but no, the idea you presented, would never work..
weakening the signal (adding resistance to the tach signal) is not going to do anything besides make your tach even LESS accurate..
so, scratch this idea, because you cant feed a gasser tach with a W signal from a diesel alt and have it read..
gas and diesel tachs use TOTALLY 100% different signals.. thats why you have to change the circuitry on a gasser tach to use it in a diesel..
basically, if it were as easy as adding a potentiometer inline to the tach, dont you think thats the way everyone would be doing it, rather than re-wiring there whole tach circuitry?
oh yea, there is an adjustment potentiometer inside every tach ever produced by VW. you use THAT to adjust the tach to read right.. but that still does not fix the issue that the gasser tach will not read off a diesel signal..
gasser tach reads 2 pulses per revolution
diesel tach reads like 35 pulses per revolution..
NO, YOUR IDEA WILL NOT WORK.