– 16 –
1. Install properly.
1. Prove the power source.
2. Correct the wiring.
3. If item B1 checks out and
modulating voltages are
still not obtained, ampli
fi
er
(RE-26) may be assumed
faulty. Replace.
4. Contact Factory.
1. Replace modulator head
if not approximately 45-55
ohms for M611 Valve and
60-80 ohms for MR212
Valve.
2. Clean or replace plunger if
necessary.
1. Adjust to proper minimum
fi
re.
2. If reading is greater than 1.5"
negative pressure, check
for clogged
fi
lters or other
inlet air restrictions. Consult
factory for other solutions.
1. Replace Outside Air
Temperature Sensor (TS-01)
or Discharge Air Temperature
Sensor (TS-03
1. Clean seat. Clean valve or
replace if necessary.
2. Clean, or if necessary,
replace plunger.
TROUBLESHOOTING GUIDE
Modulating Discharge Temperature (MDT or VDT EXPERT) Controls
Portions reproduced with permission from Maxitrol
®
Company
Symptom
Possible Cause
Field Test
Remedy
A. No Gas Flow
with DO-2
on UC-01
energized.
B. Continuous Low
Fire (Electronics
Problem).
C. Continuous Low
Fire (Electronics
OK).
D. Incorrect
Minimum Fire
Erratic or
Pulsating Flame.
E. Continuous High
Fire (Electronics
Problem).
F. Continuous High
Fire (Electronics
OK).
1. Arrow on side of valve should
point in direction of gas
fl
ow.
1. Check for 24V AC at
ampli
fi
er (RE-26) terminals
7 & 8.
2. Inspect.
3. Check item B1. Check
for 2-10 VDC input signal
from (UC-01) at terminals
1 & 2. Check for 0-20 VDC
output signal to gas valve at
terminals 5 & 6.
4. Check DC Voltage on AO-1.
1. Measure resistance across
modulator terminals with
connecting wires detached.
2. Inspect. Plunger should be
installed to operate freely in
solenoid sleeve.
1. See valve adjustments in
Section X.
2. Close main gas supply and
measure manifold pressure
with blower operating.
Reading should be less than
1.5" w.c. negative pressure.
1. Check Outside Air
Temperature Sensor
(TS-01) or Discharge Air
Temperature Sensor (TS-03)
for open internal circuit. See
Thermistor curve.
1. Remove bottom plate and
inspect valve and seat.
2. Inspect. Plunger should
be smooth and clean and
operate freely in solenoid
sleeve.
(Refer to Digital Control System User Manual for additional troubleshooting suggestions)
1. Modulating valve
improperly installed.
1. Short circuit or no
voltage to the ampli
fi
er
(RE-26).
2. Jumper not connected
across ampli
fi
er (RE-26)
terminals 3 & 4.
3. Faulty ampli
fi
er (RE-26).
4. No output voltage on
UC-01.
1. Short circuit or open
circuit in Modulator Coil.
2. Plunger missing,
jammed or improperly
installed.
1. Incorrect by-pass
metering valve
adjustment.
2. Excessive negative
burner pressure.
1. Open circuit in Outside
Air Temperature Sensor
(TS-01) or Discharge
Air Temperature Sensor
(TS-03) circuit or wiring.
1. Foreign object holding
valve open.
2. Plunger jammed.
