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keeps running down
it the Battery, Alternator, or Voltage Regulator?
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.
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
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.
the charge indicator is dark, the battery is less than 65% charged and
needs to be recharged and load tested.
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.
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
attempt to jump start or charge a battery with a low electrolyte level.
It may explode.
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:
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
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.
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).
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.
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.
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.
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).
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.
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.
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.
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.
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.
the alternator produces the specified voltage or current output after
full fielding, the problem is in the regulator (or wiring) not the alternator.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.