Club 80-90 Forums

Had a good poke around the coil with my multi meter today while running and nothing more deadly than 12v was showing ac or dc between neg and ground, pos and ground and neg and pos, also tried cranking with coil neg disconnected and could not find any high voltage

when you think about it the tacho connection is the coil neg on later vans goes via the fusebox to the dash so 400v here would be a hazard, i'm sure ze germans would have made a warning sticker

anybody else got any wisdom on this

I'm finding all this quite interesting as I've been looking into rev counters (and the circuits that are used in them).
It seems that at the point of the coil producing the high voltage for the spark, as the ground wire is 'broken', the magnetic field collapses and in doing so it induces a voltage in the primary coil in the order of a few hundred volts (but usually with zero current - as the ground path is disconnected). This high voltage lasts for a very short time (which might account for you not being able to measure it - I'd think that you'd need to use an oscilloscope).

I found this web page quite interesting on the subject .....

http://www.picoauto.com/automotivetopics/primary.html

Thanks i have acess to an osciloscope and i was thinking that would be worth a go

i still cant help thinking that this will only happen as a result of coil switching and wont happen if the neg is permanently disconnected for secuirity as was the original question on this thread

MidLifeCrisis wrote: It seems that at the point of the coil producing the high voltage for the spark, as the ground wire is 'broken', the magnetic field collapses and in doing so it induces a voltage in the primary coil in the order of a few hundred volts (but usually with zero current - as the ground path is disconnected). This high voltage lasts for a very short time (which might account for you not being able to measure it - I'd think that you'd need to use an oscilloscope).

Yay someone who can work it out.
Yes you can only get the 300 odd volts in a 12v system if it is induced and yes you will only see it on a scope.
This is a trace showing how it should be.



Having a switch in the circuit means there will be high voltage to one side if the switch when it is open, any condensation inside the switch can cause flash overs leading to miss fires. Rev counters can be a problem as well, any diagnosis for misfires always involves disconnecting the rev counter.

itchyfeet wrote:i still cant help thinking that this will only happen as a result of coil switching and wont happen if the neg is permanently disconnected for secuirity as was the original question on this thread

bigherb wrote:Having a switch in the circuit means there will be high voltage to one side if the switch when it is open, any condensation inside the switch can cause flash overs leading to miss fires. Rev counters can be a problem as well, any diagnosis for misfires always involves disconnecting the rev counter.

OK - so I think we are arriving at me understanding something (finally).
So, if the engine is running and the isolator switch was switched 'open' then there is a chance that the high voltage could jump the switch contacts (and that could potentially be dangerous to fingers near the switch)
But if the switch is open before the engine is started then the field in the primary of the coil will never be able to develop, and therefore never be able to collapse and create the large voltage, so that would be safe?
Also, I suppose the wiring from the switch to the coil would be a potential hazard if it was frayed and came into contact with you? But this would seem to be a small risk as the wire ground path would be much lower resistance than the person ground path?

So what's the correct solution - isolate the positive supply to the coil??

^^^^ thats what i said a page ago, when the switch is open the coil never gets energised, that high voltage is only created by the act of switching the coil off by the points or ecu which wont happen if it was never switched on

but i'm still not clear because if i can remember my ac theory you need an inductance and a capacitance to create that ringing, and you need that ac waveform to make a transformwer work so i know the coil is an inductor so where is the capacitor? Old points had a condenser which i think is the cap, if the cap is in the ecu on these ignition systems then is it pre charged cap system or not?

itchyfeet wrote:

bigherb wrote:You shouldn't connect any switch in the negative side of the coil you can have over 300 volts in that cable , and especially not one that earths the negative side as a immobiliser that's very dangerous it can cause the coil to explode.

when its permanently open circuit that wont be happening as that will only happen when switched and when its closed circuit the voltage wont be across the switch but across the ecu so i cant see the problem

http://en.m.wikipedia.org/wiki/Ignition_coil" onclick="window.open(this.href);return false;

agreed dont earth the coil directly

MidLifeCrisis wrote:
So what's the correct solution - isolate the positive supply to the coil??

