Electrical Maintenance
82
Interface Board Fuse F8
The Interface Board has a 300 ohm resistor and a
resistor bypass fuse (F8) in the alternator
excitation circuit. The fuse and resistor are
connected in parallel and are located on the
interface board Removing the resistor bypass fuse
places the 300 ohm resistor in the excitation
circuit as required for Bosch and Thermo King
Powder Coated alternators. Installing the resistor
bypass fuse removes the 300 ohm resistor from
the excitation circuit as required for Prestolite
alternators. See the specific unit wiring diagram
for exact details.
NOTE: The F8 fuse must be installed in the
interface board on units equipped with the
Prestolite alternator. If the F8 fuse is not in
place, the alternator will not charge properly.
Test Equipment for Checking
Voltage and Current
Always use accurate test equipment such as the
Fluke 23 Digital Multi-Meter and the Fluke
Clamp-On Ammeter accessory when checking
alternator circuit voltage and amperage. See the
table below for Thermo King service parts
numbers. Be sure voltages are measured from the
designated terminal to the alternator chassis
ground. All voltages are DC voltages unless
otherwise noted.
Alternator Load Test
Thermo King no longer recommends a full field
test for determining the alternator current output.
Full fielding an alternator can cause increases in
alternator output voltage that may damage internal
alternator or unit components. This damage may
not be readily apparent.
To test the alternator under load, Thermo King
recommends the use of a clamp-on ammeter to
monitor output current, both on initial startup and
under full unit load conditions. For example, on
multi-temp units, all remote evaporators should be
turned on.
General Diagnostic and Warranty
Evaluation Procedure
Complete the following diagnostic procedures
before replacing an alternator or the voltage
regulator.
1. When testing an alternator, use accurate
equipment such as a Thermo King P/N
204-1079 digital multimeter and a Thermo
King P/N 204-947 amp clamp or an
equivalent.
2. Make sure the drive belts and pulleys of the
charging system are in good condition and are
adjusted properly before testing the alternator.
Be sure the pulleys are the correct size. Worn
belts, loose belts and worn or improperly sized
pulleys will lower the output of the alternator.
3. The battery must be charged and in good
condition, the battery cable connections must
be clean and tight, and the 2A, sense, and
excitation circuits must be connected properly.
All charging circuit connections must be clean
and secure. If the unit battery is questionable,
a known good jumper battery should be
substituted for alternator testing.
NOTE: If the unit battery is questionable, a
known good jumper battery should be
substituted for alternator testing.
NOTE: Do not perform this test with a
battery charger connected to the unit battery.
NOTE: All voltage readings should be taken
between the chassis ground on the alternator
and the terminals indicated, unless stated
otherwise.
4. Check that the resistor bypass fuse (F8) is
installed on the interface board. Units with
Prestolite alternators must have the resistor
bypass fuse (F8) installed for the alternator to
charge properly.
Meter
Service
Part
Number
Fluke 23 Digital Multi-Meter 204-1079
Clamp-On Ammeter for above Meter 204-947
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