'91 trooper cranks, won't start

[m0loch]

So, sometimes it won't start, sometimes it will. The times it won't the 'check engine light' doesn't come on. Sometimes if I leave the key on for a period of time, the light will eventually come on and it fires right up once I turn the key to start. The only flash code that the ECM gives is 12 which seems to be un-documented in this particular model.

[m0loch]

Ok, I think I fixed it.

With the help of this thread and studying schematics in a Chilton's manual (what happened to these things, they used to be the bible of auto repair, this one sucks hard) I determined the problem to be the main relay. The fusible link in the aforementioned thread provides power to the main relay, which in turn provides power to the ECM and fuel pump relay. Apparently the relay was intermittently malfunctioning causing my symptoms.