Michael Thomas, automotive and diesel technician, is the author of the McGraw-Hill textbook "Truck and Trailer Systems."
The Alternator: Introduction
The charging system is responsible for charging the batteries and providing power to run the vehicle's electrical systems without draining the battery. The charging system consists of the batteries, cables, wiring, voltage regulator and alternator. The alternator is an electromagnetic device that turns motion into the electricity which will be used to charge the batteries and run the vehicle electrical system. The needs of the vehicle determine the size of the alternator. Alternators can range from as low as 37 amps for transport refrigeration units to 300 amps for some busses.
Note: If you're looking for more information, read "DIY Auto Service: Basic Electrical and Electronic Testing" for basic electrical theory and "DIY Auto Service: Basic Digital Volt Ohm Meter (DVOM) Electrical and Electronics Testing" for meter usage.
AC Current Production
The alternator follows the same principles as the power generators of the local electric company. A magnetic field is created by an electromagnet and that magnetic field is rotated to cross a conductor which will generate an AC current flow in the generator. Two things determine the output of the alternator; the speed at which these magnetic lines of force are moved and the strength of the magnetic field. As an example: Hoover Dam provides electricity to many communities. The 700-foot dam holds back an enormous amount of water. At the bottom of the dam are the generator rooms. Water flows through pipes to rotate the center or rotor of the generator. The fixed stator collects the lines of force and turns it into electricity to power homes and businesses.
The alternator is an electromagnetic device that turns movement into electricity.
These are the main parts of the alternator:
The housings or end frames provide support bearings at each end and a ground path for the negative connections. These aluminum housings are also used to bolt the alternator onto the engine.
The rotor is the rotating member inside the alternator. It is responsible for generating a magnetic field while rotating. The strength of the magnetic field is controlled by the amount of current flow through the rotor windings. Since the rotor is connected to the pulley and belt system, engine RPM’s will also have an effect on the alternator output.
Brush-type alternators use brushes that ride on slip rings at the rear of the rotor. The slip rings are connected to each end of the rotor coil. Brushes pass the electrical current to the spinning rotor.
Brushless-type alternators use a stationary rotor coil that sits inside the spinning rotor housing. Since there are no brushes, there are no brushes to wear out or create poor contact.
Voltage Regulator; Brush Holder
The voltage regulator, as its name implies, is used to control the voltage output of the alternator. The voltage regulator is either internal in the alternator or external mounted on the alternator or remotely mounted. The voltage regulator should control the alternator output voltage between 13.5 to 14.2 volts. If the voltage is too high it could cook a battery or damage sensitive electrical components. The voltage regulator controls the flow of current to the rotor. More current thru the rotor windings produces a stronger magnetic field and more output from the stator. Less current thru the rotor windings produces a weaker magnetic field and less output from the stator.
The voltage regulator controls the flow thru the rotor in one of three ways:
“A” Type circuit controls the ground path.
“B” Type circuit controls the power to the rotor.
Insulated type is a combination of both and controls power and ground.
The stator is the fixed ring inside the alternator. It is the collector of the lines of force and the component that the current is induced into and producing the AC voltage. The stator has 3 circuits or phases that are used to collect the induced current and keep AC voltage output consistent. shape in a schematic. The second called a Delta connection connected together at the ends of each loop and forming a triangle shape in a schematic. The delta stator windings will produce more current flow, which is the reason this is the most popular heavy duty truck alternator.
The rectifier bride is a heat sink and it contains 6 diodes, 3 positive and 3 negative. A diode is a one way electrical check valve. The diode allows current to flow in only one direction. Each loop of the stator is connected to one positive and one negative diode in the rectifier bridge. When AC current is produced by the stator, the negative portion cannot be used by the vehicle. The rectifier uses a conversion process called half wave rectification where it will remove the negative portion of the AC current and let only the positive portion out of the alternator. This can be used by the vehicle as DC current to charge batteries and run the electrical systems.
A diode trio is used by the alternator for internal power to the rotor. Once the alternator is charging, it is self sufficient. The diode trio has three diodes to rectify the negative AC current to make the stator output useable to power the rotor.
