M9000 open door M error

[Slots4me]

I was getting a open door M  error on my Bally Alpha 1. That would not clear no matter what. Used CF to clear, unplugged main door switch, booted with no batteries would not clear.
After tracing wires and using volt meter
Found out grey  wire is dead..
Tracing the wire it goes up to top box switch which works (no open door)
I jumped "M" yellow main door positive wire to the validator closed switch wirh a female plug to get a closed circuit. Discount when I need open door..
What would cause the grey wire to short if working on other switches? 
If grey - wire is working in top box switch would their be a short in harness going to M switch?
Or I'm I just over thinking it...