Solving the Mysteries of Rotating Electrical

Solving the Mysteries of Rotating Electrical

How learning the basics of starting/charging systems can help you avoid a warranty issue.

We’ve all experienced the store-brand remanufactured alternator failing its third warranty replacement or the pricey new permanent-magnet import starter making the same clicking noise as the original. Believe me, repeat failures in rotating electrical are as expensive and frustrating to the professional technician as they are to the professional counterpro.

In keeping with modern technology, counter professionals can steer clear of many warranty issues in rotating electrical by learning more about the basics of starting/charging system operation. In this case, knowledge is power simply because the parts professional can provide more relevant information to their product hotlines and also to technicians who may lack the basics of diagnosing rotating electrical problems. With the basics of starting/charging system operation in mind, the following information is dedicated to those wishing to get off the warranty merry-go-’round of rotating electrical.

Battery specifications
Before attempting to warranty any rotating electrical problem, remember that battery performance is integral to good alternator and starter performance. In the modern vehicle, the battery must have enough reserve capacity to maintain the keep-alive electronic memories in radios and on-board computers for at least 30 to 40 days and still have enough cold cranking amperes to start the engine on a cold winter morning. The 1990s and earlier electronic vehicles generally have a “parasitic” draw at least 30 milliamperes to maintain the keep-alive memories in their computers. The 2000 and later models with multiple on-board computers might draw less than 20 milliamperes if all of their on-board computers “go to sleep” or “time out” as they should after the ignition is turned off. Remember that some brands of aftermarket radios and electrical accessories can quickly discharge a battery by doubling or tripling normal parasitic draw.

Modern batteries are rated in cranking amperes (CA) for warm climates and cold cranking amperes (CCA) for cold climates. The two ratings should not be confused because an OE-spec battery must have enough cold-cranking amperage capacity to maintain at least 9.6 volts in the vehicle’s electrical system when cranking in zero-degree weather. Although some battery manufacturers publish reserve capacity specifications for their batteries, most imply adequate reserve capacity in the CCA rating of their batteries.

Battery testing
At the outset, it’s important to understand that, to prevent serious accidents, all safety rules regarding the handling and testing of lead-acid batteries need to be observed to the letter. Although these rules are too voluminous to cover in this space, suffice it to say that any battery can spew acid or explode if improperly handled and tested. Battery safety rules should be available from your battery supplier and prominently posted on your store’s battery test and charging bench.

With that said, the battery’s state of charge (SOC) should be tested with electronic battery testers that measure resistance through the battery cells or with hydrometers that can measure the specific gravity of the electrolyte in each cell. A low SOC can be caused by bad battery cells or by poor charging system performance. Although electronic battery testers can detect the vast majority of battery defects, a follow-up with a hydrometer is highly recommended if the battery has removable cell caps and has a questionable performance record. Before testing and recharging, the battery terminals should be cleaned with a terminal cleaning tool. Because an alternator can be ruined trying to charge a defective battery, new batteries should always be tested before they’re sold and placed in service. Once in service, new batteries fail most often due to vibration caused by loose battery hold-downs or by excessive under hood temperatures.

Alternator failures
Direct current electricity is produced in a copper wire as it passes through a magnetic field. An alternator utilizes this principle by using a rotating magnet called the rotor to form a rotating magnetic field. The strength of the magnetic field is controlled by the alternator’s voltage regulator. A stator assembly that surrounds the rotor “cuts” the magnetic field to generate electric current. The mild steel stator is the part sandwiched between the aluminum end frames of the alternator. The stator uses three separate loops of wire to “cut” the magnetic field created by the rotor. These loops of wire are connected to a rectifier bridge composed of three positive and three negative diodes that “rectify” or change the electricity output from alternating to direct current. In general, most alternators fail due to grinding caused by worn bearings or by worn carbon brushes failing to fully contact the copper slip rings on the rotor. Worn brushes usually cause an intermittent charging condition. Bad voltage regulators or stuck brushes generally cause a no-charging condition.

Voltage regulator failures
Voltage regulators on many older vehicles are mounted either externally on a body panel or internally in the alternator. Externally mounted voltage regulators must have good connections to the alternator and must be mounted to a clean, rust-free surface that is grounded to the battery negative terminal. Internally mounted voltage regulators are usually more convenient to service by replacing the alternator as an assembly. The current generation of vehicles has the voltage regulator built into the Powertrain Control Module (PCM) and rarely causes regulator-related charging issues.

Because modern alternators are machine-assembled, bench rebuilding is usually impractical in most situations. Whether internal or external configuration, it’s important to determine if the ignition switch activates the voltage regulator when the switch is turned on. On many vehicles, the voltage regulator circuit is protected by an “alternator” fuse contained inside the vehicle’s fuse box.

A shorted voltage regulator circuit can cause an extreme over-charging problem that’s indicated by the battery spewing electrolyte from its vents and the vehicle’s head lamp bulbs burning out prematurely. A battery with a bad cell can create an over-charging condition because the voltage regulator is essentially working overtime trying to charge the bad battery. In general, on-vehicle testing is generally most conclusive because the jarring of normal handling may temporarily re-seat worn brushes and cause the alternator to pass a bench test. The battery must be at least 75 percent charged to accurately test an alternator or voltage regulator on the vehicle. Using an adjustable carbon pile to place an electrical load on the alternator while it’s mounted on the engine is still the most effective method of testing alternators and regulators.

In most on-vehicle testing, the voltage regulator should produce 12 volts when loaded to the alternator’s rated amperage output at 2,500 rpm. At idle in an unloaded condition, the alternator’s voltage output should be approximately 14.2 volts at 70 degrees ambient temperature. The voltage should increase to about 15.2 volts at low ambient temperatures and decrease to about 13.8 volts at high ambient temperatures.  Most cases of low alternator/regulator output voltage are caused by bad alternator diodes, a bad battery, or a slipping drive belt.

Starter failures
A starter assembly contains a solenoid that connects the starter motor to the battery and a starter “drive” that simultaneously engages the starter motor to the flywheel to crank the engine. Many modern vehicles may also incorporate an auxiliary starter relay in their under-hood fuse box that activates the starter solenoid. Remember that any modern vehicle equipped with a permanent-magnet, reduction-geared starter will crank the engine at nearly normal speeds with only 7 volts of electricity available at the battery. The permanent magnets in these starters are very susceptible to cracking caused by sharp impacts. Cracked magnets will cause a variety of cranking speed symptoms. Low cranking speeds caused by low battery voltages are generally limited to older vehicles equipped with conventional field-coil starters.

If the starter clicks repeatedly, but doesn’t engage, the battery terminals are usually corroded or the battery’s SOC is very low. If the starter doesn’t click at all, the ignition switch, fuse box relay, transmission neutral safety switch, or the starter itself might be bad. If the starter clicks once each time the ignition switch is turned, the starter is usually faulty. If the starter motor spins, but doesn’t engage the flywheel, the starter drive is worn. In either of the last two cases, the starter should be replaced as an assembly. Keep in mind also that, especially on late-model General Motors products, the starter is controlled, not by the ignition switch, but by the engine computer. Many drivers mistake this lack of correlation between ignition switch and starter as a starter defect when, in fact, the condition is entirely normal.

Gary Goms is a former educator and shop owner who remains active in the aftermarket service industry. Gary is an ASE-certified Master Automobile Technician (CMAT) and has earned the L1 advanced engine performance certification. He is also a graduate of Colorado State University and belongs to the Automotive Service Association (ASA) and the Society of Automotive Engineers (SAE).

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