Home Made obd scanner ??

[iroc12valve]

on my 1991 crew cab (square body like a suburban) it is running like a diesel and the check engine light is on, I remember someone telling me that on safari's up to 1995 you could use a short piece of wire to jump the connectors where you would normally plug in a scan tool and then the engine light would flash xx many times slowly and then xx many times quickly ( like morse code almost) which would give you a code # (for example 2 long then 3 short for # 23)
I assume this works on the trucks too but i dont remember the exact spots to "jump"


Also if any one has a link to how to decipher the codes E.G. code 23 bad ign coil, that would be much appreciated