O'scoped Injectors

[quattrodave]

Hiya Guys,

I 'scoped my injectors this afternoon just so as to check against what i've found. Unfortunatly i didn't get what i expected; i'm trying to measure the opening time and also to check the PWM % is holding the injector open properly.

Could someone have a look and see if i'm missing something..... I'll attach a .bmp and an excel file to have a look at.

Many thanks

Dave

[arlo#1]

I want to scope mine now to see what I can see. How did you conect to the injector with your scope? maybe your scope isnt fast enough to see the voltage drop to 0 and back during pwm? Or maybe you have something causeing the voltage to slowly decrease as it is shut off like a capicitor or something.

[Mike_Robert]

The opening time "bump" is sometimes hard to see depending on the scope. To see your PWM signal unaltered by the injector inductance scope across a purely resistive load, a 1K resistor works fine.

-Mike

[quattrodave]

Hiya Guys,

Thanks for your replys, the scope is more than fast enough i borrowed the best one our company owns
If you look at the excel file and graph it you'll get a far better graph as the bitmap image is rather low quality....

Yeah i used a 3w 1K wire wound resistor on the +ve side of the injectors and attached the scope to either side of that. I think i could see the 'bump' as they fully opened but i could see no evidence of it closing.... or is this quite normal??

Many thanks

Dave

[Philip Lochner]

I dont understand how you guys are measuring your injectors - but I'm not saying its wrong either.

When I scope my injectors I use a 1x probe over the 0.05Ohm resistor (R37 and R38) that serves in the current limiter role. Being such a small value resistor, the voltage generated accross it is very small but this represents the current flowing through the injectors and that's what we want to see surely?

[Philip Lochner]

Here's an example of my scope (using ooooold technology....) These Bosch injectors (turquois indicator) are the one bank of 4 on a Land Rover Discovery 3.9 V8.

The scope is being triggered off the injector ouput with channel B (not visible) on the falling edge, so that's when current starts flowing.

The trace you see is measured accross R37/38 with a 1 x probe at 20mV per vert division, 0.2ms per horizontal division.

Looks to me like they open at a whisker over 1.3ms. (at the yellow line)

Its relevant (important??) to note that not all injectors open at this time. This graph is the AVERAGE of the 4 injectors. Using injector test mode I've SEEN (with mine own eyes - using the first visible fuel squirting as the measure that the injectors has openened) that some (not these but others also on a 3.9 Disco) open between 1.2ms and 1.55ms!!!!!! This means that at small PWs (close to opening time), some of your injectors may not be opening at all!

[Vicoor]

I used a Snap-On MODIS oscilloscope to do some testing and it reveals clearly where the opening and closing events occur. The waveforms show the voltage and current in the circuit. The green current trace shows the current as measured with an inductive probe and is best for observing the injection event. As my Megasquirt is not running yet these were only to test the difference with and without current resistors.

The first image is without a resistor


The second is with resistor





Once I get the Megasquirt online I will certainly post some images of those waveforms as well.

[Mad Medicine]

while i was in school, teacher warned me about hooking up a scope to injectors, if you don't wire it up correctly, you'll get strange results. he told me always back probe on the negative side of the injector (after the power traversed the injector) and the other probe of the scope hooked up to battery negative. tried it any other way and it didn't show the true injector PW. never needed any resistor or "special filters" hooked up with the scope.

[Mad Medicine]

also high and low impedence injectors don't show the same wave form.

[AR 67202]

while i was in school, teacher warned me about hooking up a scope to injectors, if you don't wire it up correctly, you'll get strange results. he told me always back probe on the negative side of the injector (after the power traversed the injector) and the other probe of the scope hooked up to battery negative. tried it any other way and it didn't show the true injector PW. never needed any resistor or "special filters" hooked up with the scope.

so effectively your measuring across the transistor in the management, which is also how all the scope pictures are made that i've seen from oem manufacturers...