Ignition oscilloscope analysis

[Keith Clements]

I thought I would try analysing the ignition system of my Jowett using an oscillosope so I bought an FNIRSI 2C53T 3 in 1 hand held tool along with a Hanktek CC65 current probe, a capacitive probe for the ignition leads, a P4100 High Voltage probe for the primary pickup on the distibutor and also a thermocouple probe. This is in addition to the two normal voltage probes, signal generator leads and multimeter leads that came with the 3 in 1. I also bought two 10x attenuators. All these extra bits needed the bnc insulating covers cut off so they could be plugged into the input to the scope.
I did a lot of viewing of youtube to see how to use the tool for analysing ignition waveforms and simply how to set up and use the device because the manual was not much use.
The first thing to do is to use a square wave from the signal generator to trim the probes when in 10x gain mode to get a good square wave on the scope.
The two channel inputs to the scope were set to 10x mode and DC coupling (so as to see the DC voltage changes).
The current and 2 normal probes were set to 10x mode ( HV probe is permanently set ).An attenuator was always used in series with the normal probes so as to protect the scope.
Each probe has an earth clip. They must be attached at the same point on the engine block to protect the scope. You can get voltages of 50 kv from some coils in open circuit and perhaps 2kv in back EMF at the points.

These two captures show the HV probe on the CB connection at the distributor on CH1 yellow and the capacitive probe on No 2 cylinder on Ch2 blue with triggering done off No 2 cylinder.

There is a lot you can tell from the primary voltage trace such as air gaps in distriburor rotor, plug lead or too large a spark plug gap or weak mixture as then the peak voltage increases. Allied to a current probe you can further diagnose faults.

The spark duration which is the plateau after the collapse of the field and spark start can show if you have a weak or rich mixture depending on if it slopes gently up or down. The ringing at the end of the burn shows there is some energy left in the coil which you need.

sa-fnirsi-1.PNG

I had saved traces from the Javelin but for some reason they were deleted when the download connection to the FNIRSI timed out.
Lots to play with. But the knowledge gained may help diagnose faults more quickly in future.

I also used the scope with a long time base and probe attached to the ignition switch input on the coil to see the voltage drop when starting. Great for diagnosing battery, starter or high resistance connections.

Lots of playing to follow to see affect of ignition timing, mixture and all manner of variables on the various waveforms.

[David Kemp]

I don't see the oscillations on either the primary or secondary pattern. It looks like a bad coil.

Screenshot_20250627_201053_Drive.jpg

Screenshot_20250627_201109_Drive.jpg

The current clamp can also be used to check the brush and pole condition of electric motors and generators. Shorted or open poles will be obvious.

Attached are a few files about ignition scope patterns.

[Forumadmin]

Thanks very much David. More relevant to Jowett than the stuff I found.

[Keith Clements]

I found this excellent course on auto electrics. It covers the basics well but also delves into some very good diagnosis . Section 16 is good on scope patterns.

https://youtube.com/playlist?list=PLEXF ... o7-aZlmz6L

[Keith Clements]

Analysis of the waveforms captured yesterday is proving difficult as they bear little resemblence to those in the Allen Scope handbook.
Whether this is due to a faulty component or the way the voltages are being measured has yet to be determined.
The probes themselves could be altering the waveforms or the setup of the scope could be at fault.

16.png

The primary waveform (yellow trace) shows no oscillation after the spark ceases but a sharp decrease. The points closing is imperceptable with no step down to zero. The secondary waveform using a capactive pickup on a cylinder's high tension lead only shows the high voltage spike with no spark duration.
This is the trace with the blue trace connected to the coil ht lead .

13.png

Why the spikes go negative here is not explained as I checked the polarity of the coil but have yet to do the polarity test!
A trace with CH2 on other cylinders.

17.png

14.png

15.png

I will repeat today to see if I can get some consistency and sense and try on my Jup as well.

