That worked fine, so the power supply seems good. With the original wiring harness all installed, it came up in service mode though. The service switch is still there but wasn't hooked up before with the JAMMA harness. I don't see any actual "switch" to flip on the service switch, probably broken off and apparently in the "on" position when it was broken off. I disconnected the molex connector that went to the service switch and the game came up as normal. So I guess I need a new switch for the service switch bracket.
Also, before with the JAMMA harness, only one of the two coin slots was working, the right-hand slot. With the original harness in there now, only the left-hand coin slot works. That is weird and I don't really have a theory about it yet.
When the JAMMA harness was in there and only the right slot worked, I thought maybe something was wrong with the left slot itself, its wiring or its switch, or possibly even something bad on the game board for the non-working left slot. But, since it is the left slot that is working now with the original harness, and the right one that is not; that theory is blown out of the water.
Edit: Unless; the JAMMA harness was hooked up backwards to the coin slots and both slots work fine in the coin mechanism itself [obviously] but there is only one working coin pin on the game board...
If that is the case, does anyone know how to fix that?