Sparky Coil

[Simon.b]

I just cant get the Griff to run properly, ever since I had the original coil failure a few weeks back.

Just been working on her again tonight and now the latest (Bosch, new last week) coil is sparking from the central post to the negative terminal. The engine is running OK ish, she starts and ticks over no problem, there is a slight misfire, though not like running on seven. But now the coil is sparking while the engine is running, big strong blue ones.

Anyone seen this before ?

[GreenV8S]

Is the body of the coil earthed properly?

[Simon.b]

I’m using the standard mount, which looks OK, though I didn’t think the body needed to be earthed.

Thinking about it I’ve re-run all the HT leads so that they don’t touch any thing metallic or run close to any other wiring, so there’s no easy ground if a lead, adapter or plug fails. So may be that’s it and I just need to find which one.

[60maxx]

simon, I suspect the fault is not with the coil but a higher resistance between the HT coil towards the spark plugs eg. - king lead - dist. cap - rotor arm - HT leads - spark plugs , resulting in the sparks you can see.
Just remember that electrical energy takes its easier path to earth - on one part of your seconday ign system (at that instant) it just happens to be between LT connection at the coil and the HT tower.
Fault diagnosis by a competent tech. with an oscilloscope should help identify the precise fault.
hope this helps.
regards Dave

[rev-erend]

It's probably the same fault that caused your original coil to fail.

If for instance the plug gap is too big then the spart energy will have to earth somewhere.. I would make a careful visual inspection of all the components :

Low tension leads, HT leads, rotor arm, cap and it's carbon button, leads and check the plug gap.

Any of those conponents may well need replacement.

[Simon.b]

New Lucas cap and rotor arm are now on order and I’m off to get some new plugs this afternoon.

I have a long copper core lead that I use to swap out suspects and was too tired to check them all last night, but I did try the king lead and it still did it.

What would a typical lead resistance be, I seam to think they were about 10k last time I checked.

[blitzracing]

The lead resistance is dependent on the cable liength, but 10k is a reasonable value. Be carefull with experimenting copper leads, the loss of resistance can double the spikes the ignition amp has to deal with on the primary, so you really need to keep some resistance in the HT system. Gunsons did a brillant HT tester that clamps on the outside of the HT lead, and displays the peak voltage reached. If a plug is firing OK you will see around 12kv at tickover on the lead feeding it (This voltage depends on engine load and compression). In your case it will be much higher on around the open circuit / high resistance area. Mind you cant find one on ebay now !

Mark

[rev-erend]

Someone posted last week that Halfrauds sell a spark plug tester that is about £9 - fits on the plug and shows what gets to the plug..

[blitzracing]

Lots of plug cap type devices that simply have a neon bulb inside, that will show HT is getting to the plugs, but not a voltage measurement but certainly better than nothing. Buy a set of eight and have your own under bonnet disco display! The Gunsons unit is now discontinued, so I must have bought the last one a few months back.

http://cgi.ebay.co.uk/HT-Lead-Ignition-Spark-Teste...

Mark

[Simon.b]

Raked through my tool box and Vola, I’ve got a simple neon tester that I acquired years ago. You just touch the lead and it lights up.

It shows that all the leads are firing but some defiantly have more juice than others. Replaced the plugs, no difference, so I swapped a plug extension from a good lead to a bad one and according to my neon tester the potential fault moved with it.

The extensions all look in good order and measure up OK on a multimeter, but can they break down with high voltage ?

Any way next on my list looks like a new plug extensions.

[blitzracing]

Be carefull with how you interperate the results. A good spark is basically a short circuit, so the voltage across the plug tip will rise untill it sparks, then the voltage falls very sharply. A bright neon is more likley to show a high voltage or open circuit condition, rather than a good spark at the plug tip. You are relying on leakage radiation from the HT lead to light the neon, when it should be firing the plug. The test condition with a meter on the caps is totally different to shoving a 20kv "spike" down it!

Mark

[Simon.b]

I accept what your saying here, but what ever the problem it’s only affecting one maybe two cylinders, the misfire is regular though minor. When I look at all the leads they show a bright neon, with the exception of two and one of these is worse than the other.

So my interpretation is that the voltage is leaking away before it gets chance to spark and hence the dull neon.

Thanks for all this, it really does help. I try and talk to Mrs.b about it and all she says is how much is it this time.

Cheers,

Simon.

[rev-erend]

It might be that the misfire only occurs when certain conditions are met - for instance lean mixtures are more difficult to ignite.

As you only running 1 coil - I would try making the plug gap smaller (say to 0.6mm).

[V8 Animal]

Thanks for all this, it really does help. I try and talk to Mrs.b about it and all she says is how much is it this time.

[Pasco]

V8 Animal said:

Thanks for all this, it really does help. I try and talk to Mrs.b about it and all she says is how much is it this time.

Oi Bugger off back to the Golf Section Paul

[blitzracing]

Simon.b said:

I accept what your saying here, but what ever the problem it’s only affecting one maybe two cylinders, the misfire is regular though minor. When I look at all the leads they show a bright neon, with the exception of two and one of these is worse than the other.

So my interpretation is that the voltage is leaking away before it gets chance to spark and hence the dull neon.

Thanks for all this, it really does help. I try and talk to Mrs.b about it and all she says is how much is it this time.

Cheers,

Simon.

This is a fair assumption if you are measuring after a very high resistance or open circuit point, or as you say a lead is tracking to earth. If you put the neon before an open circuit point, id expect the neon to be brighter than the other leads. I guess it would be interesting to start at the coil with the neon, and simply follow it down the HT lead (s)and observe the intesity. I would think the bit that makes the neon fire is the rise in voltage before the spark ignites, so at this point as the plug is open circuit, so the resistance of the leads should really makes little difference unless its fully open circuit or tracking to gound. So logically speaking tracking to ground - dim all the way on the relevent cylinder. Bright then dim, open circuit at the point of change. Difficult to be exact with HT, as it can track to start with, then stop tracking as the voltage drops, an a neon screwdriver is not an exact science but it would be an interesting experiment in this case. Post the results !

Mark

Edited by blitzracing on Friday 22 August 20:37

[Simon.b]

At last all sorted and ready for the BNG08

The first problem was defo the coil that failed, she was virtually un-drivable with violent shunting. I suspect it may have been the No 1 lead all along that caused it to do so.

After initially changing the coil and still having a minor misfire I checked all the leads which with the dissy cap and arm where only two years old and they seemed OK. However after replacing the coil again, for a Bosch and the Lucas ignition amp the problem had not gone away and the coil king terminal was now arcing to the coil neg terminal.

Comparing each lead, using a neon, with the others showed two dull and one bright. The two dull ones turned out to be the plug extensions which were also shorting and leaking high voltage, looking cafully I could see them glowing blue in the dark. After replacing the extensions, dissy cap and rotor arm the coil was still arcing occasionally, No 1 lead was still bright, but the car drove OK with just the hint of an occasional misfire.

New 8mm silicone leads on tonight and that's it, she’s running sweet as ever. I suspect that the lead was unable to pass the high voltage adequately and since I’d cleaned and re-routed all the leads there was nowhere for the current to go hence the arcing on the coil and the bright neon light due to the excessive voltage build up which must eventually have got high enough to occasionally fire the plug, which is why it didn’t show on the ECU fault code reader.

[blitzracing]

That ECU is not that smart, it takes a significant period (I think about a minute) of wrong lambda readings to flag up a fault code, so a simple misfire wont get picked up, as long as the plugs fire sporadically !

Mark