The charging system on your Ford truck performs two basic
1. Maintains the battery's state of charge
2. Provides power for all of the truck's electrical
systems while the engine is running
It's a fairly simple system, consisting of a battery,
alternator, voltage regulator, indicator gauge or warning light,
and the wiring that connects the components to each other and to
the components they serve.
It is very important to regularly check the alternator drive
belt; this can be done whenever the cooling system is serviced.
Check the belt tension using a belt tension gauge. And while
both "new" and "old" belt tension specs are provided in your
service manual, keep in mind that a belt is considered old after
just 10 minutes of use! If you do not have a belt tension gauge,
press down on the belt with your thumb midway between the two
pulleys. If it deflects more than 1/2", it is too loose.
The belt should be inspected for signs of wear. Check for oil,
grease or hard glaze on the underside of the belt; any of these
can cause it to slip on its pulleys, resulting in reduced output
from the alternator. Another sign of a worn belt is a squealing
noise. (This sound can also mean that the alternator is
developing a bearing problem.) To find the source of the squeal,
apply aerosol belt dressing to the belt. If the squealing stops
or changes pitch, the belt is the source of the noise. If the
noise continues as before, remove the belt and run the engine.
If the noise is gone, the problem is inside the alternator;
otherwise, the source of the noise is outside the charging
NOTE: A good rule of thumb is to replace the
alternator drive belt every three years, regardless
of its appearance.
It's usually easy to know when the charging system isn't
generating enough power -- just look at the warning light or
gauge. If the light or gauge is not working, the battery will
discharge to alert you of a potential problem. The charging
system should also be checked if the engine cranks slowly or if
dim headlights brighten upon acceleration.
A problem that is not as easy to diagnose is overcharging, which
occurs when the voltage regulator fails to limit alternator
output. This causes the alternator to overcharge the battery,
which in turn can ruin the components that use electricity. Also
the sulfuric acid battery fluid vaporizes at a faster rate when
too much current is being supplied. If the battery cells are
allowed to empty, the dry plates will deteriorate and the
battery will die. It is even possible for an overcharged battery
For this reason, the battery fluid level should be checked very
few weeks. If the level is frequently low, the system is
probably overcharging. Also, check the level immediately if you
should small "rotten eggs", which can be caused by vaporized
An overcharging condition can be detected with an ammeter,
though a voltmeter is much easier to use. If the ammeter shows
continuously high charge rates or of the battery voltage often
exceeds 14.5 volts, overcharging is the probable cause.
The troubleshooting procedures that follow will help you head
off charging system problems before they occur. Be sure to do
each procedure in its entirety; do not skip any steps.
BASIC BATTERY CHECK
1. Connect the positive lead of the voltmeter to the
positive terminal of the battery and the negative lead to
the negative terminal.
NOTE: Protect your eyes with safety glasses or
goggles when doing this procedure.
2. Remove the coil cable from the distributor cap
3. Ground the coil cable to the engine block by
connecting a jumper cable between the two. This will prevent
dangerous arcing of the high-voltage spark.
4. While cranking the engine, observe the voltage
reading. It should be above 9.6 volts for conventional
batteries and above 10 volts for maintenance-free batteries.
NOTE: A hydrometer can be used in place of
this voltmeter test to check battery capacity. Be sure the
hydrometer is clean, inside and out, to ensure an accurate
ADVANCED BATTERY CHECK
There are several ways to test a battery to see how good it is.
The quickest and easiest is the load test. A predetermined load
is placed across the battery terminals and held for ten seconds.
At the end of the ten seconds the voltage across the terminals
is measured and this voltage determines how good the battery is.
If the voltage stays above 11 volts, the battery is good and
healthy. If the voltage drops between 9 and 11 volts, the
battery is determined to be borderline. It will be okay in warm
weather, but may fail as the temperature drops toward freezing.
If the voltage drops below 9 volts, it's no good.
