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Someone must have done this with the huge retro modding talent out there!
i've got a v6 in my cortina which was originally 4 cyl so the rev counter reads 1/3 over, I know I could get a tacho from a capri or similar and mod the housing but surely it cant be too difficult to modify the existing tacho?? Does anyone know how to do it? Its a smiths tacho by the way.....
I'm sure it must be just replacing a resistor or something but I cant even find a wiring diagram on the net! I'm fairly handy with electronics so not worried about that side of stuff. Can anyone help!!??
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A quick look on the internet leads me to this possibly useful bit of info: "Nearly all analog tachometers work in a similar way. There will be some kind of charge pump circuit that puts a small fixed pulse of current through the tachometer meter movement each ignition pulse. The faster the engine goes (or the more cylinders) the further the tachometer moves away from zero.
Somewhere in there will be a capacitor the value of which determines how much energy is pumped into the tachometer each ignition pulse. The fewer cylinders, the bigger each pulse needs to be to do the job. When running a V8 with a four cylinder tachometer (for example), it will read x2 actual engine rpm, assuming the tachometer is being properly triggered.
So what needs to be done, that mystery capacitor needs to be located and replaced with one very close to half the value. (or 33% if on a V6) There is almost always a small trimmer potentiometer on the circuit board for final exact rpm calibration. The trimmer is there only to correct for minor production tolerance variations in everything else, and is highly unlikely to have enough range to correct for doubling the number of engine cylinders !!"I can't vouch for how accurate that info is, but it's certainly worth some thought because it all seems to make sense.  |
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it seems nice and simple. I FEAR electrical stuff though so what I would do is contact someone like Speedograph or one of the other specialists who rebuild instruments and get them to do it... Someone like www.irs-gb.com/ ? I never used them, but.... |
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1937 Austin Street Rod - 1941 Wolseley Not Rod - 1956 Humber Hawk - 1957 Daimler Conquest - 1966 Buick LeSabre - 1968 Plymouth Sport Fury - 1968 Ford Galaxie - 1969 Ford Country Squire - 1969 Mercury Marquis - 1970 Morris Minor - 1970 Buick Skylark - 1970 Ford Galaxie - 1971 Ford Galaxie - 1976 Continental Mark IV - 1976 Ford Capri - 1976 Rover V8 - 1994 Ford Fiesta
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Cheers MM, thought it should be something like that! Might have to just pull it apart and try somestuff! I resent paying for things to be done when I reckon I could do them I just need to know how!
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