Yesterday I swapped out an alternator that was making a screaming noise (assume the bearings are going out) on my 86 535. I put in a unit from a 735i, both were rated at 14V 80A. After installation the car and cleaning the battery terminals and reconnecting the car would crank, but not start. Assuming there to be a problem with the alternator, I put the original one back in - double checking all wires. Still, crank but no spark. I checked all the fuses and all visible wiring - ok.