. Chrysler first used the alternator in 1960 and copyrighted the term. GM and Ford came up with their own designs in 1964 and 1965 but had to call them something else. They use the term "AC generator" but wherever you go, they will know exactly what you want when you call it an alternator. The DC or direct current generators from before 1960 put out the same direct current that could go right into the battery. AC generators and alternators put out alternating current similar to that in your house. Alternating current goes back and forth in the wire and would just go into and back out of the battery. To be able to store it, the current goes through diodes that direct the various currents to all go the same way thus changing it into direct current. Diodes, (also called rectifiers), are one-way valves for electrical current. They can be ruined from too much current in their forward direction or from too much voltage when they are blocking current from flowing the wrong way. The AC generator inherently can not produce much more current than its design value so it's easy to pick diodes that can handle that value of current. Reverse voltage is a different story. Most diodes can safely block 20 - 25 volts in the reverse direction but when you perform the full-field test, it is possible for the system voltage to go even higher, perhaps as high as 35 volts. That will take out any light bulbs that are turned on and will likely damage most of the vehicle's computers. For that reason, when doing the full-field test, don't raise engine speed and don't disconnect the battery. The battery will resist most of the increase in voltage, and the generator can't develop its highest output at low speeds. The full-field test should only be performed long enough to determine if the regulator is causing a no-charge condition.
Many mechanics years ago removed one battery cable to see if the engine would stay running. If it did, they knew the generator was working. This is a REAL bad idea for a number of reasons. First of all, if one diode out of the six is open or shorted, you will only be able to get one third of the unit's maximum value of current. That is enough to keep the engine running but may not keep up to a pair of head lights, tail lights, wipers, radio, and heater fan. That will result in the battery slowly running down while you're driving. More importantly, disconnecting the battery removes the cushion that helps stabilize system voltage. If you raise engine speed, the output voltage will go up. That higher voltage is what feeds the field winding so the magnetic field gets stronger. The stronger magnetic field causes the output voltage to go up, and the vicious circle continues. Most of the time the regulator is not able to reduce field current enough to maintain a safe output voltage. On older cars, the only threat was to any light bulbs that were turned on. On newer cars, you WILL destroy computers.
This is the same reason you shouldn't raise engine speed during the full-field test. You're bypassing the resistance of the voltage regulator which makes the electromagnet very strong, so the only thing limiting output voltage from going dangerously high is the battery and the low engine speed.
There's everything there is to know about voltage regulators! In effect, it's the foot on the garden hose, ... Well, more like the nozzle on the garden hose. Holler back with the voltage reading but remember, the value we're after is when the battery light is on and the problem is occurring.
Sunday, October 17th, 2010 AT 1:21 AM