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System Testing & Troubleshooting
DIAGNOSTIC DISPLAYED REASON FOR DIAGNOSTIC UCM’S REACTION RESET REQUIRED
Low Pressure Control Open -
Circuit 2 (Circuit #2 40-60 Ton units
only)
Problem: The refrigerant in Circuit 2
is cold and may cause the Solution in
the Chiller barrel to become too cold.
Also, there may be a state of charge
issue with Circuit 2.
With one or more compressors
running, the higher pressure of the
two LPC switches on Circuit 2 has
opened 6 times with the LPC Bypass
relay de-energized, or it was open at
the time the LPC Bypass Relay de-
energized. On a standard unit this is
the 45 PSIG LPC switch, but can be
20 PSIG or lower on units with a lower
Leaving Solution Setpoint range.
A “Lockout Circuit # 2” request is
issued to the Compressor Staging
Control function. The Compressor
Relays, LPC Bypass Relay, Liquid
Line Solenoid Relay, and Hot Gas
Bypass Relay for Circuit 2 are all
de-energized.
(PMR) A manual reset is required
anytime after the Dia
nostic is set.
The Diagnostic can be reset by
the Human Interface or Tracer, or
by cycling power to the CGM.
LSS Adjusted to Maintain Minimum
Differential
Problem: The active Leavin
Solution
Setpoint (LSS) is closer to the
Leaving Solution Cutout Temp
Setpoint than the calculated
Minimum Differential.
The active Leaving Solution Setpoint
and the Leaving Solution Cutout
Temperature Setpoint are too close
together for proper unit operation.
The active Leaving Solution
Setpoint is set to the Leaving
Solution Cutout Temperature
Setpoint plus the calculated
Minimum Differential value.
(PAR) An automatic reset occurs
after the difference between the
active Leaving Solution Setpoint
and the Leaving Solution Cutout
Temperature Setpoint meets the
minimum difference requirement.
MCM Communications Failure
(40-60 Ton units only)
Problem: The CGM has lost
communications with the MCM.
Check: Check field/unit wiring
between CGM and MCM.
The CGM has lost communications
with the MCM.
A “Lockout” request is sent to the
Compressor Staging Control
function. The Liquid Line Solenoid
Relays and LPC Byp-energized.
(PAR) An automatic reset occurs
after communication has been
restored.
Sat Liquid Temperature Sensor Fail
Circuit 1
Problem: Circuit 1 Saturated Liquid
Line Temperature sensor input is out
of range.
Check: Sensor resistance between
830 ohms (200 F) and 345.7Kohms
(-40 F). If not, check field/unit wiring
between Sensor and SCM/MCM.
The Superheat/Subcool Module is
installed and the unit is reading a
signal that is out of range for the
Circuit 1 Saturated Liquid Line
Temperature Sensor input on the
SCM/MCM. (Temp < -55 F or Temp >
209 F).
The Superheat/Subcool calculation
is invalid and the values are set to
zero. No other change in unit
operation occurs. This diagnostic
is for information only.
(INFO) An automatic reset occurs
after the Circuit 1 Saturated Liquid
Temperature Sensor input returns
to its allowable range. In order to
prevent rapid cycling of the
Diagnostic, there is a 10 second
delay before the automatic reset.
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