Charging system complaints fall into one of three categories, a no-charge, undercharge or overcharge condition. Regardless of the vehicle or the complaint, begin every charging system analysis with a careful visual inspection. Check for a broken, loose, frayed or cracked drive belt. Look for loose alternator mounts or mounting hardware. Watch for leaves or debris blocking the alternator air vents. Watch for and correct any oil or coolant leaks that affect the alternator.
Don't overlook blown fusible links under the hood or blown fuses in the vehicle's fuse panel because these problems may involve charging system circuits.
Watch for components connected to the alternator such as electric chokes, control relays etc. If these devices or their wiring short to ground, the short will damage the alternator.
Next, load-test the battery. Recharge or replace it where necessary. (Battery diagnosis and battery capacity are discussed in other areas of this service section.) When a battery is worn out or under capacity, a good charging system must work overtime keeping that
In fact, a battery with insufficient capacity for the application ultimately may cause premature alternator failure. An under-capacity battery forces the alternator to charge more often-to work harder-than it was designed to do. An alternator that doesn't have much excess electrical capacity is especially vulnerable to an undersized battery.
An autopsy at the local auto electric shop may confirm that a failed alternator did overheat. But neither the electrical specialist nor the technician who removed the alternator may suspect that the alternator actually overheated trying to maintain an undersized battery! Always verify that the battery has adequate capacity for the application.
A shorted battery cell can also cause an alternator to overheat. In the battery story elsewhere in this service section, we explained that a 10.5-volt reading is a telltale sign of a shorted cell. When a typical voltage regulator senses 10.5 volts, it allows the alternator to charge like a runaway horse until battery voltage rises to normal levels again. But a battery with a shorted cell cannot come back to normal voltage.
Although improved battery construction has reduced the risk of shorted cells, the problem can and does occur and may destroy an alternator as well as the battery.
After the battery checks good, test the alternator's ability to regulate electrical pressure (voltage) and to produce electrical volume (current or amps). For an accurate analysis, you must control pressure when testing current output, and control current output when testing pressure.
Always refer to the appropriate shop manual for specific charging system procedures and specifications. Most tests are the same as or similar to the following generic procedure.
First, connect a digital voltmeter to the battery terminals and an ammeter to the alternator output wire (the circuit from the alternator's B+ or BAT terminal to the positive battery terminal). The battery should be fully charged and all accessories shut off. With the engine running at operating temperature and the alternator loafing (the ammeter reading about 10-12 amps or less), the voltmeter should show the specified voltage regulator ``setting.'' Usually, the setting is within 13.8 to 14.8 volts.
Second, connect a load tester to the battery. Clip the tester's inductive ammeter pickup around the alternator output wire. Start the engine and use a throttle tool to hold engine speed at 2,000 rpm. Turn on the load control and adjust it until the load tester voltmeter reads either the specified test voltage or about 12.5 volts. Now note the load tester ammeter reading. For the moment, call this reading ``X.''
Shut off the engine and clip the ammeter pickup around either battery cable. Next, note the ammeter reading with the ignition switch on and engine off. The sum of this reading and reading ``X'' equals total alternator output. Compare total output to specification. The manufacturer may indicate that alternator output is good if it's within 10 percent of specification.
Remember to check for a slipping drive belt when alternator output peaks and then drops suddenly for no obvious reason.
Sometimes alternator specifications are hard to find. Or you suspect someone installed a replacement alternator that's inadequate for the vehicle. If so, compare total output to vehicle demand.
To do this, check accessory demand with the ignition on and engine off. Clip the ammeter pickup around either battery cable and turn on as many electrical accessories as possible.
Don't forget to switch the headlights to high beam and turn on the safety flashers, air conditioner, windshield wipers, blower fan on high speed, rear window defogger etc.
Remember to turn on all running lights or auxiliary lights on light trucks, modified vehicles and/or emergency vehicles.
Note and record the peak ammeter reading, then shut everything off. A good rule of thumb is that this reading should not exceed 75 percent of total alternator output. If the reading exceeds 75 percent, the vehicle has the wrong alternator on it or it needs a more-powerful alternator.
Some alternators can produce an impressive amount of current in spite of a diode or stator problem. If alternator output specs aren't readily available, this output may trick some technicians into thinking the alternator is up to par.
There are three routine ways to spot diode-stator trouble without taking the alternator apart. First, watch the go/no-go diode-stator display or light on the load tester during the alternator output test discussed earlier.
Second, check the alternator pattern with an oscilloscope while the engine is running and several accessories are turned on. Most engine analyzer oscilloscopes are suitable for alternator testing. Plus, analyzer operating guides show examples of normal and abnormal alternator patterns.
Third, use an accurate digital voltmeter to measure AC voltage at the alternator output terminal with the engine running and accessories turned on. An AC voltage greater than about 0.5 volt at the B+ or BAT terminal indicates diode or stator trouble.
Diode-stator problems, which cause undercharging and possibly electrical noise, mean the alternator must be replaced or overhauled.
Also, remember that an open positive diode causes audible, rpm-sensitive static in the AM radio band. Meanwhile, a shorted positive diode will keep the charge indicator light on the dashboard on after the ignition switch is turned off and drain the battery.
Voltage drop checks
External wiring problems and bad connections cause many no-charge, undercharge and overcharge conditions. Always supplement the tests discussed earlier with three fast, easy voltage drop tests to isolate a suspect alternator-regulator assembly from external problems.
Perform all three voltage drop tests with the engine running and electrical accessories turned on.
First, connect a digital voltmeter between the alternator output terminal (usually marked B+ or BAT) and the positive battery terminal. If the voltmeter lacks an auto-ranging capability, select the lowest voltage scale. The voltmeter reading should not exceed about 0.3 to 0.4 volt.
Second, connect the voltmeter between the alternator housing and the negative battery terminal. The reading should not exceed about 0.30 to 0.40 volt. This test verifies that the alternator is securely grounded to the engine and that the engine is properly grounded to the battery.
On some alternators, a separate wire or strap grounds the alternator to the engine.
Third, identify the voltage regulator's battery sensing terminal, which is usually marked S. When you connect the voltmeter between S and the positive battery terminal, the reading must be less than 0.10 volt. On many charging systems, this third check does not apply because the S terminal is hidden inside the alternator.
On sophisticated charging systems such as General Motors Corp.'s CS-series alternators, the alternator won't charge unless the S terminal can sense approximately 12 volts.
Remember: When you connect the voltmeter per our instructions here, the meter displays the voltage loss or drop between the two points where it's connected.
When any of these voltage drops are excessive, trace the circuit, locate the problem and fix it. Then retest before removing the alternator.
Check for excessive ignition-off current drain when the charging system tests good and the battery continues going dead. The most common cause of battery drain problems are convenience lights that stick on, including those inside the trunk, glove box, console storage bin, or on sun visor vanity mirrors.
These light bulbs should be cool when you open the trunk, glove box, etc. If the bulb feels hot the moment you open the glove box or trunk, you know the light is stuck on.
If these lights are not causing a drain, an electronic component that remains powered up with the ignition off may be drawing too much current. To check this, use a digital milliammeter and a battery drain test switch such as those offered by Kent-Moore Tools, Warren, Mich., and Thexton Manufacturing, Minneapolis. These test switches simplify diagnosis by keeping the entire electrical system powered up during the battery drain test.
A good working limit for ignition-off battery drain is about 30 milliamps (0.030 ampere). Various electronic components draw an excessive amount of ignition-off current until an internal timer switches the device into a ``sleep'' or ``rest'' mode. Always consult a shop manual when drain testing because it takes some devices 15 to 30 minutes to ``go to sleep.''