[Keith Clements]

I noticed that the voltage at the ignition coil was sometimes 2v below the battery voltage so started to investigate. First I drew a circuit diagram as the SC has two seperate fuses inboard rather than by the regulator. There was 14.4v at the main fuse from the battery, through starter solenoid and ammeter. But by the time it went through the regulator and ignition switch back to the ignition fuse it was only 13v. The return circuit via the engine block , earth strap, chassis and cable to the battery is the next to be checked. I also noticed that the engine was difficult to start and fired when the starter button was released. This indicates a low voltage on cranking so I checked this which showed voltage dropped to 9.5 v. I will try a different battery tomorrow and check out the whole circuit for high resistance.

But back to cylinders 1 and 3 which when I removed the ht lead from the spark plugs it made no difference to the idle.
A compression check showed 1 with 185 psi, 3 with 195 psi, 2 with 190 psi and 4 with 180 psi.
I also played with the mixture screw on the 1/3 carb, but it did not make any difference. I will check the throttle setting again tomorrow as this was altered yesterday to get a low idle .
I tried checking the polarity of the coil using the pencil method but although there was a 1 cm spark I could not see the flare either way. May try one of the other methods tomorrow.

This process is justifying the diagnosis procedure indicated in the engine analysis leaflet.
I retook the traces on each HT lead with cylinders in order

4.png

6.png

5.png

7.png

I am struggling to make sense of these. I changed the distributor cap as I thought there may be some tracking as pulses appeared on other ht leads that should not be there.

Yes I know I am bouncing around and may be being led astray by the new toy!

[David Kemp]

It still looks like it could be a coil or condenser issue.

Are you able to put the scope on a different engine to see if the oscillations are shown? Even a modern vehicle will do.

It is important to make sure the primary is working perfectly before checking the secondary or fuel systems.

[Keith Clements]

Before descending to the gulag, I tried to interpret the traces taken yesterday. Click on picture to expand.

annotated-composite.PNG

This composite of the traces from each cylinder has been annotated to show which cyliNder I think is which. Because some traces show more than one peak on the ht lead rather than every fourth peak on the primary I changed the dizzie cap . But these traces were taken after changing!

Yes David, I also thought that and tried to find a spare coil and distributor. That is today's job.

And yes I have the Jav and my Jup that I have already used the scope on, but was just learning then!
I also have a very old (but quality) scope that I may try and see if it still works.

[David Kemp]

For the primary test, you can use the secondary probe on cylinder one as a trigger. You would need a second secondary probe to do this for the secondary test.

[Forumadmin]

Thanks DK.
The problem with using a probe to trigger on cylinder 1 is, as you see from the 4.png trace, the trigger was not reliable. I found triggering on the highest pulse on the primary better. Cylinder 2 HT lead would be the best to use as a trigger, see trace 6.png

[Keith Clements]

Vane 3080 ENGINE ANALYSER MANUAL reduced size.pdf

[Keith Clements]

https://www.fnirsi.com/products/2c53t?u ... =2C53T+DCQ

[David Kemp]

Thanks DK.
The problem with using a probe to trigger on cylinder 1 is, as you see from the 4.png trace, the trigger was not reliable. I found triggering on the highest pulse on the primary better. Cylinder 2 HT lead would be the best to use as a trigger, see trace 6.png

That is possibly why a trigger like from a timing light is usually used instead of the capacitive pickup.

Capacitive pickup like you would be using:

IMG_20250726_104623_(1600_x_1200_pixel).jpg

#1 triggers from engine analysers:

IMG_20250726_104607_(1600_x_1200_pixel).jpg

IMG_20250726_105256_(900_x_1200_pixel).jpg

This site shows the pattern a trigger clamp provides: https://old.autonerdz.com/https:/www.au ... kup-ta032/

[Forumadmin]

I have not been able to find that probe in the UK.
But here is a good comparison between four pickup methods. The probe I am using is the Hantek 25

https://youtu.be/niKSnzVnVNg?si=CSo34W4MQtpk23Ap

I have an inductive current probe but I do not think it should be used on the secondary circuit.

I have an inductive clamp from a strobe which I may modify for scope use.

Analysis temporarily suspended as I am replacing the Hardy Spicer universal joint on the prop shaft which is proving more difficult than normal.