Introduction
The VW injection pump is an electronicallly controlled bosch VE distributor pump. The basic design was used in all 1996-2003 North American VW diesels. 2004 and later VW TDIs used the pumpe duse injection system which had no injection pump because the fuel injectors acted like injection pumps. See for more details on distributor pump, pumpe duse, and common rail systems.
Minor changes are mounting and electrical connector changes. The biggest changes are between the 11mm automatic and 10mm manual transmission pumps. The 11mm pump moves more fuel. All top cover seals and fuel temperature sensors are interchangeable and easily replaced. The quantity adjusters are also interchangeable in all models but the electrical connectors are different in the 1Z/AHU engine for 1996-1999 engines and ALH 1998-2003 engines. The cold start injector and fuel shut off solenoid are also user serviceable. All other seals and internal parts are harder to service so professional rebuilding and maintenance is recommended.
Below is a rough translation description of a rebuild of a renault 1.6L diesel injection pump. It is similar to a VW injection pump but has slightly different internals and is not electronically controlled. I can't read French, if you can, the original post is from this French site:
Here is a picture of the pump disassembled to give an idea of how many parts are inside.

First, clean everything thoroughly.

Start with one of the fuel output hard lines. Below is shown the various parts from one of the hard line: the valve, valve end, and a new joint and threaded connection.



Assembled:



Start by reassembling the stator of the transfer pump:
(pay attention to the position of the holes relative to the holes at the bottom of the pump housing, circled in green)


then the rotor and its pallets:

first place in the rotor:

then add the pallets using a grip with needle nose pliers:


then the lid of the pump:

in position with its screws

Now the main shaft:

For assembly, note the two keys and the assembly grease.


then the disc spacer

and finally the key:

Place it in the injection pump

Assembled:


Now you need to assemble the door rollers:

Places the roller in, paying attention to the position of the hole

places the assembly in the pump housing:


Note the position of the roller: it is sometimes in, sometimes out.:



The door roller, when advanced, has to be able to fit in here:

Note that the axis is pointing up:

The piston in place:

Pay attention to its position! Note the hollow part of the piston which receives the spring and it's relation to the active hole of evacuation to the transfer pump:

Again, check the axis in its place.

Lock it with the pin

And lock the pin with its clip:


Use a new o-ring here:


On the other side, the spring and other parts.


Then finally the electric "surcalor". Some models may have this part or a stopper.

Then this part which finishes this section:


Then add the rollers. They are shown here on their support separated from the pump: pay attention to the relation of the disc which is bent to adapt to the supports:







Below is a rough translation description of a rebuild of a renault 1.6L diesel injection pump. I can't read French, if you can,

Add the center spring


Then the disc cover with cams goes over it all.



To assemble this part, the feed pin of the piston (green reference mark) must be aligned with the key.

The cams (bumps) of this disc while passing on the rollers is what moves the piston and compresses the fuel and the injection. The original author had swapped the disc from another model car with a slightly different cam profile.

now, let us move to the piston distributor:

then the drawer of regulation:

This ring, directly actuated by the regulator, will more or less, cover the exhaust hole of the piston (located in the previous photo). During the injection, cam moves, moving the piston. According to the position of the regulator, the hole is released by the depression of the piston. This controls the proportioning of the amount of fuel injected. (when the hole is open, the pressure is released and the injection stops)
The piston is known as the distributor because during rotation, via the holes, it directs the fuel towards one or the other of the exits of the pump.

Reassemble the piston distributor:

then return springs:


The end spring holder at the end of the piston:


and one puts the whole at his place in the pump:


Now reassemble the regulator levers and their return springs

the difficult part of this is due to the position of springs:


Close up of the articulation of the levers by special bolts (custom to the modifications by the original author)

installation of these bolts with their new aluminum joint

"Y' has more than A returns the whole of the levers speaks high" (couldn't translate this part)

The piston distributor (exhaust port marked w/green arrow), the opening for the injectors (red), and the 4 grooves for filling the chamber (green circle).

All the return springs in place

More than has to position back the whole, then the redémonter still one or twice because the return springs of the piston did not fall opposite, and veiled: (couldn't translate this part)


All the pieces of the centrifugal governor.

The pieces of the masselotes.


Placed into the body of the regulator.

Replace the sleeve of the regulator (notice the gauged hole to modify the beginning of injection according to the load):


Alignment of the axis of the regulator in the pump housing

The spacers which go behind the rotor of the regulator.


Then the regulator.


Then tighten the axis using a key. Use this special wrench to counter hold the axis during tightening. The position of this axis is important, because it regulates the the beginning of injection according to vacuum load. Therefore, you must have marked it during disassembly to get the same alignment.

Removal of the lever bushing


A new bushing

Securing the bushing with a bolt and washer


The control shaft and the regulator springs

The flat part (green) is engaged in the notch of the lever (red)

Put back the lid (with its new joint) and screw it in place.
Reassemble the stick control, note the exact position before disassembly.


The regulator valve and new components


The fuel shut off solenoid


And finally the main shaft front seal, you have to gently press it in with a drift or socket.


Finally assembled.