– 17 –
TROUBLESHOOTING GUIDE
Modulating Discharge Temperature (MDT or VDT EXPERT) Controls
continued
Portions reproduced with permission from Maxitrol
®
Company
Symptom
Possible Cause
Field Test
Remedy
1. Inlet pressure too low.
2. Incorrect outlet pressure
adjustment of Pressure
Regulator.
1.
Unit Enable Potentiometer
(MP-15) not set correctly.
2.
Discharge Air
Temperature Sensor
(TS-03) not reading
correctly.
3. The Heating Discharge
Air Setpoint on
VDT-EXPERT Control
System is not set
correctly.
1. Short circuit in
modulator coil.
2. Short circuit between
ampli
fi
er and modulator
valve.
1. Read pressure at inlet to
modulating valve using
a manometer with unit
operating at full
fi
re.
Pressure should be equal
to the sum of outlet pressure
setting plus pressure drop
of the valve.
2. Read manifold pressure
using manometer and
compare with the pressure
stated on the speci
fi
cation
plate.
1. Check setpoint on Unit
Enable Potentiometer
(MP-15)
2. Check Discharge Air
Temperature Sensor (TS-03)
and wiring. See Thermistor
curve.
3. Check setpoint with BACview
or PC.
1. Measure resistance across
modulator terminals with red
lead wires disconnected.
2. Inspect wiring.
1. Increase inlet pressure if
possible.
2. See valve adjustments in
Section X.
1. Change to correct setpoint
setting.
2. If readings are not correct,
replace Discharge Air
Temperature Sensor (TS-03)
or repair wiring.
3. Change to correct setpoint.
See Digital Control System
User Manual.
1. Replace modulator head if
less than 40 ohms.
2. Correct wiring if short is
found.
G. Incorrect
Maximum Fire.
H. Incorrect
Discharge Air
Temperature
I. Burned
out
Transformer.
(Refer to Digital Control System User Manual for additional troubleshooting suggestions)
