Lotus Elan and Lotus Elan Plus 2 Forums

Yes sorry, its been rewired off the -ve side of the coil, the main Tacho power is fed via a 10v solid state Voltage Stabilizer and the ground is presently connected directly to the Battery, the two white original power wires have been connected together to bypass the Tacho and a fresh signal wire has been fitted. The car its self has a 12v Ignition system (- Neg and the coil is a solid 3.0 ohms after removing the cable resistance) and its run without ballast on a Shiny new Optronic Ignition (which did actually help as you can see from the video's above)

What are you doing with these r?sistors? I hope that you are not connecting them in series with the input trigger wire.

Its been a long time since I was involved with vehicle electronics but I think if a resistor is needed it should either be a pull up or pull down resistor, try a 10K one connected between the trigger input and 12v or the stabilised voltage if the tach is run from that, if no better then try it between the trigger input and ground.

Yes i have just tried it as per the recommendation on the Spiyda instructions (Series in the -ve - Tacho sensor wire) but it didn't make any difference. Can you please explain in laymen's because i thought a resistor was a resistor

All fixed, Chris at Spiyda fixed it. Can't rate Spiyda highly enough, needed a Diode and capacitor on the power (some thing like that not seen it yet only going off what i've been told on the phone) suffice to say it working as it should on My Spiyda converted Tacho so the plan is now to replace the RVi internals with the original frame etc and fit a Spiyda kit to convert his to RVC which should be the Tacho Fixed

Phew.......... Should be another Elan on the road for Summer Just have to put mine back together now (Elan Sprint Parts car)

Thanks guys for the help. Wow nothing like an Electrical fault to cause havoc.

Chancer wrote:What are you doing with these r?sistors? I hope that you are not connecting them in series with the input trigger wire.

Its been a long time since I was involved with vehicle electronics but I think if a resistor is needed it should either be a pull up or pull down resistor, try a 10K one connected between the trigger input and 12v or the stabilised voltage if the tach is run from that, if no better then try it between the trigger input and ground.

That depends, as I said above if there is an internal resistance to ground from the trigger terminal inside the tacho, forming the bottom end of the voltage divider. In the context of the Tacho signal it is not in use as a pull up or pull down resistor.


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Most Smiths tachometers do not need to be fed from a 10v stabalised supply, the standard circuit inside includes a crude 9 volt regulator (a Zener diode) so I presume anyone converting or designing a DVC tachometer will have included an internal supply-voltage regulator.

You might not be giving it enough supply voltage.

Yes the first test I would do would be to remove the 10V power supply and connect it direct to the car 12 V normal power. I would be very surprised if it had been designed to run to run off a 0 V solid state regulated supply and this may be causing the issue with how it responds on power on or off.

cheers
Rohan

I see now Bill why you suggested a resistor. That will be relevant to the Spyda circuitry but not necessarily to other designs of which there are many.
I think the diode stops AC signals reflecting back up the wires, i.e. pulses going the wrong direction. Ignition systems are very 'noisy', hardly surprising when you think what they are doing. Rule 1 of the radio transmiiter builders code is 'No spark transmitters allowed!!'. That's because they are too 'dirty'
As with all these things, good clean connections are important.
Glad to hear it's all resolved.
Regards
Richard

Richard, As you say it must have been picking all sorts of background interference up.

Hi folks,

hope you don't mind, I thought I'd share the process behind recommending the diode / capacitor that Chris described, It might be interesting for some, or mind numbingly boring for others !

I designed the current version of the RVI-RVC conversion last August and many hundreds have been fitted without issue... but.. there were a couple that refused to play nicely and for no apparent reason the needle went to full scale.

I couldn't duplicate the problem on the bench so with the help of the unlucky folks, tried various things to solve the problem, but it wouldn't go away.. Its very difficult to fix an issue if you can't duplicate it !

Then a stroke of luck, one customer reported the problem occurred when sounding the horn !
As luck would have it, I had a pretty good idea that his car had a relay on the horn... and that was the Eureka moment..