The way we test the batteries state of charge is with a battery
hydrometer. A hydrometer measures the specific gravity of the
acid solution. The higher the specific gravity, the more charge
in the battery. A fully charged battery will have a specific
Temperature Compensation Table
State-of-Charge at 80°F (26.7°C)
Cell Specific Gravity
11.89 or less
1.120 or less
When you check the battery, you need
to check each cell. All six cells must have the same specific
gravity. If five cells test at 12.75 and one tests at 12.25, you
have may have a bad cell. Charge the battery for about 30
minutes and test again. If the low cell does not come up, it's
bad and you need a new battery. Some variance is allowed between
cells but if it is a large variance, you may have battery
The individual cells can also be tested with a voltmeter. Take a
coat hanger and make two lead extensions about six inches long
and attach them to the meters test leads. Touch the positive
lead to the positive terminal and stick the negative lead inside
the cell next to it. It should read about 2.1 to 2.3 volts. Now
insert the positive lead in the first cell and the negative lead
in the second cell. Proceed down the line until you get to the
last cell. Here you will put the positive lead in the last cell
and the negative lead on the negative terminal. All the cells
should read the same, or within 0.2 volts. If one reads 4.0 or
more, you have a shorted cell and the battery is no good. If you
get a very low reading or a zero reading, the cell is open and
again the battery is no good.
Before you do any battery testing, you need to start with a
fully charged battery. If it is not fully charged, then any test
results you get mean nothing. So always check the specific
gravity before you do anything. Also make sure the terminals are
clean and tight.
Now some batteries are sealed so you can't do a cell test or
check the specific gravity. In this case all you can do is
charge the battery for about 30 minutes and do the load test. You
can also measure the Open Circuit Voltage across the battery
terminals with a digital voltmeter. This is the only way you can
determine the State-of-Charge. In
sealed batteries you will usually see the "green eye"
or "magic eye". This a built in hydrometer which only
measures the State-of-Charge in ONE of its six cells. Don't trust it. I have seen
hundreds of bad batteries with green eyes telling me they are
This test will determine whether or not the system is charging
the battery and, if not, whether the alternator or regulator is
at fault. It requires a voltmeter capable of measuring at least
16 volts, down to tenths of a volt.
If the charging system on your car has an external regulator,
warm up the engine before performing the following test:
1. Turn off the engine, lights, and all other
2. Attach an engine tachometer according to the
3. Connect the voltmeter to the battery by attaching
the positive lead to the positive terminal and the negative
lead to the negative terminal.
4. Note the voltmeter reading. If you get a reading
of 12.30 volts or less, with all known current drains
(including lights, ignition and accessories) shut off, the
battery's state of charge is at 50% capacity or lower. This
indicates poor electrical connections, weak alternator
output, excessive current drain from accessories or a
defective battery. If it is less than 12 volts, charge the
battery. Then note the voltage reading again and record it.
5. Start the engine and slowly increase the speed to
6. Note the voltmeter reading again. If it exceeds
the engine-off voltage by more than 2 volts, the system has
a faulty regulator, a poor regulator ground, or a short
circuit in the wiring between the alternator and regulator.
7. If this voltage reading exceeds the engine-off
voltage by less than 2 volts, perform a load test. This
procedure will be detailed momentarily.
With current flowing toward the battery, voltage should read
14.8 volts or higher, depending upon the parameters of the
voltage regulator. If you see no increase in voltage from your
static battery test level, the alternator or regulator is not
performing properly. Before condemning these expensive parts,
however, troubleshoot further.
Shut the engine off and leave the meter set for the 12V range.
Touch the heavy (BATTERY or BAT) lead at the rear of the
alternator with the positive probe. Using a quality engine
ground for the negative probe, again read available voltage at
the alternator. This should be approximately the same as the
battery's voltage reading.
If you cannot read voltage here, a wiring or fusible link
problem exists. Zero the meter and set for DC and K-Ohms. Check
the wire from the starter solenoid to the BAT connection on the
alternator for continuity and conductivity by holding a probe at
each end of the wire. If no opens or shorts exist, the meter
will rest happily at the zero line. If there is too much
resistance, as with a slight open in the lead or a poor
connection, the needle will read upward on the scale. An actual
open in the wire prevents the needle from registering at all.
