Electrical Maintenance (Rev 1/99)
37
6. The timing chip for the maximum defrost time limit
(defrost termination timer), which is set at 45 minutes.
7. The fast clock jumper (W1) used to speed up the maxi-
mum defrost time limit for testing.
Manual defrost is selected by pressing the manual defrost
switch. This applies 12 volts to the 11 circuit, which is con-
nected to the 85 pin of the defrost relay coil. The 86 pin of
the defrost relay coil is connected to the 12A circuit. The
12A circuit is connected to the 12 circuit through the timer
for the maximum defrost time limit. If the 12 circuit is
grounded through a closed defrost termination switch, the
defrost relay will energize and pull in.
When the defrost relay is energized, it closes a set of con-
tacts that connect the 8B (power) circuit to the 26 circuit.
This energizes the hot gas valve. When the defrost relay is
energized, it also opens a set of contacts that connect the 8B
(power) circuit to the 26D circuit. This de-energizes the fan
relay to stop the condenser and evaporator fans. Once ener-
gized, the defrost relay coil is held in by a latching circuit
that consists of a diode (D20) connecting the 26 circuit to
the 11 circuit.
The unit will remain in defrost until one of the following
events occurs:
1. The defrost termination switch opens and removes the
ground from the 12 circuit. This de-energizes the
defrost relay.
2. The unit is turned off.
3. The 45 minute maximum defrost time limit is reached
and the timer opens the connection between the 12 and
12A circuits. This removes the ground from the 12A
circuit to de-energize the defrost relay.
The timer for the maximum defrost time limit controls a
transistor that acts like a set of relay contacts connecting the
12 circuit to the 12A circuit. Jumper W1 is used to fast
clock this timer to approximately 1.5 minutes for testing.
The timed defrost interval timer can be adjusted from 1 to
10 hours by placing the dial on the potentiometer (R16/R17)
in the desired position. The factory setting is 4 hours. The
timer is powered by the 2 circuit, but it does not start to time
until the unit is running and the defrost termination switch is
closed. The timer retains the accumulated time when the
unit shifts to null or when the unit is turned off. When the
timer accumulates 4 hours, it applies 12 volts to the 11/
DRF-DR circuit. This energizes the defrost relay to start
defrost. The timer resets to zero and starts over whenever 12
volts are present on the 11 circuit. Jumper W2 is used to fast
clock this timer to approximately 1 minute for testing.
P.C. Board Identification
Currently, three different P.C. boards have been used on the
V250.
• The first version, P/N 41-776, has four fuses.
• The second version, P/N 41-1619, has five fuses.
• The third version, P/N 41-1812, also has five fuses, but
uses a different arrangement for jumpers W1 and W2.
The fast clock jumper pins for W1 and W2 are located
in the same positions, but the standard (parked) posi-
tions for these jumpers are identified as W3 and W4
and are located below potentiometer R17.
A significant electrical change was made between the first
version and second version. Opening the high pressure cut-
out (HPCO) or the low pressure cutout (LPCO) de-ener-
gizes the power relay PR on the first version. Opening the
HPCO or the LPCO de-energizes the circuit to the compres-
sor clutch on the second and third versions.
A P.C. board is identified by its Service Part Number (P/N
or SP#), which is located to the left of the potentiometer
R16/R17. The P.C. board must be correctly identified to
determine which wiring schematics and diagrams to use
when troubleshooting the electrical system.
• For the first version, P/N 41-776, use 5D44455 and
5D44456.
• For the second version, P/N 41-1619, and the third ver-
sion, P/N 41-1812, use 5D46179 and 5D46190.
NOTE: The nameplate on the cover of the low voltage box
does not necessarily identify the P.C. board correctly. The
P.C. board must be identified by looking at its part number.
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