Table 17 — Terminal Strip J5 Connector Resistance
Reading
QUICK TEST
STEP NO.
J5 PIN NUMBERS
12345
1. to 2.5. ````0
2.6 00``0
2.7. 0 ` 0 ` 0
2.8. 0 ``00
2.9. to 3.3. 0 ```0
`—Infinity
Step 2 — Check the display LEDs.
1. Enter quick test mode.
88
2. If is not displayed, replace display board.
Step 3 — Check set point potentiometer — Advance the dis-
play to quick test step 1.9. to determine if this potentiometer
is set and connected properly.
Step 4 — Check display switch — Press switch. If switch
does not click, it is faulty and the display will be energized
continuously. The switch is an integral part of display board
and cannot be replaced separately.
ACCESSORYBOARD CHECKOUT — The accessory board
can be completely checked using quick test steps 2.0., 2.2.,
and 2.3. It can also be checked out as follows:
1. Remove the accessory board connector from the proces-
sor board and connect an ohmmeter to terminals 3 and 4
on the connector. Numbers are marked on the connector.
See Fig. 19.
2. Set the meter to 10,000 ohms. The resistance value ob-
tained should be 3,333 ohms. Adjust the potentiometers
and the resistance value should not change.
3. Connect the ohmmeter to terminals 3 and 6. As the reset
limit potentiometer is turned clockwise, resistance should
increase from 0 to approximately 3,400 ohms.
4. Connect the ohmmeter to terminals 3 and 5. As the econo-
mizer minimum position potentiometer is turned clock-
wise, resistance should increase from 0 to approximately
3,400 ohms.
5. Connect the ohmmeter to terminals 3 and 2.As the warm-up
set point potentiometer is turned clockwise, resistance should
increase from 0 to approximately 3,400 ohms.
If any of the Steps 1 through 5 result in any other ohm
reading, replace the board; it cannot be serviced.
TWO-STEP DEMAND LIMIT CONTROL MODULE
(DLCM) TROUBLESHOOTING — If a problem is
suspected in the DLCM board, use the following test
procedure:
The board can only be checked when it is connected to
the processor and the processor is energized so that the DLCM
is supplied with 5 vdc power. The terminals referenced are
shown in Fig. 20. Potentiometers P1 and P2 refer to the DLCM
potentiometers.
IMPORTANT: Be careful to avoid damaging the con-
nector or the processor board when taking the voltage
readings.
Test under the following conditions:
• No power to IN1 or IN2
Terminal 1 to 2 should read 4.5 vdc ±0.1 v
Terminal 2 to 3 should read 5.0 vdc ±0.1 v
• Power to IN2 or to both IN1 and IN2, and P2 set at 24%
Terminal 1 to 2 should read 1.5 vdc ± 0.1 v
NOTE: Voltage should vary between 0.5 vdc and 2.5 vdc
as the setting of P2 is varied between 0% and 49%.
Terminal 2 to 3 should read 5.0 vdc ± 0.1 v
• Power to IN1 only and P1 set at 50%
Terminal 1 to 2 should read 2.5 vdc ± 0.1 v
Terminal 2 to 3 should read 5.0 vdc ± 0.1 v
NOTE: Voltage should vary between 0.5 vdc and 2.5 vdc
as the setting of P2 is varied between 50% and 100%.
NOTE: If the voltages listed in these 3 tests are not obtained
during testing, the DLCM board must be replaced.
Thermistor Troubleshooting — The VAV control sys-
tem uses thermistors to measure temperatures of the enter-
ing and supply air, as well as the saturated condensing tem-
peratures of the refrigerant circuits. The resistance versus
temperature and electrical characteristics for all thermistors
in the system are identical. To obtain an accurate reading, a
high-impedance meter (such as a digital meter) must be used.
Thermistors in the VAV control system have a 5 vdc sig-
nal applied across them any time the unit control circuit
is energized. The voltage drop across the thermistor is di-
rectly proportional to the temperature and resistance of the
thermistor.
To determine temperatures at the various thermistor lo-
cations, disconnect the thermistor from the processor board
and measure the resistance across the appropriate thermistor
using a high-quality digital ohmmeter. Use the resistance read-
ing to determine the thermistor temperature.
The microprocessor has been programmed to check the
operation of the thermistors. If the measured temperature is
outside of the range of −24 to 225 F or 98,010 to 282 ohms,
then it will be treated as a sensor failure and a diagnostic
code will be displayed. See Table 11 for sensor temperatures
versus resistance drop. It is also possible to check the op-
eration of the thermistors using the quick test routine.
If a thermistor has failed or the wire is damaged, replace
the complete assembly. Do not attempt to spice the wires or
repair the assembly.
Control Wiring — For assistance in troubleshooting unit
control wiring, refer to Fig. 36 for typical unit control wir-
ing, and to label diagram attached to unit.
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