So here it is:
MCE, on their PHC 1000, used triacs to complete the path to ground, and this is how their "outputs" work.
The Door Close Front (DCF) triac brings in the Door Open Relay. If this triac is shorted, the doors will open normally, but then when the dwell timer expires, and the Door Close Relay comes in, the doors begin to close noramlly, but then as soon as the Door Open Limit changes states, the shorted DCF triac then brings in the Door Open Relay. Then the doors snap back open, they time out again, then try to close, and the process keeps repeating. The effect is that the doors just keep whacking off near full open. Changing the shorted DCF triac makes the problem go away.
Hope this was at least slightly intrigiung!
