Electrikal problem on an Audi RS6 4B B-Turbo

Hi There, I will first write the Story of the car.
Customer came for an Automatik Transmittion rebuild. to take out the Tranny i needed to take out the Engine too.
After the rebuild i put everything back together. During the Work i discoverd a few broken wires on the wiring harness. I fix them.
I test drove the car 1 hour before the customer came and everything worked fine.
I put the car car on Lift to install some Plastic Parts and since then car didnt start again.

Im searching 2 Weeks now to find the Problem and im really done! So i need your Help.

The problem is that the Engine has almost 5Volts! When i took the Battery cable out of the Alternator the Engine has 0,70Volts.
The Alternator is New.
I checked the starter and is working, but when i crank the Engine (sometimes) i hear the solenoid of the starter cliking but the starter wont move.
The customer sow the Problem himself. We both checked the Wiring Harness and found some more broken cables. I fix them all but i cannot find from where the engine takes Voltage. Fuses Relays and so on are many times duble checked.
the small wire of the Alternator and the Starter are together on one plug and are ending in a fuse and one relay, those are good.
I cannot explain more the problem and sorry for my english but i need Help

thanks in advance
Themis

Did you do a voltage drop test at the starter to see if it's getting enough power to turn?

When you say the engine has 5v, what do you mean by that? Where is that measurement taken from?

Themis wrote: The problem is that the Engine has almost 5Volts!

Make sure the engine to battery ground cable is good.

The problem was indeed the main ground cable. Corrosion every where:(

I put a new one and the engine started.
What confused me was that i had Voltage on the Engine it self(Multimeter,ground on the car and positive on the engine Block)and when i was unpluging the Control cable from the alternator the Voltage was dropping to less than 1V.
With my knowledge i couldnt expain it to me!

Thank you very much for your answers !!!

Themis wrote: What confused me was that i had Voltage on the Engine it self(Multimeter,ground on the car and positive on the engine Block)

This voltage was coming from everything that uses the engine block for a ground. It was all "trying" to get back to the battery.