My '85 535i will not start

details:i parked it and took the battery out to go purchase an '84 bmw528e.i went and got the 528 and returned to where i parked the 535i and reinstalled the battery to drive it home and it would not start.the motor turned over,but sounded like the battery was drained.with little time,i just put the batt back in the 528e,which had no problem with the same battery and drove home.

i came back the next day to further investigate the problem.i found that the battery cranked the motor slow and when hooked up to jumper cables it turned faster but didnt sound like it was firing.added ether and nada.found it was getting fuel but no fire to the plugs.the coil is getting power and it works on the other bmw.i started tracing wires from the + batt post and found three red wires going into the main relay that had no continuity.traced these wires into the main engine harness.it's like i have a dead short in my ignition circuit but the weird thing is,i dont know why.the car had been driving fine for over a year and gave no indication of a problem bf this happened.i would like to get help with figuring out the electrical system and diagnosing this problem.do i have a bad computer?a bad crank sensor?a defective distributor?and how does something like that just all of a sudden quit with no warning?ive looked at all the wires connected to the battery cable to see if i had damaged or broke one and found nothing-HELP!