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3. Check field power leads L1 and L2. If these are not receiving

power, system cannot function.

If Transformer Has High Voltage Applied To It:

1. Check low-voltage transformer leads R (red) and C (brown).

Be sure they are wired to correct locations.

2. Check output voltage of transformer secondary side R (red)

and C (brown). Be sure transformer output is between 18 and
30 vac. If transformer output is incorrect and transformer is
receiving correct input voltage (208v or 230v), then trans-
former needs to be replaced with recommended transformer. If
no problem exists with transformer secondary, proceed to
items 3 and 4.

3. Check low-voltage fuse shown in Fig. 3A. If fuse is blown,

replace it with an identical 5-amp fuse. The transformer
cannot supply power to board with fuse blown or loose. If fuse
blows when unit has power applied to it, the system most
likely has 1 of the following problems:

a. Check all 24-v wiring for an electrical short.

b. The maximum load on transformer is 40 VA. If load on

transformer is excessive, the low-voltage 5-amp fuse will
blow to protect transformer. If load exceeds VA rating of
transformer, a larger VA rated transformer needs to be
installed. Check sequencers/relays for excessive current
draw.

c. Check wiring of heaters. If a heater is miswired, fuse may

blow. If a heater is miswired, correct miswiring by com-
paring it to heater wiring label.

4. Check connections on primary side of transformer. If they are

not connected properly, the transformer secondary cannot
supply the 24-v signal to energize fan relay. If transformer is
receiving correct primary voltage but is not putting out correct
secondary voltage, transformer needs to be replaced.

IF ELECTRIC HEAT STAGES WILL NOT TURN ON BUT
FAN WILL TURN ON:

If There Is No High Voltage To Transformer:

1. Check plug connection between heaters and board. This

supplies power to transformer and fan. Be sure plug is
connected properly.

2. Check sequencer/relay number 1 and plug wiring. Yellow

wire should be connected to pin number 9 of plug and to limit
switch. Black wire should be connected to pin number 7 of
plug and to sequencer/relay number 1.

3. Check incoming high-voltage power leads. If these are not

receiving power, system cannot function.

If Transformer Has High Voltage Applied To It:

1. Check low-voltage transformer leads R (red) and C (brown).

Make sure they are wired to correct location. The unit will not
function without proper connections.

2. Check output voltage of transformer secondary side R (red)

and C (brown). If transformer output is low (less than 18 vac),
refer to items 3 and 4 of previous "If Transformer Has High
Voltage Applied To It" section.

If Traces Are Overheated on Back of PCB:

Usually whenever a trace is blown on PCB, it means either there
has been a high-voltage short or high voltage has been applied to
low-voltage circuit. This can be prevented by making sure PCB is
wired correctly before PCB has power applied to it.

IF TRANSFORMER FUSE KEEPS BLOWING:

When low-voltage fuse blows, it means transformer would have
blown if fuse had not been in circuit to protect it. The fuse usually
blows when there is a high current draw on transformer, high
voltage applied to low-voltage circuit, or a direct secondary short.
When there is a high current draw on transformer, it is most likely
because transformer has been shorted or system is trying to draw
more VA than transformer rating allows. When fuse blows because
of high voltage, the system has mixed high- and low-voltage
signals.

1. Check wiring of sequencers/relays as shown in Fig. 4 and 5.

Be sure transformer is not shorting out because thermostat
wires are miswired.

2. Check wiring of sequencers/relays as shown in Fig. 4 and 5.

Be sure low-voltage and high-voltage wiring is correct.

3. Check VA draw on transformer. If VA draw is more than VA

rating of transformer, fuse will blow. If this is the case, replace
transformer with one that has a higher VA rating and meets
system specifications.

Fig. 3A—Fan Coil Printed-Circuit Board

A97020

LOW
VOLTAGE
FUSE

FAN RELAY

FUSE

PCB BLOCK WIRING