Hussmann proto-aire Manuel De Montage

Protocol™ Installation and 

Service Manual 

meter to the two outside pins.  The voltage present at these two pins should 
be between 20 and 27 VAC.  If the secondary voltage is within limits 
replace the control board.  If the secondary voltage is out of limits, 
investigate supply voltage to the control transformer. 

 
 
 
 
Problem D:  Temperature Sensor is not reading properly. 
 
The temperature sensor used on Protocol™ is typically used to sense discharge air 
temperatures at the evaporator load.  The sensor contains a NTC (Negative Temperature 
Coefficient) thermistor, which will increase resistance as temperature falls and decrease 
resistance as temperature rises.   
 
Trouble Shooting Alarms 
 
The following section gives information on diagnosing specific alarms.  The first step in 
analyzing alarms is to determine when the alarm occurred.  The alarms will appear in the 
controller.  The cause, time and date of the alarm will be shown. 
 
There are two types of alarms:  switchback and non-switchback.  A switchback alarm is 
normally the result of some external failure as seen or interpreted by the electronic control.  
When a switchback alarm occurs, the electronic control removes itself from control of the 
compressors.  Operation and cycling of the compressors will be controlled by a low 
pressure mechanical switch located inside the Protocol™ system, which will cycle one half 
of the compressors.  The indication of alarm will be dependent upon which alarm device 
has been installed:  auto dialer, in store alarm or computer system.  Note that under a 
switchback alarm no defrosts will occur.  There are three types of switchback alarms:  (1) 
High Suction Pressure, (2) Low Suction Pressure or (3) All compressors off for more than 
60 minutes. 
 
All other types of alarms fall under the “non-switchback” category, such as high discharge 
temperatures.  The control will continue to cycle compressors and manage defrosts under 
this category of alarms.   
 
Alarm:  High Suction Pressure 
 
This alarm is one of the three “switchback” alarms.  The time delay for this condition is 30 
minutes.  High suction pressures are ignore during defrosts. 
 
Possible Causes 
One or more compressors are operational. 
High alarm limit is not set properly. 
 
Step 1 

Enter the Alarm Menu.  Observe the time and date of the alarm. You will be 
prompted as to whether or not you want to clear the current alarm.   

 

 

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