Secondary ignition image question (with pic)

Hi,
I used an old scope to show the secondary ignition pattern of both banks of my BMW V12 M70 engine.
The image for bank 1-6 seems to be perfectly OK (perhaps mixture too lean?).
The image for bank 7-12 shows an anomaly during saturation of the coil: a flat line.
Suggestions of what could be going wrong at coil 7-12?
For reference I also added the image from the BMW workshop manual.
Note: one coil with distributor per 6 cylinder bank.
Thanks and regards from Albert, Netherlands

That kind of difference in detail can be down to probe placement, or the probe itself. If bank two doesn't have a running issue, I'd suggest moving probes bank-to-bank, or try moving the bank two probe around to see if the signal improves.

From a purely theory standpoint, the lack of a coil turn-on oscillation may indicate a possible coil issue. But, the same waveform shows a turn-off oscillation, which is an indication of a healthy coil. :silly: That's why I'm suspicious of the secondary probe.

Back in the days when OE's talked about secondary ignition testing.

Can you post Primary Current and Voltage waveforms for both coils?

Yes I will do that, just waiting for my digital scope to arrive.

The current inflows of both coils follow a straight line, see image. No horizontal part due to current limiting. I guess that's OK (although most of SD's examples show a flat current limiting part).


What I noticed is that the secondary traces taken between the coils and distributors look fine (see attached movie). However, when measured at the individual plugs, many traces are worse causing a lot of misfiring. Distributor/rotor problem?

There are two legitimate air gaps in the secondary, one is the rotor air gap and the second is the spark plug gap. I f you select a very short time base of 20 or 50 microseconds/div you will be able to see the height of the voltage required to jump the rotor air gap. This should be less than the total voltage to make the spark jump between the plug's electrodes. The voltage required to jumpr the rotor gap should be constant and not change with rpm or load. Many rotor gaps will require some where between 2 and not more than 4 kV.

Your current ramp and the charging time look pretty normal. You also can see that the reading of your ampclamp is affected by the secondary ignition; what looks like a little overshoot after the primary turn off most probably is an influence from the secundary voltage as the overshoot lasts exactly as long as the spark is present, around 2 ms.

I'm not proficient in ignition wave forms, but isn't picture 1-6 a secondary wave form and 7-12 a primary wave form.

scannerjohn wrote: I'm not proficient in ignition wave forms, but isn't picture 1-6 a secondary wave form and 7-12 a primary wave form.

That's right.