Lotus Elan

Odd Rev counter jumping?

PostPost by: Grizzly » Mon Feb 23, 2015 10:29 pm

Hi all.

I'm just helping a friend with his S4, he's fitted an Accuspark ignition which meant his rev counter stopped. So to fix this he's bought a RVC Smiths Tacho of Ebay (think he said it came from a Scimitar Se6) then swapped the faces over, we just finished installing the new Tacho and it seems to work ok but when you start it the needle flicks round to max revs at idle and after a couple of seconds it drops down to display the correct RPM then when you turn the ignition off it flicks to max rpm again before turning off. So some thing is not right when it first starts and shuts down.

Its really odd tbh and knowing the car worked fine before the Ignition was fitted kind of points in that direction but at the same time the dash has been out so maybe disturbed some thing behind there?

Has anyone come across this before?
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PostPost by: types26/36 » Mon Feb 23, 2015 10:38 pm

Wasn't a Scimitar Se6 a six cylinder car? whether fitting it to a four cylinder car would cause the problems I don't know but I would expect it to be inaccurate at least.
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PostPost by: Grizzly » Mon Feb 23, 2015 10:50 pm

Sorry my mistake, i remember him saying he was going to get a SE6 Tacho but just spoke to him and he didn't know what it came off as it was bought through Ebay (8000rpm red line RVC), the face didn't look anything like an Elan Tacho it was quite plain and the unit was inside a case almost like it came off a bike?

I'll find out more about it tomorrow but it a bit late now.
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PostPost by: billwill » Tue Feb 24, 2015 12:31 am

He probably needs a resistor in the signal pickup lead.

The signals are probably too big.
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PostPost by: Grizzly » Wed Feb 25, 2015 7:37 pm

My mate has just called round in his car to show me the progress, he's fitted a Lumenition Optronic Ignition and taken the Accuspark off and its made quite a big difference, it's now working more or less correctly (think it needs calibrating) but where it was Flicking over the max revs when you start and stop the car (was holding at max revs for 2 or 3 seconds before) now it still flicks to max but drops straight away and turn off is as it should be.

I've just checked the Ohms on his coil and its bang on 3.0 ohms which as far as i'm aware is about right for a 12v set up. As he rev counter only plays up when cranking now he's got it in his head its a bad ground but i don't know it seems to turn over and start fine so it would be a surprise if it was.

Bill, What sort of Resistor would you recommend trying? i think at this stage he's just changing things for the fun of it hoping to stumble on the problem.
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PostPost by: ricarbo » Wed Feb 25, 2015 10:10 pm

The rvc type reacts to voltage pulses. Therefore the first obvious answer is a poor quality ignition switch, which doesn't switch on cleanly, but 2 or 3 seconds seems very odd. If you jiggle the key in the switch, perhaps you can reproduce this. As it only happens on cranking, how about disconnecting the starter lead, then turning the key and seeing what happens? I can't see how a resistor might help, as it's counting voltage pulses, not current. It has a high impedance, so a resistor would also need to be of a high value to affect the pulse much. As a pure guess perhaps 100 k ohms, but you could try 100 ohms, then 1000, then 10,000 etc. I doubt it will help. I saw a sticky contact breaker problem last year which was producing sparks all over the place (hardly relevant, I realise). At switch on you should have no response at all, The trigger voltage comes from the change at the breaker points or the electronic ignition, via the sensor lead. Nothing else should cause a reaction. Does it work properly with a contact breaker ignition as a base line to rule out the electonic ignition? Or on the bench with a switch for the supply and a 'dithery' hand connecting the trigger lead to plus 12 volts as fast and intermittently as you can? i'm sure you get the idea.
good luck
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PostPost by: Grizzly » Thu Feb 26, 2015 1:51 pm

I went round to see if he had any joy with it and i'd say that's a man with his mind well blown.

Thanks Richard for the suggestions, we tried disconnecting the starter lead and turning the key, wobbling the key then bypassing the ignition switch which didn't seem to replicate the effect on the Tacho, unfortunately neither of us have a stock set of point to try but i think that could well be on the cards.

Whats also a bit odd, i tried my brand new 12v Lucas gold coil on his car and it made the problem worse, after swapping back to his coil the problem was not constant, first time it started it was flicking right over to the Max revs then it would start a couple of times where it would only flick to 4k and even one time it only made it to 1k, So thinking it could be a bad connection we replaced all the spade connectors, took the connectors off the coil and gave that a good clean but it didn't seem to make much difference.
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PostPost by: billwill » Thu Feb 26, 2015 3:01 pm

Re the reduction of the signal with a resistor.

Try 100K ohms as a starter. Or use a 100K variable resistor and turn it down until the circuit works OK, then measure it ans substitute a fixed resistor.

This was discussed before in respect of the Current-sensing to Voltage sensing conversion of a tachometer. See:
lotus-electrical-f38/rvi-rvc-tacho-t31294.html

See the images of Spyida instruction sheets on that topic.
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PostPost by: Grizzly » Thu Feb 26, 2015 3:44 pm

Just been sent these two video's that i've uploaded. Please ignore the RVI on the face as that's been swapped onto the RVC internals.

This is RVC with Accuspark ignition https://www.youtube.com/watch?v=5t85im2 ... e=youtu.be

This one is RVC with lumenition optronic https://www.youtube.com/watch?v=bg2yI_K ... e=youtu.be i've just watched this a few times and noticed the Ignition light dims when the Tacho plays up?? but we have tried running it without the Alt plugged in and its the same?

