Guardian 4721, Repair Manual

Page: 38 / 115

PROCEDURE:
1. Locate Wire 4 that is routed from the circuit board and
connects to the voltage regulator terminal, third from the
top (see Figure 3). Disconnect that wire from the voltage
regulator terminal.
2. Set a VOM to read DC volts.
3. Connect the positive (+) VOM test probe to the terminal
end of disconnected Wire 4.
4. Connect the common (-) VOM test probe to the ground-
ing lug.
5. Crank the engine while observing the VOM reading. While
the engine is cranking, the VOM should read approxi-
mately 3-4 volts DC. When engine is not cranking, VOM
should indicate “zero” volts (see Figure 4).
RESULTS:
1. If normal field boost voltage is indicated in Step 5,
replace the voltage regulator.
2. If normal field boost voltage is NOT indicated in Step 5,
check Wire 4 (between regulator and circuit board) for
open or shorted condition. If wire is good, replace the
circuit board.
Figure 4. Field Boost Test Points
TEST 8 - TESTING THE STATOR WITH A VOM
DISCUSSION:
A Volt-OHM-Milliammmeter (VOM) can be used to
test the stator windings for the following faults:
• An open circuit condition
• A “short-to-ground” condition
• A short circuit between windings
Note: The resistance of stator windings is very
low. Some meters will not read such a low resis-
tance, and will simply indicate CONTINUITY.
Recommended is a high quality, digital type meter
capable of reading very low resistances.
Figure 5. Typical 1-Phase Stator Windings
PROCEDURE:
1. Testing 1-Phase Stators (Figure 5)
a. Disconnect Stator Leads 11 and 44 from the
generator main circuit breaker terminals.
b. Disconnect Stator Leads 22 and 33 from the
neutral block (00).
c. Complete a resistance test across Stator Leads
11 and 22. The resistance of a single winding
should be indicated.
d. Test for resistance across Stator Leads 33 and
44. Again, the resistance of that winding should
be indicated.
Stator Power Winding Resistance Across Wires
No. 11 & 22 and No. 33 & 44 should be:
15 kW . . . . . . . . . . . . . . . . . . . . 0.063 ohms.
20 kW . . . . . . . . . . . . . . . . . . . . 0.056 ohms.
25 kW . . . . . . . . . . . . . . . . . . . . 0.041 ohms.
e. Set the meter to a high resistance scale. Then,
test for a shorted condition as follows:
(1) Connect one test probe to stator lead 11,
the
other test probe to a clean frame ground on
the stator can. The meter should read INFINITY.
(2) Connect one meter test probe to stator lead
33 and the other to frame ground. The
meter should read INFINITY.
E1 E2
4
4
10.94
VDC
TO GROUND
S15
S16
0
6
162
PART 2
Page 34
PREPACKAGED LIQUID
COOLED AC GENERATORS
SECTION 2.4
DIAGNOSTIC TESTS