McQuay MHC Series, Installation & Maintenance Data, Page: 35 / 36

IM 985-1 / Page 35 of 36
Troubleshooting the MicroTech III
Unit Controller
General Use and Information
The­Microtech­III­unit­controller­is­provided­with­two­
drive­terminals,­R(24VAC)­and­C(0­VAC)­that­can­be­
used­by­the­end­user­to­drive­the­thermostat­inputs­(G,­
Y1,­Y2,­W1,­and­W2)­and­control­inputs­(U,­E,­and­
O) .­Any­combination­of­a­single­board­drive­terminal­
(R­or­C)­may­be­used­to­operate­the­MicroTech­III­unit­
controller’s­control­or­thermostat­inputs .­However,­only­
one­drive­terminal­(R­or­C)­can­be­connected­to­any­
individual­input­terminal­or­damage­may­result .­Some­
control­inputs­are­not­accessible­to­the­end­user­(for­
example,­HP,­LP,­SLTS,­and­COF) .­
Typically­the­Microtech­III­unit­controller’s­R­(24VAC)­
terminal­is­used­to­drive­the­board’s­thermostat­inputs­
and­control­inputs­by­connecting­it­to­the­R­terminal­
of­an­industry­standard­thermostat .­The­control­outputs­
of­the­standard­thermostat­are­then­connected­to­the­
Microtech­III­unit­controller­thermostat­inputs­and­
control­inputs­as­needed .­Any­remaining­board­input(s)­
may­be­operated­by­additional­thermostat­outputs­or­
remote­relays­(dry­contacts­only) .
All­Microtech­III­unit­controller­inputs­must­be­operated­
by­dry­contacts­powered­by­the­control­board’s­power­
terminals .­No­solid­state­devices­(Triacs)­may­be­used­
to­operate­the­Microtech­III­unit­controller­inputs .­
No­outside­power­source­may­be­used­to­operate­the­
Microtech­III­unit­controller­inputs .
To avoid electrical shock, personal injury or death, be
sure that field wiring complies with local and national fire,
safety, and electrical codes, and voltage to the system is
within the limits shown in the job-specific drawings and
unit electrical data plate(s).
Power supply to unit must be disconnected when
making field connections. To avoid electrical shock, per-
sonal injury or death, be sure to rigorously adhere to field
wiring procedures regarding proper lockout and tagout of
components.
DANGER
Figure 29. MicroTech III Unit Controller LED Status and Faults
Troubleshooting Reference
Flash Yellow LED
Read Outputs
Check Timers
Brownout
High
Pressure
R - W 1
R -Y 1
Flash Green LED
Stop Compressor
No
Yes
No
No
No
No
No
No
No
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Low
Pressure
Low Suct
Temp Sensor
Low Suct
Temp
Room Temp
Sensor Failure
Condensate
Overflow
Stop Compressor
Flash Red LED
Stop Compressor
Flash Yellow LED
Flash Green LED
Solid Red LED
Stop Compressor
Heating Mode
Run in Cooling
Mode for 1 Min.
No
Yes
Cooling Mode
Turn on
Yellow LED
Stop Compressor
Reversing
Valve On
Time Delay
Start
Compressor
30 Second
Time Delay
Request for
Water Flow