I would suggest having an adjustment feature for the alt-driven tach. That way it can be "dialed in" to perfection. Get the initial adjustment "ballpark" by using the pulley ratios, and then fine-tune the actual RPM adjustment when the engine is running. Matching the engine sound to the frequency of a tuning fork works very well with high accuracy for doing final calibration (if you've got any sort of a musical ear).
Quote from: "fspGTD"I would suggest having an adjustment feature for the alt-driven tach. That way it can be "dialed in" to perfection. Get the initial adjustment "ballpark" by using the pulley ratios, and then fine-tune the actual RPM adjustment when the engine is running. Matching the engine sound to the frequency of a tuning fork works very well with high accuracy for doing final calibration (if you've got any sort of a musical ear).How would this be possible? With extra resistors or something? I have heard that the diesel tachs had some kind of fine-tuning adjustment on them, but I don't see anything obvious looking at the back of mine (taken from a 1985 jetta diesel).My diesel tach is definitely over-reading, although I haven't yet figured out by exactly how much. The alternator (& engine) is from a 1997 Golf AAZ, and the tach from a 1985 Jetta. So I'm pretty sure the alternator pulley diameters were quite different in those 2 cars (1985 and 1997).At idle, the tach always shows 1200 rpm, and at 60 mph in 4th gear (ie. top gear) it shows 5000 rpm (!). Yeah, sure, the Vanagon is geared really low, but that's got to be about 1000 rpm higher than actual engine speed. Any suggestions on "dialling in" a diesel tach?