Chapter A: General Diagnosis
30
shorted to power in a system with an internally
grounded field, or B-circuit
• High resistance in the wiring between the battery
and main fuse panel—this results in an inaccurate
voltage indication to the field circuit from the igni-
tion switch
• Battery with an internal short that accepts a charge
with little resistance
• A faulty component drawing excessive current
System Testing
Insight: In 1996, SAE J1930 terminology was adopted
to standardize component identification. This standard
adopted the name "generator" to refer to the compo-
nent commonly known as an "alternator." This study
guide uses the term generator throughout however;
both terms are used interchangeably in the ASE tests.
On-vehicle generator system tests are essentially the
same, regardless of the vehicle. The main differences
are meter test points and specifications. This section
contains general descriptions of common on-vehicle
tests. To check a particular generator or system, use the
procedures and specifications from the vehicle manu-
facturer.
The voltage regulator limits the strength of the magnet-
ic field by changing the current going to the field coil.
Therefore, it regulates the generator voltage output ac-
cording to the electrical system demand. Bypassing a
voltage regulator for test purposes is called full-fielding
the generator. Bypassing a voltage regulator supplies
maximum output to the field coil. Under these condi-
tions, system voltage can increase above 16 volts,
which is an unsafe level. Once you determine that the
generator can or cannot produce the required voltage,
remove the bypass immediately.
Charging System Output Test
A CSB analyzer simplifies charging system diagnosis.
This test instrument combines the functions of the car-
bon pile and separate meters needed for various tests.
Use the following general procedure to test the charg-
ing system output:
1. Connect the analyzer to the vehicle as instructed in
the analyzer manual.
2. Set the test selector switch to the charging test po-
sition.
3. Switch the ignition on and note the ammeter read-
ing. This is the current required to power the igni-
tion system and accessories; it will not be includ-
ed in the maximum output reading.
4. Start the engine and run it at 2,000 RPM, or the
specified test speed.
5. Adjust the carbon pile to obtain the maximum am-
meter reading while keeping the system voltage
above 12 volts. Always keep load time to a mini-
mum to prevent damage.
6. Compare the ammeter reading to specifications.
If it is:
• Within 10 percent of specifications, go to step 7
• Not within 10 percent of specifications, go to step
11
7. Set the test selector switch to the regulator test po-
sition.
8. Adjust the engine speed to 2,000 RPM or the spec-
ified test speed.
9. Note the voltmeter reading after it stabilizes. The
length of time required for the reading to stop ris-
ing varies according to battery state-of-charge.
10. Compare the voltmeter reading to specifications.
If it is:
• Within specifications, the voltage regulator is okay
• Not within specifications, replace the voltage regu-
lator and retest
11. With the engine running at test speed, high RPM,
turn the carbon pile to discharge the battery.
12. As system voltage drops, the voltage regulator
should increase generator output.
13. When the amperage output begins to decrease,
reduce the carbon pile discharge. Turn the carbon
pile off.
14. Add the highest ammeter reading to the readings
taken in step 3, using a minimum of 5 amps.
15. Compare the total to specifications. If it is:
• Within 10 percent of specifications, no problem is
found
• Not within 10 percent of specifications, further test-
ing of the field circuit and regulator is required. With
an internal regulator, test other generator inputs for
correct voltage
Circuit Resistance Test
Replace or repair any loose or corroded connections or
damaged wiring before testing the charging system to
prevent inaccurate test results. Isolate high resistance in
the positive, or insulated, and negative, or ground, cir-
cuits of the charging system by voltage drop testing.
Test under load with the engine running at 1,500 to
1,800 RPM and the generator producing a specific cur-
rent, usually 20 amps applied to the battery. Begin by
checking insulated circuit voltage drop by connecting
the DMM between the positive battery terminal and the
battery, or output, terminal at the generator, figure 1-37.
If the voltage drop is higher than specified, move test
connections down the circuit to pinpoint the high resis-
tance.
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