Lotus Elan

Please help me understand ignition wiring & Tacho....

PostPost by: pharriso » Mon Sep 16, 2013 11:01 pm

I'm doing a RVI --> RVC tacho conversion using the Spiyda board, my car has electronic ignition & the Tacho jumps all over the place!. Spiyda document the changes to the internal wiring of the tacho well, but do not mention changes required to the Ignition wiring itself.

I've read the tech article http://www.lotuselan.net/publish/bouncing_tach.shtml but want to make sure that I am on the right track.

As built with the RVI set up, it looks like the ignition switch supplies power through the Tacho to the positive terminal of the coil:
RVI_Ignition_wiring.jpg
RVI wiring
RVI_Ignition_wiring.jpg (32.31 KiB) Viewed 5393 times

So If I detach either white wire from the back of the tacho the car will not run, but if I connect them together the car runs fine.

Now looking at the RVC diagram:
RVC_Ignition_wiring.jpg
RVC wiring
RVC_Ignition_wiring.jpg (35.29 KiB) Viewed 5393 times

It looks the positive coil terminal is fed directly from the ignition switch (i.e. just connect the 2 white wires that used to go to the tacho together :idea: ) & run a new wire from the -ve coil terminal straight to the remaining single tacho input.

Am I on the right track here? I fully admit to being retarded when it comes to electronics :roll:
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PostPost by: elanner » Mon Sep 16, 2013 11:45 pm

Phil,

Your last paragraph is entirely correct.

Take the two white ignition wires off the back of your RVI tacho and connect them together (they should have male and female bullet connectors). The ignition 12v now goes directly to the coil (passing the tacho and collecting $200).

Then run a sense wire from the negative side of the coil (the side that connects to the electronic ignition/distributor) up to the signal input of the RVC tacho. That's it.

You might see a second wire from the ignition/12v side of the coil going to the electronic ignition - this is merely supplying power to the electronics.

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PostPost by: billwill » Mon Sep 16, 2013 11:46 pm

OK, as far as I know...

On the original tacho, the white wire from the ignition switch to the coil simply passes through the tacho, on earlier tachos like mine it doesn't even connect to it, it simply is wound with one coil around a metal strap, which in turn has both of its ends connected to a loop inside the tacho. On later ones I think they found that unreliable so they used connectors (?) on the outer ends on that internal loop and cut the white wire & put connectors on it which you push onto those tacho connectors. Electrically its essentially the same. The contact breaker causes the current through the coil and hence the current through the tacho loop to pulse. Inside this pulse is transferred by magnetism of the loop to another coil, which operates a transistor circuit. The transistors square up the pulse and fix its duration so all pulses are identical. With more pulses in direct proportion to the RPM of the engine. The rest of the circuit the effectively pumps the pulses into a capacitor with a resistor across it, so the net effect is that the current draining through that resistor is proportional to the NUMBER of pulses and that current feeds the dial meter (which is basically a milli-ammeter).

OK, so now the problem is that when you fit electronic ignition you drastically reduce the current that flows through the 'points' and change the shape of the current pulse so it doesn't activate the transistors properly, so the tacho doesn't work properly.

The cure is to change the circuit in the tacho so that it is operated by the changing voltage at the points, or electronic points substitute, (I think) instead of by the current loop. The voltage on the supply side of the coil Plus on neg-earth cars won't change much as it is connected through the ignition switch to the battery, so that is no longer a suitable signal, but the voltage on the other end of the coil is moving from 0 volts when the 'points' are closed to +12v when the 'points' are open, so it is a splendid signal.

So after my long explanation.. what you said is spot on correct.

It looks the positive coil terminal is fed directly from the ignition switch (i.e. just connect the 2 white wires that used to go to the tacho together :idea: ) & run a new wire from the -ve coil terminal straight to the remaining single tacho input.

Joining the two white wires at the tacho is merely doing what the tacho used to do inside anyway and then bringing a signal back from the -Ve end of the coil to your new shiny voltage sensitive terminal on your modified tacho is exactly the right thing to do.

All in my opinion OK, but based on many years of doing electric & electronics stuff and actually having an Electrical Engineering degree.. :D :) :) :mrgreen:
Last edited by billwill on Mon Sep 16, 2013 11:50 pm, edited 1 time in total.
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PostPost by: billwill » Mon Sep 16, 2013 11:47 pm

Snap... :mrgreen:
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PostPost by: pharriso » Mon Sep 16, 2013 11:54 pm

Fantastic guys!! Funny how there are many posts on how to modify the tacho itself, but very little info on the new input wire required...