we are not alloed that information, it was sent by pm, we will have to work it out for ourselves

MidLifeCrisis wrote: OK - so I think we are arriving at me understanding something (finally).
So, if the engine is running and the isolator switch was switched 'open' then there is a chance that the high voltage could jump the switch contacts (and that could potentially be dangerous to fingers near the switch)

Not really dangerous that was Itcys interpretation it might give you a tingle. The high voltage is needed to boost the voltage in the primary windings giving a better spark, any leak off that voltage due to poor insulation will cause miss fires due to a weak spark. Thats why rev counters can cause miss fires some of the cheaper ones fitted to electronic ignition need a diode in the live side to stop the high voltage finding a path back to a lower potential.
It is dangerous if the negative side is earthed and the ignition is left on without the engine running. Then the windings become a heating element, heating the oil inside the coil which then becomes a pressure vessel and usually blows the top off, hence coils are mounted with the top pointing away from anybody working on the engine.
Electronic ignition have a current limiter to stop this happening if you put a test light on the neg side of the coil and switch the ignition on the light will be dim then within 1 sec will be bright as the current regulator switches off the primary windings.

You can get a right kick off any coil (not just ignition coils) with enough turns, even off a 1.5 volt battery.

If you get a mains step down transformer out of say an old alarm clock radio and put a 1.5 volt battery across the mains input, if you touch one of the transformer terminals as you take off one of the coil connections (doesn't matter whether +ve or -ve) you can get a "belt" that hurts a bit and jerks your hand. I know this from experience. I first "discovered" this at the age of about 7. I then used it to advantage by incorporating a mechanical buzzer into the circuit to make a bedroom burglar alarm that would give my brother a shock (off the door handle) if he tried to enter whilst I was not there In some circles this is known as a "trembler coil" and was used as a source of sparks for firing up jet engines etc (and still is for model jet engines). Yep I started my career in electronics early

Oh, back on topic, if I wanted to do this, I think I would use a relay with the feed from the (permanently) live stud in the black box and a switch in the return line to earth. The relay contacts would be in the live feed to the ignition amp.

Seems a far too academic way to tackle this though. Why not just interrupt the power to the coil altogether (ie positive side). If its not working, its not working. I can't see anyone going to the trouble of poking around to see WHY it won't start if they're about to steal your pride and joy. They'll just give up and try something easier. Keep it simple. After all, the engine bay isn't the easiest thing to get to for the opportunist and certainly not in a hurry.

You can get a back emf as the field collapses in the +ve side just the same as in the -ve side I agree though, this is all getting a bit academic !

CovKid wrote:Seems a far too academic way to tackle this though. Why not just interrupt the power to the coil altogether (ie positive side). If its not working, its not working. I can't see anyone going to the trouble of poking around to see WHY it won't start if they're about to steal your pride and joy. They'll just give up and try something easier. Keep it simple. After all, the engine bay isn't the easiest thing to get to for the opportunist and certainly not in a hurry.

I agree on reflection switching the positive is the way forward, switching the neg is a bad idea
Sorry we got way off topic, still interesting to know there is something other than the secondardy on the ignition that can bite, or at least nibble a bit

Also worth noting the ignition positive to the coil ( black wire) is not fused so if you run a cable anywhere and back your wiring needs to be 100% or add an inline fuse to the black box or you could fry your wiring with a short.

just pull the HT lead off it, put it in ur pocket who's gonna nick a T25 , unless he's a misery rider lol

Rotor arm was a good one on a type 1 engine if in a hurry but deny positive connection for coil in the T25 and thats that - its going nowhere without major surgery.

However, its important to make a GOOD job of it or you could find yourself losing all power suddenly on a motorway if a connection comes adrift. Not fun