Scope traces of secondary ignition voltages on Ferret

I was just wondering if anyone was familiar with scope traces taken from the coil wire of a contact breaker ignition system, in this case a Ferret. The ones I took are showing a lot of "noise" on the spark line part of the trace. I can't figure what is causing this.

The first photo shows the typical messy trace. This happens on most plug fires on all cylinders. The second shows the occasional "clean" trace that pops up.

This Ferret idles well and runs strongly. I was just being curious. I shouldn't have looked. I found references to lean running or a bad condenser having this effect on the spark line, so I richened the mixture, and swapped the condenser, with no improvement.

I looked at another Ferret and the traces were all much cleaner, so I don't think it's my equipment or technique.

Malcolm

Where you went wrong was fitting heigh tech diagnostic equipment to a ferret

I eagerly wait responses from some of our more knowledgeable members.

Cheers,

Terry

I know, Terry. I bitterly regret it.

But there were a lot of those Sun ignition analyzers around when points ignitions were all the rage so I'm hoping someone can shed some light.

Malcolm

Malcom interesting stuff indeed. Can you post a higher resolution image I can't read the text. But my reaction is to suspect the capacitor. Is the engine running nicely? Is there any carboning on the plugs? Is this happening from cold or only after running a while?

When Crypton engine analysers were introduced in REME workshops, we had all the adaptors for screened ignition and showed up all sorts of things, very misleading. You could have a breakdown in the insulation on HT lead from coil. Also be aware of the effect screening has on HT.

Malcom interesting stuff indeed. Can you post a higher resolution image I can't read the text. But my reaction is to suspect the capacitor. Is the engine running nicely? Is there any carboning on the plugs? Is this happening from cold or only after running a while?

If you click the images you get a large version, which is quite readable.

Malcom interesting stuff indeed. Can you post a higher resolution image I can't read the text. But my reaction is to suspect the capacitor. Is the engine running nicely? Is there any carboning on the plugs? Is this happening from cold or only after running a while?

Clive, I had a new automotive condenser in there (i.e. not Ferret-specific), so as part of my troubleshooting I did change to a NOS Ferret condenser I got from the UK recently. It made no difference to the noise on the spark line.

I did get some strange readings from the NOS condenser through, only 0.5 megohms at 250V on a megger (the automotive condenser was in the gigohm range), and 0.6 microfarads on my DMM, compared to 0.25 microfarads for the automotive one. I believe these condensers are the electrolytic type. Could the NOS one have dried out?

The engine runs well, with perhaps a slight misfire on idle (it's hard to tell with the popping from the exhaust sleeves). It runs strongly under load. Compression is 125 to 130 psi on all cylinders. But it burns oil pretty badly. It smokes badly and pulling the plugs after extended idling while taking traces showed wet oil on 4 of them. One was slightly oily. The last was dry but sooty. I ran them through the spark plug tester (without cleaning) after taking the traces and they all sparked strongly at 140 psi air pressure.

This happens cold and hot. It doesn't seem to matter.

The text is just my ramblings. Here it is:

"Coil lead at distributor. Idle, around 700 rpm.

Clamp-on probe was not fully closed due to protective sleeve around wire from coil, so kV values may be low, as maybe probe has to form conductive ring around wire. kV values certainly are low compared to individual plug wire traces, which showed very high firing values, around 40kV.

This coil wire trace shows the classic oscillations when the spark extinguishes, and also the classic oscillations when the points reclose. For some reason, the traces taken from individual plug wires don't show this stuff. Maybe dampened out by having to jump across rotor gap? So large jump up seen at end of spark line for individual plug wires is the first part of the oscillation.

Trace still shows all the messy stuff all along the spark line, also seen on individual wires. I richened idle mixture to see if it was due to lean running. No difference. I replaced condenser with NOS one from Marcus Glenn. No difference. I don't know what causes those glitches on the tan Ferret. Some spark lines don't have glitches over the whole line. In fact a few are clean.

Note zero offset. It's about 2.4kV, pretty large."

Clicking on the image makes it a bit more legible.

Malcolm

Hi Malcolm,

It sounds like you know what you're talking about and I certainly don't want to tell you how to suck eggs, but high oil usage with good compression usually points towards valve seals. Have you thought of renewing those? The B series has a reputation for high oil usage, but my B80 was rebuilt in 1981 or so and uses almost no oil. I know that at some stage during their lifecycle, they changed the type of rings used and I believe that some also had different bore liners.

Unless you're dead set on originality, get yourself a Jolley Engineering breakerless ignition kit.

http://www.classicheads.com/Electronic_Ignition.html

Clive has also produced an excellent guide to screened B series ignition, which also concludes that the Jolley kits are excellent.

http://hmvf.co.uk/pdf/IGNITIONMATTERS.pdf

Cheers,

Terry

I'm working from a phone and although I seem to enlarge things it doesn't give any more clarity to the text.

