I'm also converting a gas tach to the diesel "w" terminal. I am electing to go with a Dakoda Digitial signal converter which converts the "w" signal to the gas tach signal. I bought one on ebay $62 with free shipping, and am hoping it will do what it says. 4cralwer spent more than that on making an optical setup.
Although if this converter doesn't work, then I have wasted my money anyway, and at least his system works to an extent.
I have taken my Bentley and searched the wiring diagrams and looked at the back of my gas tach (and circuitry since I have the whole gauge cluster) and where the wire goes to the plug in. I will bring the wire from the converter to the appropriate terminal on the plug or tap into the current wire going into the plug and think this will be good. I haven't done it yet, but this is the plan. I have looked at the wiring diagrams, and looked at the wiring of my current plug (1980 caddy with speedo only cluster) and have traced the wires and where they go and what they do. I plan to match wires from the plug to the approriat terminals. I am blessed to have a 12 prong plug and a 14 prong plug. I will need to match as I get ready to install.
It seems that if you can find the wire from the tach and/or if you have the whole cluster assembly you can trace the tach line to the appropriate terminal on the plug, either through tracing or by looking at the wiring diagram and finding the tach terminal on the plug, then tracing it back to the tach terminal.
This is all theory with some experimentation at this point, but thought it would share it so that others can interact and confirm or correct that this is a good route to go.
I am hoping the converter will be the most painfree way of matching signals and getting an accurate reading. I posted a question about the converter and got some feedback, but I dont' know I have found anyone who has used it and responded. So, it might be prudent to wait until I have actually installed it, or someone else who has installed the coverter can share his/her experience. . Anyone actually used one and how does it work?
Hope this helps some.