As far as I can remember on my old A80, the central locking pump was quite a simple unit. Not like the A4 units that look after the alarm etc...
The unit operates by monitoring the voltage on one of the cables, when you operate the key lock it sends either a +12 Volt signal or Ground signal to lock or unlock.
Check the pump and cables for signs of damage, I had problems with mine where the rear window seal had gone and water was finding its way into the pump connections causing strange problems. The pump should be located in the right hand side of the boot.
It could also be a problem on the microswitch in the front door locks.
Looks like you will have to do some investigation.
Regards,
Jim.