Many thanks, I'll give it a go on Friday & update the post.
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PostPost by: rgh0 » Tue Sep 17, 2013 2:14 am

Just a comment on the issue with electronic ignition and tacho jumping.

The current pulse to the coil is essentially the same with or without electronic ignition as the same pulse is required to generate the spark regardless of whether it is points triggered or electronically triggered. I know this is an over simplification but it is true to the first order of magnitude of variation.

The tachos jump around for two reasons normally when electronic ignition is fitted to a current sensing tacho.

1. People often put the power supply to the electronic ignition module to the coil positive. This introduces extra current pulses that the tacho sees and that causes it to jump around.
2. The current tachos are all very old by now and their electronics especially the capacitors are getting dodgy. The are all failing and a failure may just coincide with the change to electronic ignition.

Replacing the current sensing tacho internals with voltage sensing internals is an fix to the above problems but it is cheaper and simpler to fix the base issue of wiring it correctly or replacing the failed capacitors IMHO

With a sound current tacho wired correctly I have not seen problems when fitting electronic ignition to my cars

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PostPost by: billwill » Tue Sep 17, 2013 12:42 pm

Bye the way, I found this circuit of the tachometer on the net some time ago,

This shows it modified for negative earth, but using current sensing (from the loop in top left corner) not voltage sensing.

http://freespace.virgin.net/tommy.sandham/tacho.gif

Image

The authors website: http://www.tommysandham.name/

And useful info that he collected at:
http://freespace.virgin.net/tommy.sandham/welcome.htm


PS: because of the style in which Mark Olsen & Glen Wallace drew this circuit diagram. It is not laid out in the traditional layout for a "mono-stable multivibrator" I can't quite get my head around how, in fine detail, it works. Presumably they drew it by analysing around the circuit board of an actual tacho.
Last edited by billwill on Tue Sep 17, 2013 1:03 pm, edited 1 time in total.
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PostPost by: pharriso » Tue Sep 17, 2013 12:55 pm

rgh0 wrote:With a sound current tacho wired correctly I have not seen problems when fitting electronic ignition to my cars

cheers
Rohan


Rohan, just found installation instructions for the Crane Cams Fireball electronic ignition fitted to my car by the DPO on the internet. It contains the following note:

"MODIFICATION OFSMITHS CURRENT SENSING TACHS
Some older British vehicles use Smiths current sensing tachs.
The wire from the coil positive terminalto the ignition switch passes through a current pickup at the tach. Installation of a Crane electronic ignition may cause erratic operation of a current sensing tach, due to the higher coil current. Modification of the current pickup to reduce the signal level will usually eliminate the problem."

Maybe you are just lucky ;-)
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PostPost by: rgh0 » Tue Sep 17, 2013 1:36 pm

yes may be lucky.

But may be some science in it also

The current will only increase if you change the coil to a lower resistance / impedance one the electronic trigger itself does not increase the coil current flow.

The loop senses the change from no current flow to current flow not the absolute current flow level also.

Reducing the sensitivity was probably around stopping the sensitivity to other current spikes if you took the electronics power supply off the coil

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PostPost by: billwill » Tue Sep 17, 2013 2:14 pm

Hunting around for a version of the circuit drawn in a more traditional manner, with no luck because all the ones I found so far are basically copies of the above layout I did find this site:
http://server88-208-245-106.live-server ... 453823.htm
which has this sentence, which make a lot of sense to me about the use of electronic ignition.
Hi All,

The problem with electronic ignition and RVI tachs is that the power for the ignition is taken from the BAT terminal of the coil. Electronics invariably have a large capacitor on the power input, to smooth out voltage fluctuations. Unfortunately this also smooths out the current pulses, for the RVI tachs, thus giving poor or no triggering. The trick is to power the ignition from a circuit that doesn't go through the current sense transformer, in the tach. The clump of connectors on the RHS side of the engine bay, near the firewall, has a spare white connector, which is switched ignition and a good point to take the power for the ignition module.
I have done this with my RVI tach and a 123 dizzy and have had no problems.

