Carrier 48XL Furnace Board, Systems Communication Failure, Model Plug, Control Fault, Oat

-Defrost operation

-Low ambient cooling

-Crankcase heater operation

-Compressor external protection

-Pressure switch monitoring (refrigerant)

-Time delays

FURNACE BOARD

The furnace board controls the following functions:

-Indoor blower operation

-Gas valve

-Inducer motor

-Remote sparker module

-Pressure switch monitoring (gas)

SYSTEMS COMMUNICATION FAILURE

If communication with the Infinity Control is lost with the UI, the

controls will flash the appropriate fault codes. Check the wiring to the UI, indoor and outdoor units.

MODEL PLUG

The HP/AC control board must have a valid model plug to operate. If a valid model plug is not detected, it will not operate and the

control will flash the appropriate fault code, shown in Table 5.

PRESSURE SWITCH PROTECTION-REFRIGERANT

The unit is equipped with high- and low-pressure switches. If the

control senses the opening of a high- or low-pressure switch, it will respond as follows:

1.De-energize the compressor contactor (HPS1 & LPS) or the

compressor solenoid contactor (HPS2).

2.Keep the outdoor fan operating for 15 minutes.

3.Display the appropriate fault codes.

4.After a 15 minute delay, if there is still a call for cooling and

the LPS or HPS is reset, the compressor contactor is energized.

5.If LPS or HPS has not closed after a 15 minute delay, the outdoor fan is turned off. If the open switch closes anytime

after the 15-minute delay, then resume operation with a call for cooling.

6.If LPS or HPS trips 3 consecutive cycles, the unit operation is locked out for 4 hours.

7.In the event of a high-pressure switch trip or high pressure

lockout, check the refrigerant charge, outdoor fan operation and outdoor coil for airflow restrictions.

8.In the event of a low-pressure switch trip or low pressure lockout, check the refrigerant charge and indoor airflow.

CONTROL FAULT

If the HP/AC control board has failed, the control will flash the

appropriate fault code (See TaMe 5). The control board should be replaced.

BROWN OUT PROTECTION

If the line voltage is less than 187v for at least 4 seconds, the

appropriate compressor contactor and fan relay are de-energized.

Compressor and fan operation are not allowed until voltage is a

minimum of 190v. The control will flash the appropriate fault code (See Table 5).

230V LINE (POWER DISCONNECT) DETECTION

If there is no 230v at the compressor contactor when the unit is

powered and cooling demand exists, the appropriate error code is

displayed. Verify that the disconnect is closed and 230v wiring is connected to the unit.

COMPRESSOR VOLTAGE SENSING

The control board input terminals VS and L2 (See Fig. 18) are

used to detect compressor voltage status, and alert the user of

potential problems. The control continuously monitors the high

voltage on the run capacitor of the compressor motor. Voltage

should be present any time the compressor contactor is energized,

and voltage should not be present when the contactor is de-energized.

CONTACTOR SHORTED DETECTION

If there is compressor voltage sensed when there is no demand for

compressor operation, the contactor may be stuck closed or there is

a wiring error. The control will flash the appropriate fault code.

COMPRESSOR THERMAL CUTOUT

If the control senses the compressor voltage after start-up, and is

then

absent

for 10

consecutive

seconds

while

cooling

demand

exists,

the

thermal

protector

is

open.

The

control

de-energizes

the

compressor

contactor

for

15 minutes,

but

continues

to operate

the

outdoor

fan.

The

control

Status

LED will flash the appropriate

code

shown

in

Table

5.

After

15

minutes,

with

a call for

low

or

high

stage

cooling,

the

compressor

contactor

is energized.

If

the

thermal protector has not re-set, the outdoor fan is turned off. If

the call	for	cooling	continues,		the	control	will		energize		the
compressor	contactor		every	15	minutes.	If	the	thermal		protector
closes (at	the	next 15	minute	interval),		check	the	unit	will resume
operation.
If the thermal		cutout	trips for		three consecutive			cycles,		then	unit

operation is locked out for 4 hours and the appropriate fault code is

displayed.

NO 230V AT COMPRESSOR

If the compressor voltage is not sensed when the compressor

should be starting, the contactor may be stuck open or there is a

wiring error. The control will flash the appropriate fault code. Check the contactor and control box wiring.

TROUBLESHOOTING UNIT FOR PROPER SWITCHING

BETWEEN LOW & HIGH STAGES

Check the suction pressures at the service valves. Suction pressure

should

be

reduced

by

3-10%

when

switching

from

low

to

high

capacity.

NOTE:

The

liquid

pressures

are

very

similar

between

low

and

high

stage

operation,

so

liquid

pressure

should

not

be used

for

troubleshooting.

Compressor

current should increase 20-45% when switching from

low to

high

stage.

The

compressor

solenoid,

when

energized

in

high stage, should measure 24vac.

COMPRESSOR

INTERNAL

RELIEF

The

compressor

is

protected

by

an

internal pressure relief (IPR)

which

relieves

discharge

gas

into

compressor

shell

when

differential between suction and discharge pressures exceeds 550 -

625 psi. The compressor is also protected by an internal overload attached to motor windings.

TEMPERATURE THERMISTORS

Thermistors are electronic devices which sense temperature. As the

temperature increases, the resistance decreases. Thermistors are

used to sense outdoor ambient (OAT) and coil temperature (OCT).

Refer to Fig. 25 for resistance values versus temperature. See Fig. 26 for OCT location.

If the outdoor ambient or coil thermistor should fail, the HP/AC

control	will	flash the		appropriate		fault code (See Table 5).
IMPORTANT:			Coil		thermistor	is factory mounted.			Check	to
insure	thermistor		is	mounted		properly.	Outdoor	air	thermistor
(OAT)	is field		mounted		and connected.		Verify that	the	OAT	has
been	properly	installed.

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