D2-4 24 VDC Electric Supply System D02023
Preliminary Checks
1. Check the drive belt tension. Refer to engine
manufacturer’s Operation and Maintenance
Manual for belt tension requirements.
2. Insure that an undercharged battery condition
has not been caused by accessories having
been left ON for extended periods.
3. If a battery defect is suspected, check battery as
specified in “Battery - Troubleshooting”.
4. Inspect the wiring for defects. Check all connec-
tions for tightness and cleanliness. Remove and
clean battery cables.
5. If truck is equipped with a battery equalizer sys-
tem, verify proper operation of equalizer and
individual battery voltages. Refer to “Battery
Equalizer”, this Section.
Test Setup
1. Discharge batteries sufficiently to insure ade-
quate loading of alternator when engine is
operated during tests.
2. Open battery disconnect switch. Remove bat-
tery cable from alternator B+ terminal.
Refer to Figure 2-2 for the following steps. Meters
should be installed directly at the alternator as shown
to eliminate variations in readings due to cable
lengths etc.:
3. Install an ammeter (negative lead) between the
battery positive cable removed in step 2. and
the alternator B+ terminal (ammeter positive
lead).
4. Install a voltmeter between the alternator B+ ter-
minal (positive lead) and the ground terminal
(voltmeter negative lead).
5. Secure all test equipment leads to prevent dam-
age or short circuits when engine is started.
Reconnect battery disconnect switch.
The following tests require working near the
engine when running. Use caution when working
near engine fan, alternator fan and belt.
Test Procedure
1. Start engine, accelerate to high idle and
observe meters.
If voltmeter reading exceeds 30.5 volts, stop
engine immediately and refer to Table II.
• If batteries are sufficiently discharged, amps
should be “high” (240 amps ±10%) and voltage
should be between 27.3 and 28.9 volts (normal
range) or may be less than 23.7 volts if the
batteries are significantly discharged.
2. As the batteries approach full charge, the
amperage should fall as voltage rises.
3. When amps and volts readings stabilize, note
readings and refer to Table II to diagnosis sys-
tem condition.
AMPS VOLTS DIAGNOSIS
HIGH LOW
Charging system is OK. Batter-
ies are not yet fully charged.
Wait for charging system to
bring to full charge; amps
should decrease and voltage
should stabilize between 27.3
and 28.9 volts.
HIGH NORMAL
Watch until amps decrease or
voltage exceeds 28.9 volts. If
amps decrease and volts
remain normal, system is OK. If
voltage exceeds 28.9 volts, reg-
ulator and/or alternator defec-
tive.
HIGH HIGH
STOP TEST! Regulator and/or
alternator defective.
LOW LOW
1. Recheck voltmeter leads. If
connections are OK, alternator
and/or regulator defective.
2. Perform Regulator Bypass
Test per instructions on follow-
ing page:
a. If volts and/or amps increase,
alternator is OK but regulator is
defective.
b. If no effect, replace alternator
LOW NORMAL
Charging system is OK.
LOW HIGH
STOP TEST! If battery and volt-
meter check is OK, regulator
and/or alternator defective.
TABLE II. TROUBLESHOOTING CHART
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