Herb Adler


So if your tacho is still current sensing and is playing up with electronic ignition, try taking the power to the electronic ignition box from the white wire that goes from the ignition switch to the tacho BEFORE the tacho, not after it at the top of the ignition coil.


~~~~~~~~~~~~
Meanwhile I'm still trying to understand that circuit and I think I understand it all EXCEPT what that low value resistor R8 at 3.3 ohms is there for.
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PostPost by: elanner » Tue Sep 17, 2013 2:24 pm

Rohan,

Yup, I agree with you 100%. To my simple way of thinking coil current is coil current, regardless of whether it's switched by mechanical points or solid state electronics. And I really like the elegance of the RVI current-sense design.

Your comment about wiring the power to the electronic ignition correctly (i.e. not through the tacho) was discussed here: lotus-electrical-f38/electronic-ignition-tach-t27618.html This is a good idea but, as it happens, didn't do anything for my tacho. Perhaps the current draw for a Pertronix system is just too low to make a noticeable difference?

Anyway, my original plan was to simply replace the capacitors on my RVI tacho on the basis that they were 40 years old, dried/worn out, and the cause of the bouncing. But when I got the tacho to pieces it seemed like a false economy to change only the caps, since all the other components were presumably equally decrepit. So I figured it made sense to change the lot. However, I never found the diagram posted above by Bill, so was at a loss to figure out what was needed - I couldn't even read the old cap values.

So, once I was on track to replace everything it was simply easier to use a modern voltage circuit (copying rjaxe's forum entry: elan-mods-f31/rvi-tachometer-conversion-t26919.html).

The new circuit works very nicely 99% of the time, but is suffering from some sort of overheating problem in very hot weather - it died a few times during Summer. So I'll have to take it apart and run it on the bench with a heat gun to figure out which component is misbehaving. A Winter project. One advantage of the circuit is that it sits on top of the movement (rather than being wrapped around it), so it's much easier to maintain! ;-)

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PostPost by: billwill » Tue Sep 17, 2013 3:05 pm

I found some real gems of original Smiths documentation, which I have posted on a fresh topic, so that they don't get lost in the depths of this topic.

lotus-electrical-f38/smiths-instruments-documentation-from-the-horses-mouth-t29442.html

Read all about them there, they are very useful (though they don't contain the internal circuit that I wanted.).
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PostPost by: billwill » Tue Sep 17, 2013 3:36 pm

I found Mark Olsen's page where the circuit above originates (I think) plus he describes two other circuits used in tachometers.
http://www.accutach.com/Pages/SmithsTac ... under.aspx

But I regret that I think the layout in which that first circuit diagram was drawn has misled him and his description of exactly how it works is not quite right. He talks of 12v pulses, but in my opinion the Zener Diode, Capacitor C3 and resistors R7 and R8 form a 6 volt stabalized power supply to the rest of the circuit so the height range of the pulses can't in fact exceed 6 volts.

It all looks a bit odd because the circuit was originally designed for positive earth cars so R7 is connected to earth instead being on the other side of the 12v supply, so the stabalized 6 volts range is tied to the +12 volts battery line and the bottom of the stabalized range floats up and down relative to earth. Just one of life's little quirks. :D
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PostPost by: billwill » Tue Sep 17, 2013 8:24 pm

Here's one alternative circuit using the ubiquitous (and generally understood) chip called a 555.

http://www.555-timer-circuits.com/car-tachometer.html
Image

This circuit is voltage triggered.

Note that the 3 right hand components form a typical 9 volt stabalized supply from the 12 volts and all the other components connect via that stabalized supply.
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PostPost by: billwill » Thu Sep 19, 2013 1:11 pm

Oh, one extra general point I had forgotten, but was reminded of while browsing about.

For everyone in general:

If you convert your tacho as mentioned above and if the electronic ignition system that you chose is of the Capacitative Discharge type, do not simply take the pulsing signal from the negative terminal of the coil. On a normal system that goes 0 to 12 volts or so, but on a C-D system it is likely to have pulses of up to 400 volts there. So you will need to either take the signal from a provided terminal on your electronic box, Or make a small circuit with two well insulated resistors, to reduce that 400 volt signal back to about 12 volts to suit your modified tachometer.


Capacitative discharge electronic ignition systems work by having an internal power supply that creates about 300 to 400 volts from the 12v car supply. Then a capacitor is charged to that high voltage and then is dumped into the ignition coil when the contact breaker (or electronic equivalent) opens. So it provides a short sharp high voltage kick instead of a longer 12 volt kick.
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