Better Electrical Diagnosis

[dadof2]

TC would not allow the file type for this image to be loaded so I photographed it whilst I had my camera handy.

It shows the waveform of a 4 stroke electronic ignition spark.

The blue line comes in from the left, where it drops is the start of coil charging, this lasts about 3 mS. Then there is about a 1mS squiggle and step as the coil current is switched to fire it. The voltage the rises suddenly to about 38 kV for about 1 mS then drops again. The very high spike at about 24.56 mS is the voltage "pressure" building up before the spark jumps the plug gap and gives the burn time. This 1 mS at 38 kV is the ignition spark burn. The 4 oscillations after it drops is called coil bounce and indicates the coil is in good order.

This type of analysis is quick and easy and a lot better than swapping parts.

For example if there were no charging section it probably means a CDI fault, if there is no switching step it could be CDI or trigger coil.

This waveform is using a single sensor on the HT lead. This system is a 4 channel system and can give 4 traces at once. Had I put sensors on CI output to coil, Ignition source coil and trigger coil as well as HT I could have found the exact area of the fault with a quick spin of the engine.

Edited September 26, 2014 by dadof2

[copemech]

So simple a caveman can do it!

Try getting some patterns on a few trials bikes, primary side between mag and cdi, cdi to coil and such, then secondary.

Problem on some with integrated cdi/coil though.

Quite honestly, at least in my end of the auto trade, we have not tought/used a scope for much other than crank/ cam input to the computer for decades now whth coil on plug things and such, a noid light will do, oh well.

[dadof2]

A noid light though useful only tells you if there is a burst of electrical activity of some sort. With the 4 channel scope you can measure for example the current and voltage to 2 injectors at the same time, the slightest discrepancy between the to is clearly visible but a noid check would not show this.

With the scope I have a coil on plug sensor can be used for COP systems.

Same thing with a HT coil, noid will show its OK and the coil may perform properly when engine is revved in garage. Put a scope on and its a fair bet you will see the defect that only intermittently manifests itself under sustained load.

Integrated HT coil / CDI does not make much difference, I would just put sensors on coil secondary, stator source coil and stator signal coil.

It is very simple to use, the only difficulty is interpreting if there is a fault when you get an unfamiliar abnormality on the waveform.

When you read the electrical problems people post on TC it would make diagnosis far easier if manufacturers fitted a scope socket, and a list of what you should see when all is well.

The modern PC based oscilloscopes are a long way from the first automotive cathode tube type scopes.

Checking CPS, ABS wheel sensors and ignition is probably most common scope use. My kit cost not far short of £2k, but there are compact combined diagnostic code readers and scopes for less than £130 Which can do pretty well all the tests needed on trials bikes.

When I get time I will see about posting some waveforms off trials bikes.