mercathode white wire on control backside

[sbb2112]

Would anyone have an idea what this wire is? There was a mercathode unit (blue) that I am replacing but it did not have this wire. The is no reference as to what it does in the instructions that came with the new black unit. I have looked and don't find much at all. this is a drive mounted style, not the pucks

[sbb2112]

I thought it might be for a remote status light but found that it is for diagnostics. It is described in a service bulletin. there are different versions of the unit and different tests.