Resistance readings help locate corrosion within wire leads,
too. Battery cables or terminal clamp connections often develop
such problems. Unseen in a visual inspection, a current blockage
cannot fool your ohmmeter. The force necessary to keep current
flowing is measurable.
1. Keep the tachometer and voltmeter connected as
they were for a no-load test.
2. Note and record the voltage reading.
3. Start the engine and turn on the heater (or air
conditioner) at high speed. Turn on all lights and
4. Increase the engine speed to 3,000 RPM and note
the voltmeter reading.
5. If this reading exceeds the engine-off voltage by
1/2 volt or more, the charging system is functioning
6. If this reading exceeds the engine-off voltage by
less than 1/2 volt, perform a full-field test.
the regulator's control function is bypassed, the alternator
runs full-field. The method of bypassing the regulator differs,
depending on the vehicle. For this reason, please note that Step
1 below of the full-field test is for Ford models only. The
remaining steps are the same for all makes.
1. Turn off the engine and remove the voltage
regulator connector. Connect a jumper wire between the "A"
and "F" terminals of the plug, as shown at right.
2. Once the regulator has been bypassed, repeat the
3. If the full-field voltage exceeds the engine-off
voltage by 1/2 volt or more, the regulator is defective and
must be replaced.
4. If the increase in the voltage is less than 1/2
volts, either the alternator or wiring is faulty.
5. Inspect the wiring for signs of wear or heat
damage. If the wiring looks good, the alternator is probably
the faulty component.
NOTE: When performing the no-load, load, and full-field
tests on a vehicle with a catalytic converter, try to finish
them within a total engine running time of 5 minutes. If more
time is needed to finish the tests, wait 30 minutes before
continuing. This will allow the catalytic converter to cool,
thus preventing it from being damaged.
REGULATOR GROUND REPAIR
Once it has been determined that the problem is with the
regulator and not the alternator, you can find out if the source
is the regulator itself or its wiring or ground.
1. Check the wiring between the regulator and
alternator for heat damage or wear. (The regulator is mounted
on the inner-right-side of the radiator support.)
2. Remove the bolts from the voltage regulator. Use
sandpaper to clean off the area around the bolts and the
spot where the voltage regulator mounts on the truck. This
will assure a good ground.
3. Clean, reinstall and tighten the bolts.
4. Perform all the troubleshooting procedures again
to see if the problem still exists. If so, the problem is
either with the wiring or the regulator itself. If you have
already inspected the wiring, try replacing the regulator.
CAUTION: The battery ground cable should be disconnected
before replacing the voltage regulator.
When the alternator is defective, your shop manual will
provide test information and overhaul procedures. Any concern
about alternator output should begin with checking the drive
belt tension. Next, inspect all wire connections. To test the
alternator and regulator output, test the current flow from the
alternator with an induction ammeter. Compare the ammeter flow
to the OEM specifications for your truck, beginning at an idle,
then 1500 RPM, and then finally 2000 RPM. An induction ammeter,
although not as accurate as more expensive test equipment,
provides a quick sense for alternator output. Since the meter
simply fits over the cable or wire insulation, you can test
without removing any electrical component.
NOTE: Perform the induction meter test with
the battery charge low. You want the alternator
nears its maximum output.
NOTE: NEVER disconnect battery with engine
If starter and alternator circuits check okay and the battery's
cells read normal specific gravity at a full charge (applicable
for open-cell batteries), the chronic low battery voltage is
still possible. Accessories like the clock or the improper
hook-up of an aftermarket sound system can cause the battery to
go dead. (Be sure to wire you sound system through a fused and
ignition-switched accessories (ACC) source.)
A defect in the ignition switch, air conditioning clutch,
lighting equipment, the turn signal switch, a radio/tape deck or
hazard lamp can each draw excessive current from the battery.
The dome, underhood and hazard lamps operate without the
ignition switch on, so check these areas first. When an
aftermarket accessory taps directly into a battery source,
disconnect the accessory and see if the problems resolves.
If a current drain persists, suspect the ignition switch. A
shorted ignition switch can deliver current, even with the key
is in the off position. Current may be passing to the coil (or
ignition module, if so equipped). You can confirm current flow
at each of these areas by taking voltage readings with your VOM.