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.
