Maintenance & Repair | Service Repair

Search Autoparts/Motorage/Maintenance-repair-service-repair/

Alternator design and diagnosis

With more demands placed on the electrical systems, changing your approach can make repairs easier.
Thursday, September 1, 2011 - 00:00

With more demands placed on the electrical systems, changing your approach can make repairs easier.

alternators electrical systems repair shop training technician training A/C training automotive aftermarket

The modern automobile is a study in escalating expectations. Consumers purchase new cars with the idea that this year's model will do more than last year's, whether it is quieter, more powerful or any number of other criteria. Government regulations are the same, in that each new year brings a set of higher standards regarding fuel economy, emissions and/or safety. Truly, the automotive industry is being pushed hard on all fronts, and for the most part is rising to the challenge.

The common denominator on many of these increased expectations is electronics. Cars can do much of what they do because of the enabling power of computers. Computer control also implies more extensive use of electric actuators. While we used to use mechanical or hydraulic drives for many automotive systems, we now are doing more of the heavy lifting with electric motors and solenoids. Past that, new accessories constantly are being added to the vehicle, and the virtually all of these are powered with electricity. One serious downside to all this is increased electric power consumption. The power has to come from somewhere, and we still rely on an engine-driven alternator to provide the wattage we need to make it all work.

While the basic idea behind the alternator hasn't changed much since it replaced the direct current (DC) generator in the early 1960s, it has evolved to meet the new demands that have been placed on it. Calls for increased output and greater reliability have brought about changes in its design and the way it is integrated into the remainder of the automobile systems. Computer-control now is the norm, and this has had an impact on charging system diagnostics as technicians use scan tools and oscilloscopes alongside dedicated battery-starting-charging system testers. Yes, times have changed, and while the game is getting more challenging, it also is becoming more fun for the savvy automotive technician.

A Retrospective

When the first automotive alternators were introduced 50 years ago, they were replacing DC generator technology that clearly was inferior. One major issue was the generator's diminished output at engine idle, which led to chronic undercharging of the electrical system. Beyond that, the DC generators of the day were controlled with complex mechanical regulators that were unreliable at best. The alternator was an elegant and powerful design by comparison, and quickly was adopted industry-wide. The earliest designs used remote-mounted mechanical voltage regulators, which then gave way to transistorized designs. Eventually, solid-state voltage regulators were integrated into the alternator housing, further increasing reliability.

As time went on, automotive engineers included progressively more electrical accessories in vehicle design, increasing the demands on alternator output. To further complicate matters, the space allotted for alternator installations was becoming smaller. Alternators were going to have to become more power-dense, meaning that they were going to pack more generating capacity into a smaller package. This also implied a redesign of the alternator drive system, which was being asked to transmit more mechanical power to facilitate the increased output. Multi-ribbed serpentine belts with automatic tensioners became the norm, which turned out to be a major improvement in service life and serviceability. The drive system was further refined with the inclusion of overrunning alternator decouplers (OADs), which reduced NVH and extended life of the accessory drive. One step at a time, the automobile charging system is getting better, in terms of both performance and reliability.

Theory of Operation

The basic alternator produces three-phase alternating current (AC) using a multi-pole rotating magnetic field that cuts across a set of stationary windings (known as the stator). Sending DC current through a field coil that is integrated into the rotor assembly produces the magnetic field. The field coil current is supplied through carbon brushes running on slip rings that are concentric with the rotor axis. Iron claw poles surround the field coil and concentrate the magnetic field around the circumference of the rotor, which then induces an electric current in the surrounding stator windings.

The stator is made up of three separate windings. The current that is produced in the stator windings is known as three-phase AC, because it is made up of three separate waveforms that are 120 degrees out-of-phase with each other. The AC is converted into DC using a rectifier bridge made up of a pair of diodes for each stator winding. The rectifier bridge acts like a railroad switchyard of sorts as it sends all positive electrical pulses to the battery positive and the negative pulses to ground.

The voltage regulator is responsible for maintaining electrical system voltage, and it does this by controlling the DC current flow in the rotor field coil. Increased current in the field coil produces a stronger magnetic field in the rotor, which in turn induces more current in the stator windings. The voltage regulator adjusts alternator output by monitoring the vehicle battery voltage and will increase field current to compensate for decreased voltage. As battery voltage rises, it will decrease field current to prevent overcharging. The voltage regulator also will adjust charging voltage according to ambient temperature. If temperatures are lower, charging voltage will be increased to compensate for a slower chemical action in the battery.

