Do Diesel Injectors Usually Group Fire During Cranking?

While I was doing a voltage based virtual compression test on my friends 2010 Peugeot Partner II 1.6 hdi Turbo Diesel I hooked a probe up to an injector so I would have a reference point to know which cylinder is which during the virtual the test. What I got was the injector firing on every turn of the engine.

So is it a common thing for diesel injectors to group fire during cranking, if that's in fact what I'm seeing here? Or is something strange going on here?

These are voltage readings... I'm using a 20:1 attenuation on the injector and a 10 kilohertz low pass filter... I tried probing both wires and got the same confusing results... I know diesel injectors are usually tested with amp meters, but since I'm just using it as a reference point I figured just back probing it would work fine....



Another bonus question, is there some other way I could get a reference point for knowing which cylinder is which?

EDIT I went back and retested with three of the four injectors disconnected and I still got basically the same thing. I'm wondering if maybe this is a result of a fried ECU... The replacement EGR apparently fried the diode for the solenoid control and I'm wondering if this is somehow connected to that...

I have very limited experience with diesels, but I know the basics. And since no one else has replied, I will give you my input, based on what I know. An injector in a diesel engine is only supposed to fire near TDC on the compression stroke for that cylinder, similar to a spark plug firing in a spark ignition engine. Assuming this is a 4 cylinder engine, if an injector was firing every time any cylinder came up on compression, that would mean it would be firing at BDC intake and exhaust and TDC exhaust/intake. The engine would smoke like hell, and probably knock severely. If this engine was running somewhat decently before, it’s unlikely that was the case. I don’t know what the circuitry is like on an electronically controlled diesel, so I can’t make sense of what your scope is showing. My first thought was that perhaps you connected to the wrong wire, maybe a common power feed, but since you said there are only two wires to the injector, and you got the same results with both, I can’t figure it out.

As far getting a cylinder reference, is it not possible to use the cam/crank sensors?

SailorBob wrote: EDIT I went back and retested with three of the four injectors disconnected and I still got basically the same thing.

That has got you a classic one of this isn't like the other three which is likely the injector which is still plugged in. Guessing this Pug is a 4 banger? Also looks like you've got a repeating two weak holes pattern on the relative compression. I'm no diesel guy but that's what I see in the traces.........

I have tested diesel injector voltage and got a similar result.

Unlike a petrol injector where it is just ground switched B+, in a modern diesel the ECU (or separate control unit) switches on a voltage supply of 50+ volts (140v for piezo) and grounds the other side at the same time. You may be seeing the voltage spike but only the active injector is being grounded. This could be resolved by testing across the injector rather than to battery ground... or using an Amp clamp.

Good info here
markstammersdiagnostics.com/reference-wa...ms/diesel-injectors/

Clearly has bad compression in one of the four cylinders. Was running before, but smoking black smoke like hell.

Thanks Wightscope, I'll go back when I have a chance and try testing across the injector and see what results I get.

Voltage compression test and an amp clamp on the injector is answer. But on a common rail diesel if a bad compression on a cylinder you can usually identify the cylinder by looking at injector corrections in diagnostic data at idle speed. On an old style diesel you would have to remove an injector or glow plug which really isn't worth the effort