Lotus Elan and Lotus Elan Plus 2 Forums

Trying to see why my tach jumps around, I put a little scope on the distributor side of the coil (the white wire that runs through the tach), and get this messy looking trace:


The vertical is 10v/division, so you can see where it flattens out at 12V, but I have those huge messy spikes going off the screen when the points close and the pulse starts (top of screen is 60v). It would make sense for that to be making the tach act skittish.

Is this normal? Assuming not, what is it a symptom off? Bad condenser in the distributor? I do not have that metal can "supressor" connected at the switch side of the coil (it is supposed to be a 10 microfarad 500V capacitor to ground, I am still searching for one), but I thought that was only for radio noise. Might it also help to control this?

That kind of wave form is normal for ignition. Well the middle one is. I'm not sure about the ones with the downward spike.


See the article referred to in the first message of this topic:
lotus-twincam-f39/using-oscilloscope-for-engine-diagnostics-t20967.html?hilit=ignition%20scope

billwill wrote:That kind of wave form is normal for ignition. Well the middle one is. I'm not sure about the ones with the downward spike.


See the article referred to in the first message of this topic:
lotus-twincam-f39/using-oscilloscope-for-engine-diagnostics-t20967.html?hilit=ignition%20scope

Thanks. Any idea what else could cause a "skittish" tack, where the needle is not steady? Little random movements, with twitching around the reading?

For ages I thought the condensor across the distributor points was merely a suppressor and contact burn limiter, but discussion about ignition circuits on this forum last year or the year before made me realise that it is an essential part of the system. It causes the coil plus capacitor circuit to oscillate (ring) in a decaying high frequency 'sine wave', which is essential for a good fat spark.

Misbehaving tacho is common when electronic ignition is fitted: often cured by making sure that the 12v power line to the electronic circuit is NOT routed through the white wire, which would add various other pulses on the wire.

If no electronic ignition, I suspect that capacitors inside the tacho have faded well out of spec, so the 'smoothing' circuit is not converting the above waveform into one pulse per spark, but occaionally several.

Has above the spikes are normal it is the back emf as the current collapse in the coil when the points open
Ian

billwill wrote:Misbehaving tacho is common when electronic ignition is fitted: often cured by making sure that the 12v power line to the electronic circuit is NOT routed through the white wire, which would add various other pulses on the wire.

If no electronic ignition, I suspect that capacitors inside the tacho have faded well out of spec, so the 'smoothing' circuit is not converting the above waveform into one pulse per spark, but occaionally several.

This is sounding likely, I guess I will delicately poke in there and see what I can refresh. Yes, I have that kind of supidity/nerve (choose one).

elansprint wrote:Has above the spikes are normal it is the back emf as the current collapse in the coil when the points open
Ian

But the messy bits are on the rising voltage when the points close. I can see the coil inductance "fighting back" against the current increase at that time, while when they open it collapses cleanly.

Hi JG

I think there are a number of reasons why your waveforms look odd and suspect there is not a real problem ( with the ignition at any rate ) when the points open , close, and the spark occurs at these 3 time points current around the circuit changes rapidly -- this can induce voltages in what might be thought of as "0V or earth" connections. Depending where the scope probe earth or 0V is connected this can give the appearance of some ringing as well as appearing to produce a voltage that seems to go in the other direction. the leading edge of the first pulse seems to show this effect.

The capacitance of the hIgh voltage plug leads determined partly by how long they are and how close to earthy stuff, it is not unusual to see each cylinders pulse looking a bit different. Rarely are all four plug lead identical in layout. Also in the dizzy the volts have to jump the dizzy gap as well and they may be variation here

Not sure what coil you have but they typically step up the voltage around 50 to 60 times so to get say 10000 volts, (probably enough volts at fast tick over with a high vacuum) at the secondary probably means around 180V at the primary. So your scope vertical amplifier system would be overloaded. Depending upon the scope how it responds to this overload may give false readings AFTER the high voltage pulse has occurred.

From your waveform it looks though the points closed period is around 2/3 of the time between pulses and this seems about right for a 60% or so dwell.

Regarding the tacho if you have an RVI type with the inner trigger loop ( when you open it up if there is only a single transistor then it is) then it is probably the capacitor across the base - emitter of the transistor. I seem to recall a 2.5uF at 10V tantalum or electrolytic will substitute for it OK. When mine went iffy I changed this capacitor and it is now OK.

Hope this helps best of luck

Bob

bob_rich wrote:Hi JG

I think there are a number of reasons why your waveforms look odd and suspect there is not a real problem ( with the ignition at any rate ) when the points open , close, and the spark occurs at these 3 time points current around the circuit changes rapidly -- this can induce voltages in what might be thought of as "0V or earth" connections. Depending where the scope probe earth or 0V is connected this can give the appearance of some ringing as well as appearing to produce a voltage that seems to go in the other direction. the leading edge of the first pulse seems to show this effect.

The capacitance of the hIgh voltage plug leads determined partly by how long they are and how close to earthy stuff, it is not unusual to see each cylinders pulse looking a bit different. Rarely are all four plug lead identical in layout. Also in the dizzy the volts have to jump the dizzy gap as well and they may be variation here

Not sure what coil you have but they typically step up the voltage around 50 to 60 times so to get say 10000 volts, (probably enough volts at fast tick over with a high vacuum) at the secondary probably means around 180V at the primary. So your scope vertical amplifier system would be overloaded. Depending upon the scope how it responds to this overload may give false readings AFTER the high voltage pulse has occurred.

From your waveform it looks though the points closed period is around 2/3 of the time between pulses and this seems about right for a 60% or so dwell.

Regarding the tacho if you have an RVI type with the inner trigger loop ( when you open it up if there is only a single transistor then it is) then it is probably the capacitor across the base - emitter of the transistor. I seem to recall a 2.5uF at 10V tantalum or electrolytic will substitute for it OK. When mine went iffy I changed this capacitor and it is now OK.

Hope this helps best of luck

Bob

Thank you Bob, very clear explanation. Very cheap little scope (half the size of a phone), so I do only take it's waveforms to be a very general guide. I will be pulling the tach tonight.

This tread turned out to be very informative for me, I never thought about the capacitor interacting with the inductance of the coil on the rising signal, makes great sense.

My recently converted Electronic Ignition Rev counter does exactly the same jumping about, come to think of it so does the speedo, ( that's just been overhauled as well!)

Regards,

Phil

Speedo jumping about is almost always a bad speedo cable. They can look OK & spin ok by hand but still cause the jumping about.

Could also be a minute speck stuck to the magnetic disc..

John

I think you have a tacho problem, not an ignition problem. I also think Bob Rich is on the money concerning a failing capacitor. I think you should bear in mind the electronics of the original tachos are now very old and have lasted longer than it is perhaps reasonable to expect. When i got my elan I added electronic ignition, principally to eliminate the high maintenance issues of the points in the standard set up. This stopped the revcounter working, so I revised the electronics of that - No more problems. There are various options here, but I suggest you junk the original circuit for something new. Well worth the effort, in my view.
Regards
Richard

Sorry John
No magnetic disc! Speedo?s use the Eddy current principle, there is a magnet involved but it?s in a bar form.
Most likely cause of problem is, as Bill, with the cable.
Ron.