This applies to an 89 Aero 3.0l A/T...but may apply to other models as well. I had a starter stay engaged on my dads van...eventually engine stopped either from a seized starter or low battery voltage. Once I got it home by pulling it back. Parts wise, I changed the starter, solenoid and ign. switch all at one time. Before I fired up the van, I decided to check for voltage on the crank wire at the solenoid. With wire unplugged, key on..had almost 8 volts which I assumed was not good. With the key in start postition, voltage rised to 12.3..which is what you want, but when I released key still had the 8 volts on the wire. So, I start looking for short to power causing this residual voltage to stay on the crank wire with the key in the run position. When I unplugged the neutral switch voltage went away, so I untaped the harness to find my problem. Found no wires melted together...hmmmm? Plugged the neutral back in and went elsewhere. Eventually, I unplugged the ECM and voltage went away again. I ran continuity tests throuh the ECM connector and could not find any continuity from CKT 30 to any other wires..power or ground...hmmm. My buddy had Taurus ECM, hooked it up, with key on I had only 5 volts through my crank circuit wire, not 8 like the van,s ECM...some improvement I thought...but couldn't be 2 bad ECM,s on that circuit? Keep in mind I left the small wire off the solenoid and key on through this whole troubleshoot and here is the catch....when you plug the wire into the solenoid, the residual voltage is absorbed apperently by the diode in the solenoid. I wasted a bunch of time searching for no reason. I just went by what I learned from older Ford vehicles and GM's....
Thanks,
Phillip