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Ever since I've owned my Caddy, I really wished it had a tachometer. The one option that is available on later model VW diesels is the "W-terminal" alternator. This alternator has an extra terminal (labeled "W") that allows an RPM-proportional signal that can drive a dash-mounted tachometer. It is common on the turbo diesel models and requires a full gauge cluster swap to have the tach replace the stock clock.
Unfortunately, my alternator is not the "W" type and it is still working fine, so I don't really want to replace it, same with the instrument cluster. The alternator is driven off the engine by a v-belt, so the readings provided by the tachometer are probably not really accurate. In any event it would have to be calibrated in some manner (I guess by a mechanical tach?). So, I decided it was best to design a fully digital tachometer system to avoid all these problems.
One of my old engineering projects I worked on at NASA involved using optical shaft encoders to pick up shaft speed and rotation for use in position sensors. These encoders use a series of concentric circles of alternating light and dark marks to detect motion. For speed only, one set of marks is sufficient. In the case of the VW diesel, the injector pump and cam shaft are driven by a toothed belt off the crankshaft. The crank shaft sprocket is 1/2 the diameter of the rest of the sprockets, so they spin at 1/2 the engine RPM. The crankshaft sprocket is not easy to get to, but the one on the fuel pump is accessible by an existing hole in the timing belt cover. Since the injector pump sprocket is turning 1/2 speed of the engine, it is necessary to have the pickup wheel make two on-off cycles per revolution.
DP-680-RATE Rate Meter/Tachometer- $74.00
EE-S Reflective photomicrosensor- 7.60
Optical pickup wheel, s/s sheet metal - 5.00
I fabricated the pickup wheel from a sheet of stainless steel sheet metal. I sized it to fit inside the sprocket on the fuel injection pump, thie made the outer diameter 118mm with a 43mm hole cut out of the center. I masked off two opposite quardants and spray painted the remainder with flat black enamel paint.