Guardian 4721, Repair Manual

Page: 97 / 115

DC CONTROL
LIQUID COOLED
ENGINE UNITS
SECTION 4.4
DIAGNOSTIC TESTS
PART 4
Page 93
PROCEDURE:
With “Utility” source power available to the transfer
switch, connect the test leads of an AC voltmeter
across Terminals N1 and N2 of the control board ter-
minal board. Normal rated “Utility” source voltage
should be indicated.
RESULTS:
1. If voltage reading in Test 28 was good, but reading in this
test is bad, go to Section 3.3.
2. If voltage reading in both Tests 28 and 29 are good, go
on to Test 30.
TEST 27 - TEST SENSING TRANSFORMER
DISCUSSION:
Normal line-to-line “Utility” source voltage is delivered
to the primary winding of the control board trans-
former. Transformer action reduces this voltage to
approximately 14 volts AC. The reduced sensing
voltage is then delivered to the control board. This is
a test of the control board transformer.
PROCEDURE:
1. See Figure 16. Connect an AC voltmeter across Primary
Input Leads N1 and N2 (two Black Wires). Normal
“Utility” power source voltage should be indicated.
2. Now, connect the meter test leads across the Red and
Blue leads to check the Secondar y Output..
Approximately 18 volts AC should be indicated.
RESULTS:
1. If normal “Utility” source voltage is indicated in Step 1,
but not in Step 2, replace the control board transformer.
2. If the transformer checks good, but startup and transfer
still occur when “Utility” power is available, replace the
control board and test automatic operation.
Figure 16. The Sensing Transformer
BLACK
BLACK
YELLOW
RED
YELLOW
BLACK
BLACK
N1
N2
PRIMARY
240 VAC
SECONDARY
14 VAC
RED
BLUE
TR1