AUTOiNFORM ONLINE MAGAZINE: Testing Piezo Injectors with Frank Massey

AUTOiNFORM ONLINE MAGAZINE: Testing Piezo Injectors with Frank Massey


hello and welcome to Auto inform online magazine my name is Frank Messer and this how-to feature I’ll be discussing how to test piezo injectors the vehicle were chosen is one of the latest generation bosch common rail direct injection systems that have now been employing piezo injectors for probably around two years or so the problem with these léo injectors is mainly that once an injector develops a fault it closes down the power output stages from the ECU now that in many ways defeats some of the opportunities we have of actually testing the circuit and by that I mean they’re driven they’re power driven by a capacitor discharging in effect quite a high voltage into the injector about 120 volts this then creates a positive current flow in the PDL crystal of around six or seven amps which is the opening event the opening period then is controlled by the ECU and there are several opening period seven several fueling events which take place with piezo injectors typically from three up to I believe now around eight individual fuel delivery events in one power cycle once a fail is occurred the power stages are then shut down on all injectors effectively the engine will not run that then defeats any possibility of measuring current through a circuit which is the most accurate way of testing functionality so we then remarkably then resort to using a multimeter so what I’d like to do first of all is demonstrate the correct operation of a piezo injectors using current measurements so that you’re aware of what the pattern what the events look like and then the simple test of proving then what the fault is how the fault exists and how to diagnose it now of course the most obvious opportunity for diagnosis is serial communication there will be a DTC the DTC will be accurate and we will identify which cylinder injector is faulty but I think it’s also of interest to be able to measure each injector perhaps to save the potential of a failure on another unit so let me begin by first of all setting the equipment up which is the current clamp with the cell scope and we’ll do a normal running test first of all and then unplug the injector and then do the multimeter tests first of all I’m going to use a inductive clamps actually a Hall effect clamp but by means of induction we’re going to measure the current flow and where to set it to a range of 20 amps I’m only anticipating 7 8 amps maximum I’ll need to zero the calibration eventually but for now I’m going to place that around one of the conductors and now need to set the scale to cover a 10 ohm range now have a 10 amp range set we now effectively need to run the vehicle because I don’t know which way around that clamp should be in other words which potential we’re going to pick up we need the positive event first sometimes we’re going to run the vehicle pick up the image and then use a trigger to actually stabilize the image on the screen for you but we have an image of several injector events what we need to do first is stabilized one particular events I’m going to use a trigger and I’m going to select a repeat trigger in the bottom left box I’m going to set the trigger event at around 50 percent of the current event so it’s a nice stable trigger point I’ve also set the trigger right over to the left hand side of the screen because I’d now like to focus on one of these events which seem at the moment to be a singular event in fact they are not I were to drop the time based initially to 1 millisecond 1000 for the second per division and you can now see that right with three events each power cycle of the engine takes place with three individual injected fuel events I think we can probably go down a little further just to see there we are so we can just about get these three events on one screen what you can see from this is that we have a positive opening or charge event of around 8 amps and the closing or discharge event of around 6 amps and we have three individual events a pilot intermediate and main fuel event the timing of which the space ratio between it between each event is adjustable and the amount of fuel quantity which is normally adjusted in the main fuelling event also is adjustable through the software and I believe that the adjustment of fuel quantity is a square of the increasing rail pressure obviously has variable rail pressure and there is an additional adjustment to fuel quantity which is a a mathematical equation based on the square of the increase in fuel pressure so we have three events the way to look at piezo injectors is that the opening event is to be viewed or understood as charging the crystal this creates a small expansion within fuel crystal and with indirect control this then acts on what is known the hydraulic coupling or a servo coupling that multiplies the movement ratio which then effectively opens the injector bar means of hydraulics there are however some new development of piezo injectors which are direct acting where my understanding that is that the p0 expansion itself can directly open the nozzle there direct acting piezo injectors so even even though piezo injectors are relatively new they are already developing and evolving very very quickly because we can now have multiple events with one power stroke emissions power torque is all very very much improved I believe that we can now have up to eight her power cycle of an engine so it’s an incredible change of taking place in the way diesel fuel is delivered into the engine your pressures are obviously increasing as well as the accuracy of control of the actual injector that’s as far as we can go with an engine that’s functional current is the way to go we don’t measure voltage current is really the effects of around a hundred and twenty volts being fired at the injector Bank capacitor but ultimately we want to monitor current in the circuit the catch is if current isn’t present because there is an injector fault then the engine will be prevented from running by all the power surges closing down so the next stage now is a turn the engine off and show you how to do this simple test with a multimeter I’m going to just wait a few seconds until the PCM powers down completely and that’s simply to avoid installing a DTC we can remove the clamp and of course go man when you do use a chrome clamp it’s around one circuit only and the orientation the polarity should ensure that the positive current cycle comes first so we now have a situation where the engine isn’t running and what to confirm the actual condition of the injector that what when to do is a theoretical measurement across both terminals of the injector we’re going to remove the vehicle harness for this to the injector there’s a theoretical figure of 200k ohms which isn’t in itself definitive what we need then to do is to measure from each pin to ground because when these crystals fail they tend to go short to case in other words a short circuit between the PSL crystal and the case of the injector so it’s a simple measurement across each of those circuits so first of all remove the socket connect the meter to resistance and I want to do a quick calibration check that’s fine now my first intention is to go across both pins of the injector and I’m going to place a multimeter so I can see it I’m going across both pins and it will take some time to make this motion which is quite a high reading and I’m expecting around 200 K now we’re on 175 177 and I’m trying to keep the pressure on the terminals are quieter away in these probes are actually a little bit large that we are having up a good connection 173 174 175 175 K ohms quite happy with that that’s quite close to the expected value now the definitive measurement noise from each pen so I’m going to go initially from the left pen actually so ground and I can’t reach the body of the injector so I’m going to go to the engine block and the critical measurement here is infinity there should be no reading whatsoever if there is any value less than infinity then that injector is Fulton you must measure across each pin and the reason for that is that a small error would not be seen across the first measurement we measure done done 75 K originally so it’s important that we measure from each pin to ground and there is as you can see there infinity across both pins should that injector be faulty you will have a reading so any reading at all less infinity that’s the problem and that is as simple as it needs to be it will confirm the injector has formed a potential partial shunt or short to the body of the injector or ground and that will instigate a complete closure of all the power output stages to the injectors as there will be dtc for that I think it’s always and wise it may be the intermittent fault perhaps related with heat that often does happen where the injector may fail intermittently first but it’s a test that is very accurate in terms of diagnosing the injector thank you for joining me in this feature if you’re interested developing your diagnostic skills please visit the auto informed website for details of our face-to-face training DVDs and learning modules we’re also able to supply a range of tools thank you and I hope to see you again soon


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