Lotus Elan

Help with Tach and Vacuum Switch

PostPost by: tvacc » Sun Jan 18, 2015 11:59 pm

Hi all,

Working on the Elan S2 I have here, sold to a friend and fellow LOONY. A few issues I need some help with. The tach is not working so a few weeks ago I posted that the wires were not hooked up to it. so today I did hook up positive earth and negative power. Tach is still not working. I have verified that I have 12 volts to the tach by a volt meter, positive being grounded to the tach case and negative to the terminal on the tach. Now there is the sensing white wire that goes down to the coil. I assume that it goes to the negative post on the coil. And if my memory serves me that is also the power to the coil. So if the car starts up and runs, this must all be correct as far as continuity is concerned. I need some more help in diagnosing what is at fault here. Is there a way to test the tach? Can I test the signal coming from the coil? What is the correct wiring to the coil? Wait..I should be able to see that on the wiring diagram. Thanks in advance for he help.

My other issue is that the headlight pod vacuum switch does not work. I know these are not available from what I remember. So I need help in either rebuilding what I have or a work around. Again, thanks for the help.
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PostPost by: billwill » Mon Jan 19, 2015 12:34 am

Hmmm, tricky, you probably don't have the equipment to do an independent test of the tacho. Ideally that involves applying power as you have done and then feeding the loop with square pulses from an electronic pulse generator.

Usual 'easy' method is just to try a known working tacho in its place & see if that works.

It's possible that your internal circuit has been destroyed by having power connected the wrong way around.

Basically, as you deduced, the power to the coil goes from the ignition switch to the coil down that white wire. But the current flow is not continous it flows only when the distributor points are closed so the current flow is being pulsed at a rate corresponding to RPM. The pulses are somewhat ragged. The tacho internally triggers a nice rectangular pulse of fixed width from the white wire ragged pulses and it then averages the 'power' in those nice pulses to make a steady current to drive the needle around the dial, showing RPM. An internal variable resistor is used to set the calibration, i.e the needle position for a given rate of pulses.

