Johnson Controls AHU control signal problem

[dave1234]

I take care of a building with 2 AHUs serving VAVs each controled via Johnson Controls AS-AHU103-0 (at least thats what it says on the board) it also says AHU103 on the outside enclosure. Its a Metasys system. Ok both AHUs have vortex vanes for static regulation. The control board AOs have been converted from 4-20ma signal to resitive via 500ohm resistor. Both resistors are good and test at the correct value. At a 0% cmd signal the lowest the signal will go is around 4vdc- thats true for both AHUs. With a 100% cmd the dcv signal ranges to the full 10vdc? Its like its putting out a 4-10vdc put it should be 2-10vdc as thats what the resitors for. The actuators for the vanes work off a 2-10 signal, so they never close all the way and overpressurize the ducting when the majority of the boxs are at min. Any thoughts?

[fxb80]

Disconnect the cable at the controller and put just the resistor across the output and see if its 2V at 0%.

[Cagey57]

It sounds like the "Output Range" for the AO's, which are only 0-20 mA hence the 500 ohm resistor, are not 4-20 which equals 2-10VDC. You likely need to access the controller and check/correct that low end of the output range. It is most likely 8 (mA) instead of 4 mA.

As fxb80 said disconnect the output wire and check the voltage with only the resistor in place .