I have a 1994 90CS Quattro, with a V6 & a 5-speed manual transmission. It starts fine before the doors are locked with the key. The central locking system works fine, from the inside of the vehicle. But once the doors are locked with the key from the driver's door, when I unlock it with the key on the driver's door, & insert the key in the ignition switch & turn it to start it, the dash warning lights come on, like they're supposed to, but then starter will not crank the engine. Also, I notice that the central locking system will not unlock the other doors after being locked with the key. Even if I try to unlock it from the inside, the central locking system will not unlock the other doors. If I manually unlock a door, it will still lock it again. But the locks are not the main issue. I just mention that because I believe that it's related to the starter issue, & can help with diagnosis.
Having said all of that, if I remove & reinsert the central locking / Anti-theft fuse from the main fuse/relay box, under the hood, then it seems to reset the system, & everything works again, including the starter, until the next time the doors are locked from the driver's door with the key. Then, I have to remove the fuse again to reset.
So I have already checked the starter cutoff relay (#8 in the main relay box), by swapping it with another like relay from another system. I've also replaced the central locking switch on the lock cylinder of the driver's door with a brand new unit from Audi. But neither had any effect. I am pulling my hair out, trying to figure this out. Does anybody have any suggestions about what else it could be? Is there anything else that could affect the anti-theft memory. Maybe an anti-theft module that isn't sending the reactivation signal, when the doors are unlocked. I really thought that the lock switch on the lock cylinder was the ticket. The sypmtoms were consistent with that. But I guess not. Any advice any1 can offer would be greatly appreciated. Thank you.