It is possible to make a calibrator using a mains transformer to bring mains voltage down to a safe level then squaring up the sine wave of the AC supply (60 Hz in UK, 60 Hz where you are) and then calculating what RPM would correspond to 60 Hz. You may need to find an electronics enthusiast to make such a calibrator for you, and to mend your internal circuit if it is broke. The circuit has been published on this website so will need finding, but basically all the Smiths tachos use the same circuit.

~~~~~~~~~~~~~~~~

On your headlkight vaccuum, you first have to check that you have no air leaks elsewhere and that the non-return valve is working correctly, only suspect the Control 'switch' as the last element to be tested. There are picctures on this site of a dismantled 'switch'.

Try rigging a vaccuum cleaner with a large cork with a thin pipe pushed through it and some plastic pipe which the then use to 'suck' the headlight pod(s) to see if they work properly. Their internal rubber diaphragms are prone to perishing.
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PostPost by: Craven » Mon Jan 19, 2015 1:28 am

Distributor runs at ? engine speed and produces 2 contact breaker openings per engine revolutions,
The rev counter/Tacho is calibrated in Rev per minute so at an indicated 1800 rpm engine speed would be integrating 3600 pulses per minute or 3600 divided by 60 to give 60 Hz.
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PostPost by: tvacc » Mon Jan 19, 2015 1:45 am

Are there two separate wires that go to the coil, one for the power to the coil from the ignition switch and one for the tach signal? Or is it the same wire? I dont have an S2 wiring diagram here to look at. Does the tach "signal" wire go to the negative or positive side of the coil on a positive ground car? I never hooked up the tach on this car, do if someone hooked it up backwards it was not me so I cannot say if it was ever hooked up wrong.

I don't have another positive ground tach to swap out here. I did at one time, but sold all those parts back when I bought my Elise in 2005 and thought I had sworn off these old Lotus cars, boy was I wrong. Just like that line in Godfather 3, " They keep pulling me back in!!!"

Oh well.

I have tested both pods with a vacuum pump, and the left one is sort of leaky and the right one is fine. When I draw down on both without the switch in the line, the do open (or raise). When I include the switch, they don't raise at all. So the switch has to been the open problem. Is there a modern fix for the switch?
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PostPost by: nomad » Mon Jan 19, 2015 3:17 am

I've had enough problems with these old tach's that I think I would just go with the unit from "Spyda" that eliminates the old circuitry. I have one I will be fitting one of these days but I believe there are folk's here that have had personal experience with them. I have not as of now but have heard good thing's.

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PostPost by: nomad » Mon Jan 19, 2015 3:34 am

Make that spelling "Spiyda"


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PostPost by: tvacc » Mon Jan 19, 2015 6:16 am

I thought you meant Spyder cars but I looked on their website and did not see a tach. Can you be more specific in who has a tach that operates on s different system?
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PostPost by: tvacc » Mon Jan 19, 2015 6:38 am

Got it. It was the exact spelling. I am having s hard time figuring out how it works, but II think my guy would prefer to keep the tach original.
Tony Vaccaro
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PostPost by: billwill » Mon Jan 19, 2015 11:47 am

tvacc wrote:Are there two separate wires that go to the coil, one for the power to the coil from the ignition switch and one for the tach signal? Or is it the same wire? I dont have an S2 wiring diagram here to look at. Does the tach "signal" wire go to the negative or positive side of the coil on a positive ground car? I never hooked up the tach on this car, do if someone hooked it up backwards it was not me so I cannot say if it was ever hooked up wrong.



There's pnly one wire. If you had two it would probably by-pass the tacho & it would not see any pulses.

Image ignore the polarity of this diagram, it was drawn for a neg earth car.


However.... I found that in my car, the white wire goes all the way to near the coil, then is connected to another white wire which goes back to form the loop at the tacho, then comes forward again in the same part of the loom to connect to the coil, so if you looked in the loom you would actually see 3 white wires.

~~~~~~~~~~~~~~~
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PostPost by: billwill » Mon Jan 19, 2015 11:52 am

The thread/topic for mending the vacuum switch is here:
lotus-twincam-f39/vacuum-switch-t13457.html#p60892
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PostPost by: billwill » Mon Jan 19, 2015 12:03 pm

The kit from Spiyda converts the Tacho from current sensing with the loop as discussed above into a voltage sensing circuit, this is often required if the car is converted to electronic ignition, but will work with normal ignition too.

In the voltage sensing version the removable springy half loop on the back of the tacho is replaced by an ordinary single connector, which is then fed via a chosen resistor from the bottom end of the ignition coil (i.e. the end that connects to the distie contact breaker. The white power wire for the top end of the coil then goes direct from the ignition switch to the coil and not via the tacho.
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PostPost by: Quart Meg Miles » Mon Jan 19, 2015 1:08 pm

On the S2 there is no electrical connection between the coil wire and the tachometer. The white wire to the coil passes one turn through a plastic block with a magnetic metal strip inside it which pushes into a matching receptacle in the tach to form a pulse transformer. In later cars there were bullet connectors splitting the white wire. So stimulating the early tachometers isn't easy to do.

I suspect you may have a white wire with a bit of plastic on it floating around behind the dashboard! There is a fiddly round nut clamping the plastic to the tach if I remember correctly, it may have fallen off.
Meg

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PostPost by: tesprit » Mon Jan 19, 2015 10:35 pm

tvacc wrote:I thought you meant Spyder cars but I looked on their website and did not see a tach. Can you be more specific in who has a tach that operates on s different system?


Here is a link to the Spiyda kit on ebay to convert your tach: http://www.ebay.com/itm/Smiths-RVI-RVC- ... 589c3770c7 As long as the meter part of your tach still works this kit will fix it because it replaces all of the old circuit boards/circuitry with a modern driver for the meter.

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