The sensor on the front of the engine I've always believed was the tach sensor..... it does indeed run to the diagnostic port in the left wheel well area (round black thing with a cap that screws on).... but there is also a wire or possibly wires leaving that port that join a large bundle going into the cabin. Seems funny to have more than one sensor for the same thing but I suppose it's entirely possible. The only thing I could see in the area described on the bell housing looked like a braided ground cable but I didn't have a flashlight so it was hard to see.

If you run out of options for looking for the tach sensor let me know and I'll unplug the sensor on the front of my engine and see if my tach works or not then.... I would do it right now but it doesn't look like it'll be too easy to do (plus it might quit working altogether if it gets messed with).

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k