There is a bit about different capacitors and plugs in Clives Corner in Bright Sparks for Land Rovers. Can't do a link from the phone I'm afraid. I need to add a section on reduction of insulation as a coil heats up. Experiments so far indicate that after 20 mins of continuous current the insulation leakage will be one tenth of what it was when cold when tested at 5kV. If it gets down to 25meg it will no longer produce a spark. I have yet to determine the lowest tolerable resistance.

When Crypton engine analysers were introduced in REME workshops, we had all the adaptors for screened ignition and showed up all sorts of things, very misleading. You could have a breakdown in the insulation on HT lead from coil. Also be aware of the effect screening has on HT.

Richard, by adapters, do you mean clamps you could clamp around the screened ignition cables? I can see how that would affect readings. Or do you mean the adapter was a length of unscreened cable that went between the screened cable and the plug so you could clamp on to that?

In my case, these traces were taken using a short length of unscreened 7 mm cable between the screened cable and the plug.

I don't think I meggered the wire between the coil and the distributor. I'll check that.

I have seen a couple of cases where the coil wire wasn't securely pressed onto the piercing pin in the cap, and there was evidence of arcing, but I confirmed that was good.

Malcolm

Richard, by adapters, do you mean clamps you could clamp around the screened ignition cables? I can see how that would affect readings. Or do you mean the adapter was a length of unscreened cable that went between the screened cable and the plug so you could clamp on to that?

 

In my case, these traces were taken using a short length of unscreened 7 mm cable between the screened cable and the plug.

 

I don't think I meggered the wire between the coil and the distributor. I'll check that.

 

I have seen a couple of cases where the coil wire wasn't securely pressed onto the piercing pin in the cap, and there was evidence of arcing, but I confirmed that was good.

 

Malcolm

Hi Malcolm,

We are talking about nearly 30 years ago, but as I recall, there was a short adaptor lead that fitted in to the sparkplug and HT lead and another that fitted in the coil lead. I think you are correct about the clamps, they may well have been connected on the unscreened adaptor leads.

Richard

I'm working from a phone and although I seem to enlarge things it doesn't give any more clarity to the text.

 

There is a bit about different capacitors and plugs in Clives Corner in Bright Sparks for Land Rovers. Can't do a link from the phone I'm afraid. I need to add a section on reduction of insulation as a coil heats up. Experiments so far indicate that after 20 mins of continuous current the insulation leakage will be one tenth of what it was when cold when tested at 5kV. If it gets down to 25meg it will no longer produce a spark. I have yet to determine the lowest tolerable resistance.

Clive, I read through Bright Sparks. An excellent document. Thankyou.

I have been leery about meggering coils at 5000V since I was concerned the primary winding insulation could be damaged, but I see you have done it with no problems. I just wanted to confirm it's OK?

Malcolm

Malcolm the thing is that whatever voltage is applied to the HT output of the coil, as it is an auto-transformer, inevitably is connected to the primary via the junction at CB of the primary & secondary. I have tested several coils in this manner & they run on the respective vehicles just fine even when hot. I suppose in an unscreened system one could make a case for the coil to be mounted on an insulating block so there would be no incentive for HT to want to leak to the case? I have heard of some people mounting a spare coil in readiness for failure when the coil has got hot.

But a cold coil it seems should have an insulation of many gigohms at 5kV to allow for the very significant reduction when hot & it looks so far like the insulation drops by a factor of ten.

I'm back home now & easier to read things. I have a feeling that a fuzzy trace like that is a sign of a break in the cable or a breakdown in insulation but still allowing the plug to spark. Or might it be like one of those plugs in the article with a spark at the gap but also to carbon in the insulator or an internal break in the plug itself or leakage through a cracked insulator?

Clive, That's good news about being able to megger test the coil at 5000V.

So resistance checks of both primary and secondary coils with a DMM (assuming you know the spec resistances) to confirm continuity and no major winding shorts, and a cold megger test at 5000V showing gigohms pretty well guarantees the coil is good. But if you want to be 100% confident, a hot megger test is good. What temperature do you recommend? I could see the internals of a Ferret coil getting up to 60C in the 30C weather we are having here.

Your comment about a break in the wire reminds me I have just found the HT coil wire very burnt in the distributor cap of another Ferret. This was causing serious misfiring and the engine wouldn't rev beyond 1500 rpm. Either the wire wasn't properly located on the piercing pin, or the piercing pin was oxidised, or the wire had been removed and replaced so there was no longer good contact between the pin and the wire. I wonder if this is what is causing the fuzzy spark line. Seems like random arcing at the pin could cause this type of noise on all cylinders.

By the way, your difficulty measuring a logical temp on capacitors with an IR gun, even with the mandator black paint, will be due to two things; the effect of the sharp curvature of the capacitor (IR readings are very sensitive to perpendicularity to the gun), and the area scanned by the gun is cone-shaped with the cone expanding as distance between the gun and an object increases. For something as small as a condenser, you would have to have the gun jammed against the condenser. You would be better using a contact pyrometer.

Malcolm