If the pump still runs, but doesnt create vacuum/pressure, it is most probable that the graphite impeller has shattered.
This normally happens if there is a slight leak in one of the pipes, either at the front doors, the connector behind the drivers seat, or at the pump.
The pump has to work overtime to attain the pressure/vacuum, and eventually becomes damaged.
You can swap the pump for any with a similar part number and a suffix greater than 'A' (which doesnt have RF interface).
Depending on where the pump is from you may or may not have to reset the central locking 'softcode' number.
I am using a 'G' suffix pump from an A3 in my A4, and it did change the code number after fitting the replacement pump.
From the Rosstech site:
35-Central Locks: -- KWP-1281
VAG-COM Works fine (note: pre-'97 A4's may not have this module).
Recode locking options
[Select]
[35 - Cent. Locks]
[Recode - 07]
Enter the coding below
[Do It!]
Write down the existing coding for future reference.
A4 pre-1998 with Delta radio and anti-theft system
Standard Coding: 03468
A4 pre-1998 with Delta radio, anti-theft system and keyless entry
Standard Coding: 12172
A4 1998+ with Concert/Symphony radio and anti-theft system
Standard Coding: 03340
S4, A4 1998+ with Concert/Symphony radio, anti-theft system and keyless entry
Standard Coding: 16140
Add the following options to your coding number:
+64 Windows up, sunroof close, and windows down with remote.
-04 One touch unlock of all 4 doors and trunk
-1024 Remove horn honk when arming
+32 Auto lock/unlock of doors (Doors lock at 15 mph and unlock when key is removed)
So, if you want to have a 1999 A4 with Concert radio and anti-theft system and keyless entry, and you want to add window control with the remote along with auto-lock, then:
add 03340+64+32= 03436
You can also buy a cheap audi 80 etc pump on ebay and swap over then actual motor/pump assembly into your existing casing, they are virtually identical.
If your pump was not working for lmore than 4 or 5 seconds before the problem occured, it could also be a tube disconnected inside the pump.
Although physically similar the twin outlet '8DO' and '8LO' types have a slightly different signal/pinout, and are not compatible with each other.