Is it the Battery, Alternator, or Voltage Regulator?
It could be any one of the three, or an undetected
voltage drain caused by a trunk light, underhood light, or
glovebox light that does not go out when the lid is closed.
An alternator is based on the rotation of a magnet inside a
fixed-loop conductor. The output circuit and the field
circuit make up the automotive charging system.
The first thing that should be checked is the battery state
of charge. If it has a built-in hydrometer (charge
indicator), a green dot means the battery is 65% to 75%
charged and okay for use or further testing.
If the charge indicator is dark, the battery is less than
65% charged and needs to be recharged and load tested.
On 1985 and later model Chrysler vehicles, the charge
indicator on some batteries also contains a red dot which
shows if the battery is less than 50% charged.
If the charge indicator is clear or yellow, the level of
electrolyte inside the battery has dropped too far to give a
reading. It also means the battery will need to be replaced
soon. Once water level drops below the top of cell plates,
they dry out and lose their ability to hold a charge.
Never attempt to jump start or charge a battery with a low
electrolyte level. It may explode.
The state of charge of a sealed top battery without a
built-in charge indicator can be determined by measuring its
open circuit (no load) voltage:
Open Circuit Voltage State Of
Charge
12.6v
100%
12.4v
75%
12.2v
50%
12.0v
25%
11.7 or less
Discharged
A low charge level does not mean anything is wrong with the
battery or charging system, it simply means the battery is
low and needs to be recharged.
Performing a load test would be the next step. This checks
the battery's ability to deliver current. The battery must
be at least 65% charged before load testing. If not, a good
battery may fail the test.
A conventional load test is performed with a carbon pile
battery tester. The load created by the carbon pile is
adjusted according to the battery's cold cranking amp (or
amp/hour) rating. The carbon pile is usually set to one half
the battery's CCA rating (or three times its amp/hour
rating).
Temperature compensation is also important because a cold
battery puts out fewer amps than a warm one. The load is
then applied to the battery for 15 seconds while voltage
output is observed. If voltage remains above 9.6 volts, the
battery is good. If it drops below 9.6 volts, the battery
can be recharged and retested, or given a three-minute
charge test.
A three-minute charge test checks for a sulfated battery.
Slow charge the battery at 40 amps for six minutes, then
check voltage across the terminals with the charger on.
If the voltage is above 15.5 volts, the battery is not
accepting a charge. Slow charging for 20 hours can sometimes
reverse the sulfated condition, otherwise the battery is
junk.
If the battery check is okay, the next item to check is the
charging system. A properly working system produces a
charging voltage around 14 volts at idle with lights and
accessories off (refer to a shop manual for exact charging
specs).
When the engine is first started, charging voltage should
rise quickly to about two volts above base battery voltage,
then taper off and level out at the specified voltage.
Exact charging voltage will vary according to battery state
of charge, load on vehicle electrical system, and
temperature. The lower the temperature, the higher the
charging voltage. The higher the temperature, the lower the
charging voltage.
On a GM application, for example, accepted voltage charging
range is 13.9 to 14.4 volts at 80 degrees F. At 20 degrees F
below zero, charging range is 14.9 to 15.8 volts. At 140
degrees F, the charging voltage is 13.0 to 13.6 volts.
Charging output can also be checked with an adjustable
carbon pile, voltmeter and ammeter. The carbon pile is
attached to the battery and adjusted to obtain maximum
output while the engine is running at 2,000 rpm.
If charging voltage is low, the alternator or voltage
regulator could be faulty. To find out which component is
bad, a procedure called "full fielding" can be used to
bypass the regulator.
If the alternator produces the specified voltage or current
output after full fielding, the problem is in the regulator
(or wiring) not the alternator.
The exact procedure for full fielding an alternator varies
from vehicle to vehicle depending on how the alternator is
wired. Basically, the regulator is bypassed by connecting a
jumper wire between the field (FLD or "F" terminal) and
battery positive (BAT) terminal on the alternator.
On older GM applications with Delco integral regulator
alternators, inserting the tip of a screwdriver through the
D-shaped hole in the back of the alternator full fields the
unit.
Either voltage or current output can be compared against
manufacturer specs to determine if the alternator is
functioning at full capacity. Generally speaking, alternator
output should fall within 10 amps or 10% of its rated
capacity at 2,000 rpm.
For several reasons, it is important to follow full fielding
test procedures exactly. If only one diode or stator winding
is bad, for example, the alternator may still make enough
electricity at high rpm to keep the battery charged, but not
at idle or low speed. The alternator and/or regulator can
also be damaged if the wrong test procedure is used.
On Chrysler externally regulated alternators, for example,
you do not apply voltage to the "F" terminal. This system is
full fielded by grounding the green wire at the regulator
connector. On externally regulated Ford alternators, the
alternator is full fielded by disconnecting the four-wire
connector from the regulator and jumping across the "A" and
"F" terminals.
If charging output goes up when the regulator is bypassed by
full fielding, but otherwise fails to produce voltage, check
the regulator for a poor ground. This is especially
important on Ford and Chrysler systems. Poor or open wiring
connections between alternator and regulator can also cause
a charging problem.
A slipping fan belt is one of the most common causes of
under charging. A fan belt that holds at idle or low rpm may
slip when the alternator is under load. Glazed or burned
streaks on the belt are an indication of slipping.
If the battery and charging system are okay and the battery
keeps running down, check for a voltage drain somewhere in
the electrical system. To isolate the cause, remove one of
the battery cables and connect a volt meter or amp meter
between it and the battery.
A voltage drain will cause a reading on the meter.
Disconnect fuses one by one until the circuit is found that
causes the reading to disappear.
On-board electronics such as the computer, an electronic
clock, etc., will draw a few milliamps all the time, but
should not be enough to run the battery down unless the
vehicle is not driven for long periods of time.
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