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Electrical: Engine Starting, Charging
regulator/alternator diagnosing and adjusting easy. Be aware that many of the
tests mentioned in this article may not work on your specific application. Some
alternators can be damaged by full fielding for instance, others have a pulse width
modulated field controlling charging. When in question, always follow the
manufacturer recommendations.
An alternator generates current and voltage by the principles of electromagnetic
induction. Accessories connected to the vehicle's charging system require a steady
supply of direct current at a relatively steady voltage level. You cannot charge a
battery with alternating current, so it must be rectified to direct current.
Checking Ripple Voltage
Ripple voltage or AC voltage can leak past the rectifier bridge diodes and actually
cause the battery to discharge. It can be measured by switching your DMM to AC
and connecting the black lead to a good ground and the red lead to the BAT
terminal on the back of the alternator. Do not connect the leads to the battery, as
the battery will absorb or dampen some of the AC. Run the engine at 2,000 rpm
and read the meter's display.
You may want to also load the system by turning on the rear window defogger and
headlights. A good alternator should measure less than 500 mV (.5 VAC). A higher
reading indicates damaged alternator diodes and may cause problems in the ECU.
Another way to check the integrity of the diodes on a Bosch regulator is to check
the voltage readings at the D+ terminal and B+ terminal. The voltage reading
should be the same at both terminals. A difference of more than one volt would
indicate faulty diodes and the need to replace the regulator.
Use the Vehicle's Radio to Check Alternator Diodes
Have you ever heard a whine from the radio that changes with engine rpm and
isn't rap music? You can use the vehicle's radio to test the alternator's diodes. Turn
on the radio and select a quiet FM radio station. Turn up the volume and rev the
engine from idle to 2,000 rpm and back down to idle. Listen for a small whine or
siren noise in the background that follows the rpm change. The noise usually
indicates excessive ripple or AC voltage leakage from the rectifier bridge diodes.
Verifying a Good Alternator
The battery must be fully charged before testing the alternator. Run the engine and
verify that the no-load voltage is 13.8 to 15.3 V. Next, load the alternator to its
rated output current with a carbon pile across the battery. If you don't have a
carbon pile, load the alternator by turning on as many accessories as you can. Run
the engine at 2,000 rpm and check the current output with a current clamp. You
may find that someone has put a number of additional loads on the charging
systems increasing current demand from the alternator. Make sure that the
alternator is rated to the application.
Where Is the Best Ground?
Technicians ask me this all the time. With the engine shut off, the battery supplies
power to accessories and is the source of the best ground. After the engine starts
and the alternator takes over, the alternator becomes the source of all power and
the battery becomes a load and stabilizer. The best ground now is on the alternator
file:///C|/Users/Steve/Documents/Volvo%20FAQ%20Updated/ElectricalStarting.html[01/13/14 10:02:47 PM]
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