A capacitor is an electrical storage device like an accumulator. It stores the voltage spikes and releases voltage when it is low. The three loops or phases of the stator produce a pulsing voltage as each loop produces its voltage. With a capacitor in the output circuit, the result is a smoother voltage output from the alternator and less radio noise.
A fan is located behind the pulley or inside the case to draw airflow thru the alternator for cooling purposes. As the amperage output goes up, so does the heat in the stator and rectifier bridge.
The alternator is driven by a belt drive system.
V-belts used to drive most alternators, but this has been replaced by a serpentine belt with an automatic tensioner.
Depending on the design of the alternator, different amounts of disassembly are required to test the internal components. Brush-type alternators typically require the most disassembly to test components. Delco Heavy Duty Brushless alternators have a plate on the rear of the alternator to access most of the components for testing.
This is the procedure to perform a typical AC Delco SI series alternator repair.
Once the alternator is removed from the engine, scribe a mark across the front and rear half of the end frames. This will aid in reassembling the alternator later.
- Remove the nut that holds on the pulley and fan. Remove the pulley and fan and any spacers that are on the shaft.
- Remove the 4 through bolts that hold the two halves of the alternator together.
- Pry the two halves apart with the stator staying with the rear half.
- If the front bearing is being replaced, press the rotor out of the front end frame. Remove the bearing plate bolts and press the front bearing out.
- The rear housing contains most of the components. The three stator leads bolt to the rectifier bridge. Remove the three nuts, then remove the stator from the rear housing.
- Before removing the rest of the screws that hold in the rectifier, brush holder, diode trio and voltage regulator make a diagram of where the insulated screws and the non insulated screws go. Note: If the screws are put into the wrong position the alternator will short out.
- Remove the screws that hold in the rectifier, brush holder, diode trio and voltage regulator.
- If the rear bearing is to be replaced, press it out at this time.
Alternator Component Testing
- Visually inspect the components for damage, corrosion and overheating. Electrical parts that have been overheated may look black and have a burnt wire smell. To test the alternator components follow the guidelines below as a general rule.
- The Brush Type Rotor is the rotating member inside the alternator. Slip rings at the rear of the rotor are connected to each end of the rotor coil. With an ohmmeter on its lowest scale test across the 2 slip rings for continuity. The meter should read very low resistance on a good rotor coil. Test for continuity between the slip rings and the shaft. There should be no connection.
- The Brushless Type Rotor has a stationary rotor coil. With an ohmmeter on its lowest scale, test across the 2 wires for continuity. The meter should read very low resistance on a good rotor coil. Test for continuity between the coil and ground. There should be no connection.
- The stator is the fixed ring inside the alternator. To test the stator windings, with the ohmmeter on the lowest settings, test between each of the eyelets. There should be very low resistance. Test between the eyelets and the stator frame and there should be no connection.
- The rectifier bride is a heat sink and it contains 6 diodes. The rectifier bridge is divided into a positive side (insulated) and negative side (grounded). With the meter set on the diode test function, place one test lead on the negative side of the bridge and test the diodes first one direction then, switch the meter leads and test again. The diodes should flow in one direction but not the other. Switch to the positive side and perform the same test. The meter will read the amount of millivolts required to turn the diode on. A bad diode will read in both directions or not in either direction.
- The diode trio has three diodes to rectify the negative AC current and one lead after the diodes. With the meter on diode test, place one meter lead on the single terminal side and test all three leads on the other side. Switch the leads and test again. There should be a reading to all the diodes in one direction but, no reading in the other direction. A bad diode will read in both directions or not in either direction.
- The voltage regulator usually cannot be tested. The performance of the voltage regulator is tested with the alternator running. Its job is to keep the voltage between 13.5V and 14.2 volts. I f the voltage was outside this range a voltage regulator may be needed.
- A capacitor is an electrical storage device like an accumulator. Test the capacitor with an ohmmeter. There should be no continuity between the shell and the lead.
Alternator Electrical Diagram
Charging System Testing and Diagnosis
The charging system starts and ends at the batteries. The batteries should be the first things checked along with battery cables. If the alternator is tested without knowing the condition of the other areas of the system, it could lead to an alternator being replaced that is good or a problem still exists. To find a problem in any system a logical path must be followed to eliminate what is good and find what is bad. If the proper testing procedures are not followed, inaccurate diagnosis will be the likely results.
