| RETURN PLENUM SENSOR FAILURE |
The return plenum sensor is used on units with units shipped with a Return Fan. This diagnostic occurs when the return plenum pressure sensor input is out of range and one of the following is occurring:
- The return plenum pressure sensor value has risen above +3.5 IWC
- The return fan is on, the return fan speed is 100%, and the return plenum pressure sensor value has fallen below -0.75 IWC.
- The return fan is off and the return plenum pressure sensor value has fallen below -0.75 IWC.
This diagnostic can also be generated if the calibration solenoid is not functioning properly. For one second every minute, the calibration solenoid is energized to zero out the transducer. It connects the high port and the low port to outside air static reference. The controls expect to see 0.53 VDC @ 0.00 IWC during this time. Any difference from this value is used as an error amount and used to adjust the sensor reading for the next minute. If the calibration solenoid is not functioning, the controls will not receive the correct error signal and will interpret the sensor reading as zero pressure. In these cases, if power is cycled to the unit, the sensor will read correctly for one minute and then read decline to 0.00 IWC before giving the diagnostic.
Items to check include:
- wiring
- sensor
- calibration solenoid
- control module 1U52
- pneumatic tubing connections
Here are the steps for Intellipak Rooftop
Step #1 Wiring Errors
The return plenum transducer connects to the Multi-Purpose Module (MPM) at pins J3-1 (+5 VDC), J3-2 (signal from transducer), J3-3 (common). Verify that the wiring is correct, all the plugs are connected, and the voltages are present at the transducer. Refer to the unit schematics for wire numbers. (As a note, on units with Variable Frequency Drives (VFD), it is very important for the sensor raceway wiring to route high through the unit and the power wiring to route low. If this is not the case, the VFD can interfere with the operation of the sensor, and the wire routing will need to be corrected.) If the wiring checks good, continue to the next step.
Step #2 Pneumatic Tubing
Refer to above diagram for proper tubing connections for the transducer. Verify that factory installed tubing is correct and there are no air leaks. Also verify that outside air static pressure reference is properly installed. If the outside air static pressure reference is not properly installed, rainwater can enter the tubing. If this is the case, correct the installation and dry out the pneumatic tubing. The customer installed tubing should be connected as shown in the above diagram. If not installed, the sensor will see a large negative pressure when the supply fan is energized and report an error. The customer installed tubing should also be checked for kinks and obstructions. If all appears to be correct, continue to the next step.
Step #3 Sensor Checkout
-.75 to 9 IWC Transducer
Tee a manometer/magnehelic into both the high and low ports on the transducer. Verify that the voltage read at J3-2 and J3-3 on the MPM module corresponds to the pressure indicated on the manometer using the chart above. If it does not and Steps 1 and 2 have been performed, replace the transducer. If the voltage corresponds to the pressure, proceed to the next step.
Step #4 Calibration Solenoid Checkout
The calibration solenoid is energized for one second every minute. During calibration, the MPM should see 0.53 IWC @ 0.00 IWC at pins J3-2 and J3-3. The coil voltage is approximately 26VDC and is supplied from the MPM at pins J4-2 and J4-3. If a squeeze bulb is connected the Normally Closed (NC) port and pressurized, it should be relieved for one second every minute. If not, verify that the MPM is supplying the 26 VDC every minute. If not, replace the MPM. If the MPM is supplying the correct voltage but the solenoid does not function, replace the calibration solenoid. If the solenoid checks out, proceed to the next step.
Step #5 Control Module
If all the previous steps have been performed, the diagnostic remains, and no errors are found, replace the MPM module.
MPM Schematic
IntelliPak 1
IntelliPak 2