The collapsing magnetic field of a relay when it is turned off can generate big pulses on the supply and these can be a negative voltage... Many relays have a flyback diode to prevent these spikes, and certainly it is good practice to fit them in electronic circuits, but cars of the vintage we are dealing with didn't have them.

(other inductive devices such as electric motors can do the same)

I knew I had designed the tachometer board to withstand connecting the supply the wrong way round, but I had never tested for very short duration high voltage (over 60V) pulses as could be generated by a relay.

I set up a cheap relay in the circuit, and managed to duplicate the problem.

Once duplicated, fixing was easy, a diode to conduct any negative pulses to ground, and a capacitor to smooth out any pulse that is too fast for the diode.

The same components will fix similar problems on other instruments, but will not fix all needle jumping problems !

To make it easy to source the parts needed for this, I will send them out pretty much at the cost of the parts, postage and packing.

paypal ?2 to [email protected]" target="_blank" target="_blank" target="_blank" with a note explaining what it is for..

or the parts from maplin are QL73Q and KQ70M

the line on the diode and the + wire of the capacitor go to the +12 input on the tachometer
the plain end of the diode and the - wire of the capacitor go to the grounded screw on the tachometer case.

It would be fine to connect these to the source of the interference, but easier to pop them on the back of the tach.


Chris (-=Spiyda=-)

Thank you, Chris.

Strange that the interference got through the 10v stabalizer.

I'm surprised it would work at all if it gets its power off the traditional stabiliser. That is only intended for slow responding guages e.g. hot wire ammeters, and works by cycling the full battery supply on and off. More an 'average meter' (if that's a word or phrase) than a stabiliser So for some of the time there is no supply at all, but when it's on it would pass through everyting including the interference. Electronic stabilisers are different, I don't know their details, but they could, in theory, produce a smooth 10 volt supply, or be what's called switched mode where they mimic the traditional type, but probably at a much higher frequency. Car electrics are very hostile environments, as well as the ignition radiating pulses we have dynamos, alternators, relays, various electric motors, horns - all having a go at it. If you try troubleshooting 2 speed wipers or testing the voltage at the horns, using an analogue volt meter, you will see what I mean. Strange voltages all over the place.
regards
Richard

ricarbo wrote:I'm surprised it would work at all if it gets its power off the traditional stabiliser.
Richard

I think Bill was referring to one of the linear solid state voltage regulators that I supply for the smaller gauges.. they give a regulated steady 10V, at the expense of being less efficient than the old bi-metal ones.

The strange thing with the problem discussed here is that the circuit was designed to cope with the environment was extensively tested and that the problem only cropped up after probably 600 successful installs and then only on a handful of cars. The pulses in question are certainly over 60V, but of such short duration they are hard to quantify on my Oscilloscope.. probably in the region of a couple of hundred volts negative ! I certainly wouldn't want my phone plugged into a phone charger in the cigar lighter socket on a car with those kind of pulses !

The next version (probably 2016) will include a flyback diode to cover those few examples...

Chris (-=Spiyda=-)

Grizzly already said it was a solid state regulator.

Grizzly wrote:Yes sorry, its been rewired off the -ve side of the coil, the main Tacho power is fed via a 10v solid state Voltage Stabilizer and the ground is presently connected directly to the Battery, the two white original power wires have been connected together to bypass the Tacho and a fresh signal wire has been fitted. The car its self has a 12v Ignition system (- Neg and the coil is a solid 3.0 ohms after removing the cable resistance) and its run without ballast on a Shiny new Optronic Ignition (which did actually help as you can see from the video's above)

As you say, it would not work at all sensibly with the older vibrating regularor. That produces square 12v pulses of either variable width or variable frequencey to produce an AVERAGE of 10 volts out, suitable only for slow instruments.

Hi,
For those of us electronically inclined, may like to add any extra info here to their knowledge base.
http://www.accutach.com/smiths-tach-cal ... background
Ron.