Code 85: Demand Limit Potentiometer (P4) Failure; Code 86: Minimum Position Economizer Potentiometer Failure; Code 87: Warm-Up Temperature Set Point; Electronic Controls Checkout - Carrier 48DK024 Controls Operation And Troubleshooting

be displayed if display button is pushed, and reset will be
terminated. The full-scale resistance of potentiometer is
10 Kohms, but when installed on the accessory board in par-
allel with the other 2 potentiometers, measured resistance
will be 3.3 Kohms. This failure will automatically reset once
potentiometer returns to normal. If a failure occurs, one of
the following is the probable cause:
DIP Switch Problem — DIP switch 2 is in the ON position
and the accessory board is not installed (accessory board is
standard on these units so it should always be on the unit).
Incorrect Potentiometer Setting — A potentiometer turned fully
clockwise or counterclockwise is outside the valid range and
will result in a failure.
Faulty Wiring — If the wiring between the potentiometer and
the processor board is incorrect, a failure will result.
Potentiometer Failure — If potentiometer is shorted or open,
a failure will occur.
CODE 85: DEMAND LIMIT POTENTIOMETER (P4) FAIL-
URE — Used only if demand limit is being used. If demand
limit is used, DIP switch 5 must be in the ON position.
Two types of demand limit are available: a field-supplied
and installed single-step control consisting of a 10 Kohm,
3-wire linear potentiometer and an accessory 2-step control
are available from Carrier. The single-step control has a single
potentiometer while 2-step control has 2 potentiometers
(mounted on the demand limit board, see Fig. 20).
For both types of demand limit, the control uses only 80%
of the total potentiometer resistance. If resistance of poten-
tiometer is less than 10% or greater than 90%, alarm light
will be energized, a diagnostic code of
when the display button is pushed, and demand limit will be
terminated. If a failure occurs, it is probably due to one of
the following:
Potentiometer Failure — If a potentiometer is shorted or open,
a failure will occur.
Incorrect Potentiometer Setting — A potentiometer turned fully
clockwise or counterclockwise will put potentiometer out of
range resulting in an error.
Faulty Wiring — If wiring between the potentiometer and
the processor board is incorrect, an error will occur.
DIP Switch 5 — If DIP switch 5 is in the ON position and
potentiometer is not installed, an error will occur.
CODE 86: MINIMUM POSITION ECONOMIZER PO-
TENTIOMETER FAILURE — If potentiometer P5 (on ac-
cessory board) setting is less than 0% or greater than 100%,
alarm light will be energized, a code of
played when display button is pushed and economizer out-
door air dampers will move to the fully closed position.
The potentiometer full-scale resistance is 10 Kohm, but
when installed in parallel with the other 2 potentiometers on
the accessory board, measured resistance will be 3.3 Kohm.
This failure will automatically reset when potentiometer
returns to normal.
If a failure occurs, one of the following is the probable
cause:
DIP Switch 3 — If this switch is in the ON position and the
accessory board is not installed (accessory board is standard
on these units, so it should always be on the unit).
Incorrect Potentiometer Setting — If potentiometer is turned
fully clockwise or counterclockwise, potentiometer will be
out of the allowable range, and an error will result.
Faulty Wiring — If wiring between the potentiometer and
the processor board is incorrect, an error will occur.
85 will be displayed
86 will be dis-
25
Potentiometer Failure — If potentiometer is shorted or open,
potentiometer will be out of range and an error will result.
CODE 87: WARM-UP TEMPERATURE SET POINT FAIL-
URE — Applicable only if morning warm-up is used. Whether
or not unit is equipped with electric resistance heaters, use
of the morning warm-up function is recommended if the unit
is shut down at night or over weekends. In this application,
cooling will remain off and the outdoor-air damper will stay
closed until heat load from the occupied space elevates return-
air temperature to the warm-up set point. If warm-up func-
tion is used, DIP switch 4 must be in the ON position. The
potentiometer (P6) is located on the accessory board. If po-
tentiometer is set at less than 0° F or more than 95 F, alarm
light will be energized, a diagnostic code of
pear on the display when display button is pushed, and con-
trol will use a default value of 40 F. If setting is between
0° F and 40 F, control will use a value of 40 F but no di-
agnostic code will be displayed; if setting is between 80 F
and 95 F, control will use a value of 80 F but no diagnostic
code will be displayed.
The potentiometer full-scale resistance is 10 Kohm, but
when wired in parallel with other potentiometers on the ac-
cessory board, measured resistance is 3.3 Kohm.
The failure will automatically reset once potentiometer re-
turns to normal. If a failure occurs, one of the following is
the probable cause:
DIP Switch 4 — If this switch is in the ON position and the
accessory board is not installed (accessory board is standard
on these units, so it should always be on the unit).
Incorrect Potentiometer Setting — If potentiometer is turned
fully clockwise or counterclockwise, potentiometer will be
out of the allowable range, resulting in an error.
Faulty Wiring — If the wiring between the potentiometer and
the processor board is incorrect, an error will occur.
Potentiometer Failure — If potentiometer is shorted or open,
potentiometer will be out of range, resulting in an error.
Electronic Controls Checkout —
help determine whether a processor board, a relay board, dis-
play set point board, accessory board, or 2-step demand limit
module is faulty.
Before checking out any board, do the following:
1. At initial start-up, enter the quick test mode. This test will
determine if all components are connected and operating
properly.
2. If system has been operating and a malfunction occurs,
check display for diagnostic codes. Use diagnostic chart
located on inner panel of access door to control box sec-
tion of unit; this chart will help determine probable cause
of failure.
These 2 steps will help determine if a component other
than a board is at fault or if the problem is external to
control circuit.
A volt-ohmmeter will be needed to troubleshoot boards. A
digital meter is preferred but a Simpson 260 or equivalent
will work.
To prevent damage to solid-state electronic components
on boards, meter probes should only be placed on ter-
minals and test points listed in following sections. Do
not short the electrical components, and use extreme care
while working on the processor board.
87
will ap-
The following will