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My electric tacho on my (whisper it) MGB had the white ignition wire run across the back and the wire had a single loop in it. I'm assuming that this induced magnetic field in the innards which cause the needle to move.

 

But no doubt someone clever will be along in a minute to explain that properly... ;)

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Roger my understanding of the early ones as follows:

 

A pulse taken from the contact points triggers a mono-stable multivibrator . The DC value of the mono stable pulses causes a normal DC voltmeter to deflect.

 

As the pulse from the points is relatively fast and short you need something to lengthen the pulse and give a reasonable DC level thus the Mono-stable. Usually not much to go wrong and only a handful of components.

 

Modern electronic tachos will be far more sophisticated.

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270 degree moving coil movements abound, usually well damped for an automotive application.

 

I can't say for sure but I'd guess that Graham is right for modern instruments.

Edited by peejay4A
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However seriously, it depends on where the taco signal is being taken from, what type of ignition and the signal interpretation damping at the receiving unit.

 

If the signal is being taken from the coil It's not uncommon for their to be a double pulse at the coil, the second may be much smaller but if within the receiving range of the unit it can be taken as two sparks, sometimes it occurs for every spark, sometimes only on one or two cylinder firings. Its why sometimes its hard to fit modern rev limiters to old ignition systems as because they are trying to prevent over revs their signal processing is very hair trigger, so if at 3-4k its see's a double pulse from a couple of sparks it thinks the revs have suddenly leapt by 50% or more and immediately starts cutting sparks.

 

Taco displays are normally configured to only read a strong rising or falling signal and I think also have some form of damping circuit, so that whilst the signal pulses may apparently be floating about by a few hundred rpm, the display needle remains virtually static.

 

I know that some modern taco displays are designed to be fed a clean signal from ECU's (already smoothed) I suspect that they would have the same problem as Rev Limiters if not fed a clean signal.

 

If you google rev limiters and data capture device installation on older ignition systems you will find that people have devised many methods of cleaning up the signal. I found the cleanest source was always a single plug lead rather than the coil.

 

So the answer to the question is "it depends"

 

Alan

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Hi Folks,

thanks for that. I knew what Tim meant :blink:

 

Moving coil meter sounds sensible.

 

I like Grahams stepper motor - food for thought.

 

Roger

 

Roger

 

See this one

 

http://www.tr-register.co.uk/forums/index.php?/topic/56126-electronic-tachometer-conversion/

 

Have thought about converting but other jobs are higher on the list. Rogerguzzi seems pleased with the result.

 

Website at

 

http://urevco.com/

 

Phil

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Hi Phil,

I have re-read that thread and it is a good/interesting topic.

Although I named this thread about tacho's I have another application in mind and wanted ti understand how they operated.

 

I already have an electronic tacho but I'm sure it is not working properly as it hardly gets beyond 3000rpm even when I race the engine.

 

Roger

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Hi Alan,

not that sort of racing but when I put my foot down and the engine races (car stationary).

 

I can actually do this on the TRack or on a hill but the car will still be stationary

 

Roger

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Richard, about the erratic needle movement.. :(

When I did modify the engine of the 6 to EFI I had the tacho replaced by an electronic item, done by TR Revington at their workshop. The tacho displayed the same symptoms, to say erratic needle movement.

Following that Neil fitted a diode in the wire between the coil and the instument, problem solved, works perfectly for 17 years.

But I have no idea about the diode capacity he fitted.

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