"Old" pinball tech here.

I have a Shadow that has a partial column (vertical) failure of all switches after the plumb bob tilt, basically the last four switches on the column (all return and outlanes, switches 15-18 ).

All remaining switches on the machine work, including optos, normally closed switches, and no “phantom” switches are occuring.
Should be a straight forward fix.
I have not found a wiring problem, and continuity has been tested downstream successfully up to the point of break (Switch 15, Right Outlane)

Is it possible that the cabinet interconnect board could be causing issues after running back from the plumb bob?
Alternatively is this an IC U20 issue? (unlikely, but I guess it *could* happen)
This is not a diode failure, if there is no continuity problems based on this particularly common issue.

Its remains *unlikely* that this is a CPU issue (all other switches work in column and all remaining switches in game), but it is still *possible*.

Both J connectors checked no issues (this was one of the first things I did).
No cold solder joints, proper continuity.
There was battery damage on CPU board (on holder ONLY), but after inspecting the traces with a jeweler glass, I could not find any areas that went beyond the battery holder itself. It was removed years ago onto to an external battery pack and board was cleaned.

I keep suspecting U20, but I will find it, any other suggestions are welcome.
This is one of those rodeos, I have ridden before, but I have plenty of other work to do as I overhaul my machines from being in extended storage.

I DO have continuity from the CPU to switch 15 (first break in the chain), that is why this is odd.
It is the green-brown column wire that is the culprit.
Common sense tells me in this case, there IS a wiring problem.

Thanks for reminder about the switch wiring though, because I have seen this on other games were the switches were reversed at the factory, hence it is once of the four switches, unless there is a IC chip failure.

Yes.
Still testing, I will figure out were the problem is, the problem is NOT INTERMITTENT, so its not a loose wire.
I just keep checking off my list, until I find it, board issues are always last (ICs not wires or connectors).

I have FULL continuity and operation of EVERY OTHER switch in the game including optos.
Only Switches 15-18 are non-operational, and the continuity to EACH switch is good to go to the CPU board.

I already pulled and reseated every switch connector properly, including inspecting the individual connector pins, along with my usual careful attention to not being brash and bending any pins which is easy to do on the CPU board or offset the connector itself.
I physically bypassed switches 15-18 by properly and manually testing the wires, as I don't test through diodes which would not work anyway.

I do appreciate the ideas and help, as its rare for me not to find a problem quickly.
I just want to eliminate all potential physical and basic electrical issues before I start testing the WPC-95 CPU Security board.
If it is wire resistance, it has to be a fluke, because that is uncommon right now, as this is not an early SS machine, nor has this machine been in an arcade for nearly 15 years. I replaced many of the connectors over 10 years ago.

I try not to overlook the "simple things".
When I get around to more testing, I will post here.
I have no doubt I will find it, its part of the "detective story" of pinball, and addition of experience as it always is.

I guess I am not used to "tugging at the wires" with an IDC connector, as that is a big "NO NO", and I don't do it.
It just seems unlikely as I swear I replaced the connectors on the switch matrix 10+ years ago.
If something is loose, it will reveal itself.

After magnifying glass inspection, I believe that is what has occurred due to a combination of age, although I am using a remote battery pack, and I still cannot see any battery acid anywhere near, inside, or outside the connector.
When I got the game, I pulled the battery board OFF, because it did have battery damage (not severe, but sometimes even the smallest amount is enough to cause problems).

At this point, it can only be the pins themselves on the IDC (with potential leakage INSIDE the connector I cannot see which now finally killed the wires), or the IC chip itself. ALL other basic potential causes have been eliminated.

The support is appreciated, I plan to completely replace the connector this weekend, and see if this problem is finally over.
I still always prefer to start simple and work up the chain.

After replacing the J212 connector with an upgraded molex one, all switch issues were resolved. I extracted all pins from the old connector. There was surface corrosion on the pins could not be seen until pins were removed from inside the connector. Personally, I am happy this time I did not have to replace LM339 IC chips with sockets.