24VNA6 / 25VNA4: Service Manual
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Code 33-15 Compressor Low Discharge Shutdown
Code 33-55 Compressor Low Discharge Lockout
Code 33-15 will become active if compressor is running and discharge
pressure is below the lower safe operating boundary for a period of time.
Discharge low pressure boundary only applies to the 5 ton model. Active
envelope management will attempt to modify system operation to
correct this condition before fault is set, but once time limit beyond the
limit is exceeded the system will shutdown. 6 occurrences of code 33-15
within 2 hours will result in code 33-55 and 2 hour lockout.
Low discharge pressure can be caused by malfunctioning reversing
valve, low charge, missing or stuck TXV/EXV, or compressor not
operating properly.
Code 34-11 Compressor High Temp Limiting
Code 34-58 Compressor High Temp Lockout
Code 34-11 will be set when discharge temperature exceeds 230F
measured near the compressor. Low charge or very high superheat are
contributors to this fault condition. For each occurrence of 34-11, the
next startup will operate at a 25% reduction in capacity. Each hour of
operation without additional faults will allow system to increase
available capacity by 25%. Once the system is running at minimum
capacity and discharge temperature exceeds 230F, code 34-58 will occur
and shut down system for 2 hours.
Code 35-11 Compressor High Compress Limiting
Code 35-58 Compressor High Compression Lockout
Code 35-11 occurs when the high compression boundary is crossed and
AEM cannot adjust system operation to reduce compression ratio. For
each occurrence of 35-11, the next startup will operate at a 25%
reduction in capacity. Each hour of operation without additional faults
will allow system to increase available capacity by 25%. Once the
system is running at minimum capacity and the high compression
boundary is crossed, code 35-58 will occur and shut down system for 2
hours. High compression ratio can be the result of overcharge or coil
blockage/loss of airflow across heat exchanger.
Code 36-15 Compressor Low Compress Shutdown
Code 36-55 Compressor Low Compression Lockout
Code 36-15 occurs when the low compression boundary is crossed and
AEM cannot adjust system operation to increase compression ratio.
Before this fault is set, AEM will automatically reduce outdoor fan
speed (cooling only) to increase CR, then increase compressor speed. 4
occurrences of code 36-15 within 2 hours will result in code 36-55 and 2
hour lockout. Low compression ratio can be a result of compressor not
increasing pressure adequately or low system charge.
Code 38-13 VFD Compressor Fault - Start Fail
Code 38-53 VFD Compressor Lockout - Start Fail
Code 38-13 originates in VFD after additional compressor starting
failures. While this code is active the unit opens PEV in an attempt to
equalize pressure across compressor and continue start retries. After 3
consecutive code 38-13, malfunction 38-53 will be set with a 2 hour
lockout
Check compressor wiring for secure connections, wire damage, and wire
order. This code indicates that the VFD is not able to rotate the
compressor rotor enough to complete starting process, indicating
compressor damage or high pressure differential. If malfunctions
continue to be generated and technician confirms pressure differential
across compressor is <40psi, replace compressor.
Code 38-18 High Diff Pressure Start Disable
Attempting to start the compressor against a high pressure differential
may result in start failures with an otherwise healthy compressor. The
PEV is included to ensure compressor will never start against a high
pressure differential. This fault will become active if a demand exists
and the PEV is not successful in lowering pressure differential below
100psi. The fault will clear and unit will start normally once pressure
differential is below 100psi and demand is still present.
The most likely cause of this fault is a non-functioning PEV. Possibly the
PEV suffered a mechanical failure, or the coil is not mounted to PEV.
Verify PEV coil is attached to PEV valve body. If coil is attached to
body, replace PEV harness and coil first, and PEV body if problem is not
fixed.
Code 38-31 VFD Compressor Fault - Estimator
Code 38-71 VFD Compressor Lockout - Estimator
The VFD Estimator is used in the compressor starting process. While
this fault is active the PEV will open and compressor will continue
attempts at restarting. Three continuous code 38-31 will result in code
38-71 malfunction and 4 hour lockout. Repeated malfunctions indicate
high starting torque and possible compressor damage. First, ensure the
VFD model and compressor models are correct for unit being serviced as
incorrect compressor motor parameters will not allow the VFD to
complete the starting process correctly. If all parts are correct for the
model being serviced, compressor damage exists and should be replaced.
Code 38-54 Compressor No Pump Lockout
This diagnostic checks to ensure the compressor is running in the correct
direction and pumping properly. Once the compressor speed is >400rpm,
the compressor has up to 30 seconds to increase the pressure differential
(discharge pressure - suction pressure) by 20psi. If this pressure
differential is not met, code 38-54 sets by locking operation for 30
minutes.
The most likely cause of this fault is the compressor leads being wired
incorrectly. Check wire colors against wiring diagram to ensure UVW
wires are attached to the correct terminals. Less likely but still possible is
compressor motor shaft breakage allowing compressor motor to rotate
but is disconnected from compression mechanism.
Fan Motor Faults General Info
The fan motor used on these models require 5 wires. 240~373 VDC is
supplied through Pin 1 of 3 pin connector (VDC) and Pin 1 of 4 pin
connector (GND). Pin 2 of 4 pin connector is Vcc signal for IC power
supply (~15Vdc). Pin 3 of 4 pin connector is Vsp signal, which is
0-5VDC input speed control. Pin 4 of 4 pin is FG or feedback signal,
sending rpm information back to VFD. The motor is controlled using a
closed loop control where variable speed is increased or decreased by
VFD depending on the RPM command from PCM to VFD along with
actual RPM information read through FG pin. Use
damaged electronics within fan motor. Note - winding resistance cannot
be measured on fan motor due to embedded electronics. Note -
resistances are approximate, temperature may affect measurements so
only declare motor bad if results differ greatly from values in
Though the fan motor has internal current protection, an external
replaceable fuse is located on VFD for short circuit protection. See
wiring diagram for fuse replacement specifications.
Code 39-13 Fan Start Failure
Code 39-53 Fan Start Failure Lockout
When a demand startup begins a fan speed is commanded to VFD. After
25 seconds of FG (fan speed feedback signal) = 0, the PCM will reset the
VFD and try to start fan again. After 3 VFD resets Code 39-13 will be
set. PCM will continue to reset VFD a total of 6 times before declaring
code 39-53 and locking out for 1 hour. A successful fan start will reset
the fan start failed counter.
Inspect all wiring and connectors for damage and loose connections.
Check for fan blade blockage and free spin. Check external fan fuse. If
no physical blockage is present and fuse is not open, ohm motor and
replace if necessary.