Both using a Lucas 12v coil fresh plugs and Leads with the Tacho being fed from the battery direct (so clean power and ground) and a fresh wire from the coil to the Tacho as sensor wire.
Last edited by Grizzly on Thu Feb 26, 2015 3:55 pm, edited 2 times in total.
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PostPost by: Grizzly » Thu Feb 26, 2015 3:46 pm

billwill wrote:Re the reduction of the signal with a resistor.

Try 100K ohms as a starter. Or use a 100K variable resistor and turn it down until the circuit works OK, then measure it ans substitute a fixed resistor.

This was discussed before in respect of the Current-sensing to Voltage sensing conversion of a tachometer. See:
lotus-electrical-f38/rvi-rvc-tacho-t31294.html

See the images of Spyida instruction sheets on that topic.

Funny thing is i have a Spyida kit fitted to my Elan and its much less agro than this, that actually might not be a bad idea to eliminate the Tacho (swap it as i know that works)
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PostPost by: Grizzly » Fri Feb 27, 2015 4:12 pm

I was asked this but want to make sure i have it right, if you put a Multimeter from the battery to the Engine block (get a good a low Ohm reading) then crank the engine with the gauge still attached and the reading shoots up until the Starter is turned off at which point it drops back to more or less the original Ohm reading does that point to a bad ground? I thought it does, but having said that i went and checked mine knowing i have a ground from the battery through the chassis to the starter and it does go up when you crank the engine which i wasn't expecting?
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PostPost by: ericbushby » Fri Feb 27, 2015 9:15 pm

Chris, no, I think that is wrong.
If you connect the ohmmeter from the earth side of the battery to the earth side of the engine you are effectively making the return current path slightly bigger and some current will flow down the new wiring through the meter. It has too, following the rule of parallel resistors.
No matter how good the existing earth return is, some current will flow through the parallel path. Even a few milliamps may cause a reading on the meter depending on its sensitivity.
A better way to check the earth return would be to measure the volt drop between engine and battery when cranking. There has to be some volt drop there because even a large copper cable would have some resistance never mind a steel chassis.
I have not done that so I do not know what would be considered normal.
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PostPost by: billwill » Fri Feb 27, 2015 11:32 pm

Grizzly wrote:I was asked this but want to make sure i have it right, if you put a Multimeter from the battery to the Engine block (get a good a low Ohm reading) then crank the engine with the gauge still attached and the reading shoots up until the Starter is turned off at which point it drops back to more or less the original Ohm reading does that point to a bad ground? I thought it does, but having said that i went and checked mine knowing i have a ground from the battery through the chassis to the starter and it does go up when you crank the engine which i wasn't expecting?



Oh dear...

Assuming you mean from the Earthed terminal of the battery to the block:

That could damage your ohm meter if the earth continuity was really bad.. The increased reading isn't the ohms going up it is the volt drop of the earth cable feeding back into the ohm meter & giving a false reading.


You fed some volts into it when it was switched into a mode using its internal battery and that expected to be connected only to unpowered equipment!

~~~~~~~~~

Wildly changing anything is not going to get you (or your friend) anywhere, you need to be systematic and analytic.

1. Ignoring the tach, is the engine starting and running OK? if so don't change anything else on the main ignition circuit. Coil, contact breaker, ballat resistor (if any).

2. The problem will be something to do with the tacho itself. In particular the signal feed from the CB end of the coil to the trigger terminal of the tacho. It is likely that on the car it came from there was either a feed-out from its electronic ignition box (if any) or some sort of filter circuit between the coil and the trigger.

3. The simplest filter as mentioned above would be to reduce the magnitude of the signal by inserting a series resistor. This assumes that there is an existing resistance inside the tacho to provide the 'bottom end' of a resistive voltage divider.

4. Other possible filters might include a series capacitor so that only the edges of the pulses from the coil are received by the tacho.



~~~~~~~~~

Most voltage triggered tachos would work by detecting each trigger pulse and then ignore all but the leading edge of that pulse using that edge to generate one nice short fixed length internal pulse per trigger. The rest of the circuit then uses the short pulses to feed into a capacitor that is deliberately 'leaky' by having a resistor accross it. The effect is that the voltage across that capacitor averages the pulses and so the voltage is proportional to the rate of the trigger pulses. This voltage is then fed to the dial meter to produce a deflection, which is now proportional to the rate of trigger pulses & hence RPM.

The symptoms you describe would occure if the internal circuit was not triggered by the EDGE of the signal pulse but by its magnitude. When you switch on the ignition the engine is stopped or spins slowly on the starter motor. so the pulses from the contact breaker are looooooong. So it would make looong pulses in the internal circuitry and the voltage across the 'leaky' capacitor would rise to maximum and then take a long time (seconds) to leak away. Meanwhile the pointer would be hard over in the max RPM indication.

You can only check for this by inspecting the internal circuit of the tacho and/or by testing/calibrating it using a controled signal source, such as a signal generator and it hels if you can see what is happening by using an oscilloscope.


In developing their conversion kit Spyida designed a neat way of using a computer as a signal generator by using the audio output (speaker/headphone) socket and making the computer play specific .WAV files which simulate clean trigger signals.

~~~~~
I would be inclined to take the Tacho out of the car and check its characteristics on the bench.
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PostPost by: elansprint » Sat Feb 28, 2015 12:55 pm

As the elan uses an RVI tachometer and you have changed to RVC you have different wiring you just take the signal wire to the -ve (contact braker) side of the coil .
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PostPost by: billwill » Sat Feb 28, 2015 2:36 pm

elansprint wrote:As the elan uses an RVI tachometer and you have changed to RVC you have different wiring you just take the signal wire to the -ve (contact braker) side of the coil .
Ian



Oh, true , Chis hasn't actually described in detail how they wired the new tacho into the circuit. I for one certainly assumed that they had already done what Ian describes.
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