Strange secondary ignition waveform

Hi,

I have a 1996 Jaguar XJ6 3,2 inline six with COP ignition. I bought two new coils (two because I wanted one in spare and I got a good price) because the rubber insulator of the old one was partly damaged (only complete coils was available). On the new coil I checked the secondary ignition with a COP probe and an oscilloscope and saw a rather strange waveform. I swapped a coil from another cylinder for comparison and this waveform looked good compared to the other cylinders and known good waveforms on the internet. Does anyone know what could be wrong with the new coil by looking at the waveform? I can also mention that the waveform of the new coils looks more like the old good ones if they are not inverted. The waveform on the old coils are inverted on the scope. I have also attached a waveform from one of the old coils that I think looks good (the burn time was an average of near 1ms but on the picture it is a little less), measured in the same cylinder as the new coil.

Can you post voltage and current waveforms of the primary?

I have the current waveform which looks ok and similar to the other cylinders. I dont have the primary voltage, but I will perform this and post the results.

Increasingly I am seeing aftermarket coils don't match the specification or design of the OEM part add the problem of getting good secondary wave forms from COP coils and I am not surprised at your questions. Using a COP probe also throws a spanner in the works as the exact position of the probe and probe type can produce very different waveforms.

With a 2 wire COP going for the primary voltage and curent is often the easier and more reliable option, the other thing you could do is use a extension plug wire and an old style DIS probe.

I have now performed both current and voltage measurements on the primary side. On the oscillograms the coil in cylinder no.3 is the new one and in cylinder no.2 is an old which seems to be ok. I notice that there are fewer oscillations at the end of the waveform on the new and also that the waveform are different at the end of the spike. Since I wanted to capture the short spike I used a high samplerate and therfore the traces looks a bit thick. Any comments to the new waveforms? In my experience the COP probe gives quite good readings and I think getting repetitive results is not very dependent of the position, but I have experience with this car only since I am now to car-diagnosing with oscilloscope. I have a DIS probe so I will keep this in mind, thanks for the tip. I also measured the primary resistance of the coils with a DMM and got 0,9 Ohm on the old one and 0,8 Ohm on the new coil which probably is ok.

Based on the answers here I think that the conclusion could be that the new coils are not up to correct specs. About a year ago I replaced another coil with an aftermarket one. On the coil I replaced the rubber and center pin was ok so I took these parts and substituted them onto the old coil which I now use. The waveform of all the coils are ok. Thank you for your input to my problem.

linken wrote: Based on the answers here I think that the conclusion could be that the new coils are not up to correct specs. About a year ago I replaced another coil with an aftermarket one. On the coil I replaced the rubber and center pin was ok so I took these parts and substituted them onto the old coil which I now use. The waveform of all the coils are ok. Thank you for your input to my problem.

That replacement rubbers aren't available is a pain I have the same issue with MG-Rover coils I have a whole pile that test out Okay but the rubber insulator is goosed. I was changing a lot of Rover plug top coils but started setting the spark plugs to a reduced spark gap and haven't changed a coil since.

Regards COP secondary wave forms I think it is a lot to do with the way the components are physically arranged in the aftermarket coils being different from the OEM coils.

At least now I have two new sets of rubbers in case the other ones will go bad I dont think I will use the coils because of the strange waveform even still I can`t here or feel any differene when the engine is running. Without the scope I would not have noticed it. On an aftermarket coil I bought a year ago the waveform looks good. That coil was more expensive so maybe "you get what you pay for" are valid in this case. The new ones I bought now have Lucas stamped on them which is the OEM brand.

Building your own in-cylinder tool is easy and you get real satisfaction from doing it,
Be aware that the hoses supplied with compression gauges don't last long if you are doing running compressions. There are now 200psi sensors available on eBay that are ideal for the job as well as being the correct range for cranking or running compression PSI somehow seems a better sized unit than bar for engine compressions.

Link to a DIY Pressure Transducer Thread.

Thank you for your tip on the cylinder compression tool. Actually I saw this DIY pressure transducer when I was searching about the theme so I had it in mind. I have a 100 PSI transducer which is of the same type as in the transducer you have linked to. I plan to use it for fuel pressure measurement in combination with an injector to display fuel pressure and injector performance based on the fuel pressure drop. Good advice on the hoses. I think I will get a kit which also has suitable metal tubes.