02 Return Air Sensor Open Circuit MP4000

Description

Return Air Sensor Open Circuit  

• When the sensor circuit resistance is higher than 1300Ω. 
• Indicates: 
• Open circuit. 
• Defective or wrong sensor. 
• Defective wiring. 
• Defective controller. 
• Check for damaged sensor wires. 

Corrective Action 

• Check sensor connections at controller. 
• The sensor is a pt1000 – 2 wire sensor, connected to the MP-4000 at connector J3 pin 3 and 4. CM-4000 upper left connector J3, 17 pin wide, pin number 1 is the right pin, seen at the backside of the controller. 
• The 2 sensor wires can be switched without affecting the measurement. 
• Disconnect the sensor, use an Ohm (Ω) measuring device, measure the electrical resistance between the two sensor wires. 
• The sensor can’t be examined without disconnecting it. 
• The electrical resistance towards chassis must be above meg ohm (MΩ) range. 
• The sensor is a pt1000 – positive temperature coefficient, which means that the electrical resistance of the sensor increases with temperature. 
• The sensor is defined to be 1000Ω@ 0°C. 
• Normal condition measuring with disconnected sensor is 960Ω@-10°C, 1000Ω@0°C,1039Ω@+10°C, 1058Ω@+15°C, 1078Ω@+20°C. 
• The valid measuring limit for this pt1000 sensor is -100°C(602Ω) +75°C(approx 1300Ω).