Perform the following battery tests that are available for the batteries you are working with as outlined in the previous chapter. This will give you a clear picture of the battery conditions. Repair any battery problems before going on to the alternator. A previous Hub covered battery testing and diagnosis (DIY Auto Service: Battery Diagnosis and Repair) for procedures and specifications.
- Specific Gravity Test
- Static or Open Circuit Voltage Tests
- Battery Load Tests
- Capacitance or Conductance Tests
Charging System Voltage Drop Tests
Voltage drop tests are used to check for bad connections and excessive resistance in the circuit. To perform an alternator voltage drop test the engine must be running. Caution should be observed while working around a running engine.
In many cases the alternator shares the battery cables from the starter to the batteries. In some cases the alternator is wired directly to the batteries.
To perform the alternator voltage drop tests:
- Connect a voltmeter from the battery positive post to the alternator positive terminal while the engine is running. Record the reading in volts.
- Connect a voltmeter from the battery negative post to the alternator negative terminal or housing while the engine is running. Record the reading volts.
- Add the two readings together. Do they exceed 0.5 volts?
- Does either reading exceed 0.25 volts?
- If the answer to #3 or #4 is yes, perform more voltage drops with the meter leads closer together to find the major source or sources of the voltage drops.
- If the answer to #3 or #4 is no, there is not a problem with the connections, cables or corrosion in the alternator circuit.
Alternator Output Tests
Alternator Amperage Output Tests
The alternator test must be performed with the engine running at 1,500 RPM. The alternator wiring could be all the way to the batteries or the alternator could be wired to the starter positive and negative which will share the battery cables back to the batteries. The batteries may be connected to the starter with one or two positive cables and one or two negative cables. For accurate results the Inductive Amp Clamp must be on all the positive cables or on all the negative cables. Since amperage is the same anywhere in the series starter circuit, it can be measured on either the positive or negative cables providing all the positive or negative cables are in the amp clamp. The VAT tester is clamped to the battery and the inductive lead around the negative or positive cable. The typical VAT amp clamp will only fit over 1 cable so if there is more than one cable you may need to use a different amp clamp or a meter that has a larger amp clamp.
To perform the test:
- Hook the VAT up.
- Start the engine and run it at 1,500RPM.
- Using the load on the VAT, load the batteries until the battery voltage reads about 12.5 volts and observe the amperage reading. _________A
- Compare the reading to the specification _______________A (Specification)
- The amperage output should be within 10% of the alternator rating. Example: An alternator rated at 130A, should have at least an output of 117Amps.
- If all the other tests on the batteries and voltage drops pass, this will usually indicated the alternator is bad. A pulley or belt slipping could also cause low output.
Alternator Voltage Tests
The voltage regulator controls the voltage range of the alternator. With the engine running test the alternator voltage at the batteries. It should be 13.5 volts to 14.2 volts. If the voltage is not within this range, a voltage regulator may be the problem.
Alternator Output Tests
- Disconnect the negative battery cables.
- Mark then remove the positive and negative connections to the alternator.
- With V-belts loosen the 2 thru bolts holding the alternator and back off the tension screw, if equipped.
- With a serpentine belt, it’s always a good idea to make a diagram of the belt routing if one is not provided on the vehicle. Use a breaker bar or ratchet that fits the automatic belt tensioner, rotate the tensioner to loosen the belt. Remove the belt from the alternator pulley.
- With the 4 bolt mount, remove the 4 bolts and remove the alternator from the engine.
- With the 4 bolt mount, install the 4 bolts and that connect the alternator to the engine.
- With a serpentine belt, use a breaker bar or ratchet that fits the automatic belt tensioner, rotate the tensioner to loosen the belt. Install the belt on the alternator pulley and relieve the tension. Double check that the belts are aligned on ALL the pulleys, before starting the engine
- With V-belts, loosely install the 2 thru bolts and the tension screw. Install the V-belts and start tightening the adjustment screw. As the belts start getting tight, check the tension with a belt tension gauge. Tighten the thru bolts.
- Connect the positive and negative connections to the alternator.
- Connect the negative battery cables.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.