CRSMP5 AFAIK, when I had a Passat B4 TDI, the tach get it's signal from the ECU which gets it's RPM signal from the crank position sensor, not from the 120A alternator W terminal. The B4 cluster tach is stepper motor driven, not an analog movement like idi diesel and gasser tach's. I have never looked to see if the 120A B4 Passat alternator has a W terminal output or not. My guess is it does not since the W is not needed for anything I can think of in the B4 Passat.It's hard to follow what configuration you have in terms of wire harness and cluster. If you pm me I can give you my phone # and talk you through what I know regarding VW diesel tach's.
If the alternator you are using is from a TDI engine such as the 1Z, it will not have a W terminal. With TDIs, the alternator has D+ and DF terminals. DF reports alternator load to the ECU so it can control high current loads such as coolant glow plugs. Since TDI tach signals are generated in the ECU the W terminal is not needed.You may have to use the AAZ alternator to get the tach to work or get a Dakota Digital converter to make the signal compatible.Paul