We have quite a few 90's era 5FBCU15's. The two other things that have caused the CO issue for me that I didn't see mentioned here is bad main drive motor brushes or problems with the commutator, and the small DRD diode that sits on the connector bars of the TM (main transistors). This DRD looks like a small jumper with a diode that has clear shrink tube around it, both ends have small loop connectors, but one is red. Other than those two, it is usually the MS contactor sticking on or failing, bad TM transistors (I never get the readings the book says so I just look for one that reads different than the other two), and a couple circuit boards.