Time was when voltage regulators used mechanical switches to control rotor field current. These effectively duty-cycled the alternator field by switching the current on and off rapidly. A huge step forward was achieved when transistorized voltage regulators were introduced. These operated similar to the mechanical versions, but did it without any moving parts. Nowadays, many of the latest charging systems have gone one step further and assigned responsibility for voltage regulation to the powertrain control module (PCM). Like previous systems, these are controlling electrical system voltage by switching field current on and off. The more things change, the more they stay the same.

Diagnostic First Steps

Customer concerns that will lead you to test the charging system include dim lights, slow cranking or no cranking. These concerns may or may not be in combination with an illuminated MIL. A good first step is to check battery condition and determine whether it is undercharged. If it is undercharged, could it be caused by excessive parasitic drain? Keep in mind that what is considered to be too much parasitic drain on one vehicle may be normal on others. Make sure to do a thorough visual inspection of the wiring and alternator drive mechanism. Try starting the car using a jumper box. How did the vehicle respond? If the car starts and runs just fine when boosted, a logical next step is to do a check on the charging system.

If your shop has a battery-starting-charging system tester, it probably is easiest to use it to run a comprehensive check of all these systems at this point. In the absence of that, a simple test of the charging system can be performed using a digital multimeter(DMM) with a MIN-MAX function. With the vehicle ignition off, set the multimeter to read DC volts and then attach the leads to the battery terminals. Make a note of the battery voltage, and then press the MIN-MAX button once to begin recording.

Start the car and run it for several seconds, then press the MIN-MAX button again. The first reading that will appear on the meter is the minimum voltage the meter saw during the starting event. Press the MIN-MAX button two more times to get the maximum voltage, which will give an indication of whether or not the charging system is operating correctly. A maximum voltage reading of 13.5 to 15 volts indicates that the charging system is functioning, but you should confirm by running the engine at 2,000 rpm and turning on as many electrical loads (lights, blowers, etc.) as possible. Does this cause the system voltage to drop significantly? If so, further checks might be necessary.

A better way of doing the same test is by measuring battery voltage with an oscilloscope. Set the scope for a slow sweep (500 mS per division) and connect one channel across the vehicle battery terminals. Have a fellow technician start the vehicle and then pause the screen when you have a complete trace of the starting event. The parts of the trace that will be of most interest to you will be the battery voltage when the key was off, and the maximum voltage the system achieved once the engine started. Another twist on this procedure is to use an inductive amp probe with the oscilloscope to measure DC current at the battery ground cable. When the engine is cranking, you will observe the amount of current leaving the battery. Once the engine starts, the electricity will reverse direction and you will be able to see the amount of current that is recharging the battery.

It Might Not be the Alternator

If you have determined that the charging system is not working correctly, there are a couple of things you should check before condemning the alternator itself. Think carefully about what conditions need to be in place for the alternator to do its job. Above all, the alternator needs to have a good electrical connection between itself and the vehicle battery. With the key off, connect a headlamp across the B+ (output) terminal and the alternator frame. The headlamp should be lit brightly. If not, why not? Leave the headlamp attached and use a voltmeter to determine if it is the power or ground side (or both) that is causing the problem. If the power side is open (0 volt at the B+ terminal), take a careful look at the vehicle wiring diagram to determine the location of all fusible links in the charging circuit. Also, keep in mind that it is possible to have a corroded fusible link that allows small amounts of current to flow but prevents full alternator output (thus the headlamp test).

A similar test can be done with the vehicle running. To check the voltage drop on the power side of the charging circuit, connect the voltmeter between the alternator B+ terminal and the battery positive. With the engine running and the headlights on, this should not measure any higher than 500 mV. The ground can be tested by connecting between the alternator frame and the battery negative. Ground-side voltage drop should be under 200 mV. Use your digital multimeter to narrow down where excessive voltage drop is taking place.

For alternators with integral voltage regulators, be sure to check that the connectors that attach to the alternator are supplying proper signals. For instance, many alternators need a separate connection to battery voltage in order to charge properly. If this circuit is open, either low or no-charging can be the result even if the alternator itself is in good condition!


The alternator has been refined over the past 50 years to become a highly powerful and reliable automotive component. It is important to remember that when charging system failures do take place, checks should be made to be sure the alternator has what it needs to do its job. Following these steps will keep your customers happy and your comebacks few.

Tony Martin is an associate professor of automotive technology at University of Alaska Southeast in Juneau, Alaska. He holds Canadian Interprovincial status as a Journeyman Heavy Duty Equipment Mechanic. He also has 19 ASE certifications, including CMAT, CMTT, L1 and L2.

Print Article
blog comments powered by Disqus