Troubleshooting
Table 8 provides LED status indicator information
for troubleshooting the VMA18 controller.
Note:
If you experience short circuits in the 24
VAC power supply causing protective devices
such as breakers or fuses to trip, make sure
that the power connections on the VMA are
not reversed. The most common cause of this
problem is when the 24 VAC power supply
on the VMA is reversed but not reversed on a
connected secondary device. Improper wiring
of this power terminal may cause a short circuit
across the 24 VAC power supply on -1 models.
Table 7: Troubleshooting
Problem
Possible cause
Correction
Verification
Power at
Primary of
Transformer, 0V
at Secondary
Transformer has
tripped
Controller is
Off
Power at
Primary of
Transformer, 24
V at Secondary,
0V at Fuse/
Breaker
Breaker/Fuse has
tripped
1. Transformer is
shorted
2. 24VAC powered
sensor is not
wired with
the same
polarity as the
controller
3. SA bus device
is not wired
with the same
polarity as the
controller
1. Ensure polarity of ~24 V
COM / ICOM / + 15VCOM/
SA BUS COM on the
controller, auxiliary
devices and I/O is the
same
2. Ensure OUT1-OUT3
terminals of binary
outputs are not
connected to ~24
VAC COM, verify that
OCOM1-OCOM3 are not
connected to ~24 VAC
HOT (these terminals are
internally sourced)
3. Verify the short circuit has
been resolved with an
ohm-meter
4. Reset the breaker/fuse or
replace the transformer
1. Disconnect the
secondary of the 24
VAC transformer
2. Use an ohm-meter
to measure between
~24 V HOT and COM;
there should be no
short circuit
0–10 V output
is set to 10–
100%, but 0
V is at output
terminals
Output is in
protection mode -
a state the analog
portion of the
configurable
output goes into
when it detects a
wiring problem.
The analog
output is set to
0% regardless
of the command
whenever a wiring
fault is detected
Power polarity
mismatch between
connected device and
configurable output
Ensure polarities of ~24 V COM/
OCOM match and that the
connected end device uses the
same polarity
Configurable
output - analog
mode is invalid
0–10V output
has an
undesirable
offset of up to
1V
Common
Reference is
incorrect
OCOM terminal is not
connected
Connect OCOM terminal of
the configurable output to the
common of the connected end
device
1. Measure the output
and verify that
it matches the
command
2. Disconnect the
connected device
and verify the
commanded value is
present
VMA1826/1832 VAV Controller Installation Guide
22