MUA-SVX01A-EN • Direct-Fired Make-up Air
83
Trouble Shooting
Table 17.
Trouble shooting guide for Modulating Discharge Temperature (MDT)
Controls
Symptom
Probable Cause
Field Test
Solution
1. No gas flow.
Modulating valve
improperly installed.
Arrow on side of valve
should point in direction
of gas flow.
a. Install
properly.
2. Continuous low fire
(electronics problem).
Short circuit or no
voltage to the amplifier.
Check for 24V AC at
amplifier terminals 7 & 8.
a. Prove the
power source.
Open circuit in remote
temperature selector
circuit wiring.
Inspect for loose or
broken wires between
amplifier terminals 1 & 2
and remote temperature
selector terminals 1 & 3.
a. Tighten
connections or
replace wiring.
Short circuit in
discharge air sensor
circuit or wiring.
Connect test resistor as
described on page 37.
a. If
modulating
voltages are
obtained,
check
discharge air
sensor circuit
for short.
Replace if
necessary.
Faulty amplifier.
Check all items listed in
Symptom 2.
3. Continuous low fire
(electronics ok).
Short circuit or open
circuit in modulator coil.
Measure resistance across
modulator terminals with
connecting wires
detached.
a. Replace
modulator
head if not
approximately
45-55 ohms
for M611 valve
and 60-80
ohms for
MR212 valve.
Plunger missing,
jammed or improperly
installed.
Inspect. Plunger should
be installed to operate
freely in solenoid sleeve.
a. Clean or
replace
plunger if
necessary.
4. Continuous high fire
(electronics problem).
Short circuit in remote
temperature selector
circuit or wiring.
Inspect for shorts at or
between amplifier
terminals 1 & 2 or remote
temperature selector
terminals 1 & 3.
a. Correct
wiring if shorts
exist.
Open circuit in
discharge or inlet air
sensor circuit or wiring.
Check discharge or inlet
air sensor for open
internal circuit. Connect
test resistor as described
on page 66(MDT
Controls). Follow
procedure outlined.
a. If
modulating
voltages are
obtained,
check for open
circuits.
Replace
sensor(s).
Jumper not connected
across amplifier
terminals 2 & 3.
Inspect
a. Correct the
wiring.
