Why can't we get a signal out of our summing junction box?

The wiring, installation, and output signals expected for the load cell system should be the same regardless of the type of summing box you use.  If you have the 215IT version, your weight controller needs to be IT capable as well.  If the C2® voltage is missing to the IT summing junction board,  the relays will open. This condition will show only the loop back resistors and could result in no change in signal voltages with a change in weight.  If you are using the 215IT version you MUST connect the C2+/- wires from your Hardy controller.  This is a dual usage for C2 calibration and test control communications to and from the IT summing card.

The trim pots in the summing box can have some effect on the excitation voltage, but the effect is very minor. They could prevent you from receiving a signal from the load cells if they are damaged or missing. The trim pots will be explained later in this tech note.

To check out the load cells, measure the following voltages on the load cell connector of your Hardy weight controller or in the summing junction box with everything connected for normal operation: 

-Exc to +Exc should be +5 vdc
-Sen to +Sen should be +5 vdc
-C2  to +C2  should be +4.8vdc
-Sig to +Sig should be from 0 to +15 mV. 
The mV readings on the Signal lines (-Sig to +Sig) should change with a change in the weight on the scale and with a positive voltage and in the proper direction; added weight, higher Mv, lower weight, lower mV. Check these signals with all the load cells wired and installed.  At that point the load points should all be in parallel.

If the Excitation voltage is good and the other voltages are incorrect or missing, it normally indicates a problem with the load cell wiring or the C2® wires not being installed.

Checking the individual load cell signals in the summing box.  You need to disconnect the +Sig and -Sig wires from each load cell and read the signals individually for the response from that load cell.  Depending on the summing junction box you may need to remove the wires or  unplug the connector.

If the Excitation voltage is missing, disconnect the load cell connector at the module and check for Excitation voltage out of the module (no load cells). If it is still missing, then there is a problem with the module. If the excitation is present without the load cells and not present with the load cells, look for a short to ground for any load cell wire.  If all the voltages are correct at the module and you are not getting weight readings with the load cells installed, then there may be a problem with the module.  Welding can open or short all the load cells on a vessel.

Info on the trim pots:
Trim pots are used to balance the output of multiple load cells. Trim pots should only be used on scales system where the load is not self leveling or can be placed in multiple locations. A platform scale would be a good example. On a platform scale the weight distribution to different load cells depending where you placed the barrel. Using load cells that are not matched and balanced can cause a different weight reading from point to point.  With trim pots the load cell outputs can be trimmed to have the same weight output as any other position.  This position difference is what you are balancing with trim pots. You should never use trim pots to compensate for improper loading due to a shifting load cell. This is a structure problem and shimming or load reorganization maybe required. Trim pots should not be used on scales that have self leveling/centering loads.  Try and avoid trim pots as much as possible. They are a source of noise and mistreatment. Adjusting t