driving a tacho

Honest John Back Room » Technical matters
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Sat 8 Jan 2005 22:46
driving a tacho - itchy
OK, bit of a general question so havent put any make/model. I need to find a suitable signal to drive a rev counter (aftermarket).
I know on a petrol this would drive off the coil or ignition module / ecu. The car its going in has neither. I have been told before it will drive off the 'w' terminal on the alternator. Can anyone confirm? Also what kind of voltage would a coil's negative terminal give off? I want to check the alternator gives a similar output.
I also need to find out how many pulses per revolution the alternator would give out. I know it turns on a 2:1 ratio with the engine going on pulley size's. Any help is much appreciated.
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Sun 9 Jan 2005 08:36
driving a tacho - Andrew Moorey (Tune-Up)
I dont think the W terminal is going to help you as this is the warning light terminal. I fitted a rev counter to a diesel Renault once and it used a sensor strapped to the body of the alternator and presumably sensed the magnetic disturbance of the rotor and used a little box of tricks to approximate the engine speed and pulse the tacho. Negative voltage available at the coil can reach around 400v. Pulses from the alternator, well possibly 15-20 per revolution?
Andrew
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