Blown Injector FETs

[Sprocket]

In the space of a week I have had two Injector FETS fail. They have failed to ground, so the injector is on all the time when power is applied to the injectors. This last one, I can see in the logs exactly when it happened, which was right before the tank ran dry, which im sure has no bearing on it

I have pulled the board from the case. It is obvious that something had failed as when i opened up the case, you get that distinctive electronics burning smell.

Now the question is, what is the likely cause?

These FETs were on the old board and I unsoldered and re soldered onto this board, this could be a problem? too much heat on the FETS?

The other thought was that the Transient Supression diode was either the wrong one, or inverted. The diode is installed onboard with the cathode tail soldered to the board in the middle of the flyback diode array, and the anode tail soldered to EC36 pin23. The diode used is a 1.5KE30A, which is a unidirectional 30v diode. Everything appears to be in order. However, to eliminate this, I have cut this out, bridged the gap with wire and installed the 1.5KE30CA Bidirectional 30v diode in the loom just before the EC36 connector, as it was on the last loom. The flyback wire is direct from the fuse box, fused side for the injectors.

The injectors were cleaned and tested, they did stick, but are free now, all now work and all have a resistance of around 15 ohm.

The two FETS that failed are channels 1 and 3 (binary 1 and 4)

Is there anything else I should be checking.

At least i can now diagnose a smoked FET, fuel pours out the TB

[[email protected]]

Hi mate, those FQPF20N06L FETs are rated at 15A.
So something must be very awry, can you be sure that theres no shorting between power and ground?

Rob

[Sprocket]

There is a ten amp fuse feeding all the injectors and that is not blowing.

I dont think there are any ground faults, all the wiring is brand new. The live feeding the injectors is daisy chained between the injectors.

I was thinking flyback after reading this statement

'Flyback circuits are used to protect the switching equipment (the injector driver FETs) from extremely high voltages that could occur when the injector is switched off. While the load is being switched off, voltage builds up and must be dissipated somehow so that we don’t burn up the FETs'

http://megasquirtavr.sourceforge.net/manual/Detailed.Output.Flyback.html#Operate


I will tonight replace the two smoked FETs but use other channels for now, at least I can then smoke the suspect FETS . I just dont know if its because these FETs have been exposed to too much heat desoldering.

I might get the clamp on out and see if they all draw the same current.


On another note, I have been blowing fuses on the coils, but I think that was down to the dwell being too high, again, I need to get the clamp on onto those.

It does run and runs well then it suddenly takes a dive.

[[email protected]]

Flyback issues are possible - but why only on two channels?  I'd expect them all to pop in quick succession - but its certainly worth checking.

Rob

[GintsK]

I have experience with no flyback diode connected (my fault). FETs survives long time. And fails only one at time. At redline.

[Sprocket]

Right, something is disturbingly amiss

I replaced the two smoked FETs, replaced the Transient diode, and re installed everything. Set up for Alpha N and then tried again.

Again #3 cylinder flooded within seconds of starting. Injector channel 3 again down to ground and that was one of the new FETs

Something deffo amiss.

I am ordering some new injectors tomorrow, and I will tear the loom apart to check everything is ok there. The only other thing I noticed different is the injector settings. In the pic bellow the left settings are from a known good working set up with same family injector, but larger cc, and the right is the settings I have. Will any of these differencies cause the FETs to fail?

The loom is all new modern thinwall 1mm wire, the injectors were tested and cleaned last year, they did stick at first, but freed up, they all work fine, the coils measure 15 ohms.

This is going to be a painfull excersise

If I need to, can I use IGTBs instead of FETs, not the cheapest, but, I have four spare IGTBs ond no spare FETs at the moment. Not that I want to get to the stage where I have to start pissing about with that.

[GintsK]

Not about FETs, but config:
Digging through wiki and on bench tests, I found:



Injector opening time @13.2V usually sits between 500 and 1000us
Battfactor is added time @7V. Here 600us/6.2V=96us/V - still low setting. Usually 100...200us/V -> 620...1240us. Most of configs I ever see have less. It can cause bad idle!

Rampup settings here is disabled. Most ECUs haven't such setting.
=====
Are you fire all injectors simultaneously?

[Sprocket]

Thanks for that, I altered the settings similar to the above.

Checked the injector drivers, cylinder3 500 ohm to ground, cylinder 4 800 ohm to ground

Well, the saga continues. I swapped out the injectors for some larger ones of the same family, recently cleaned and tested, re configured. Tested the wiring for faults, no resistance between 12v feed and injector channel, with ECU disconnected. With ECU connected, testing to ground, both the live feed and the injector channel read 2.5 megohm, I assume the reading on the live feed is due to the flyback? Flyback wire has continuity from the fuse to the ECU. Swapped round the injector channels.

While I had the fuel rail out, I checked the injectors for operation, all ok and firing as they should. Started the engine, and it ran, all be it all over the place, and rich, EGO helped when it warmed up. Logs show the pulse width every now and then drop to zero and imediately return to normal, for no apparent reason? things were seemingly looking good, the engine was warming up slowley which was better, but then it started running rough again, switched off and looked at the TBs, fuel pouring out od #2 TB injector channel 2 now FUBAR'd at 13k ohm to ground


Please help. I am after hard advice here, as I just cannot see where the problem can be

[Sprocket]

Not about FETs, but config:
Digging through wiki and on bench tests, I found:



Injector opening time @13.2V usually sits between 500 and 1000us
Battfactor is added time @7V. Here 600us/6.2V=96us/V - still low setting. Usually 100...200us/V -> 620...1240us. Most of configs I ever see have less. It can cause bad idle!

Rampup settings here is disabled. Most ECUs haven't such setting.
=====
Are you fire all injectors simultaneously?

While I was looking at Robs guide, it says that Injector open time should be set to 0

I had previously thought that the ramp up time is the bit you tune for the characteristic of the injector, IE if it takes 9ms to ramp up, you would set that in effective ramp up

There is conflicting information everywhere, an i do not know what is right or wrong any more.

Rob, if you read this, can you please see if one of the powers that be, can suggest anything. Im close to throwing this in the bin. This is the new board and is its first use.

Im considering DTA, But i really do not want to go down that route unless I really really really have to. This is a great product, but information is pants.

Any of the development team thought about building circuit protection into these units Simple current sensing circuit thats all.

Sorry, rant over,

[[email protected]]

As a rule of thumb in the guide I suggest:
Inj open time should be 0
Inj rampup_battfac must be 4080 (which diables the feature)
The wiki is full of contradicting information, and should largely be ignored, the User Guide has been peer reviewed by a wide range of people and I hope to keep it spot-on.

The values you had in the screen shot
Inj open time 0
Inj effectiv 912
injrampup_battfac 4080
The battfac should be higher than 0, try 512 - this scales the opening time due to low voltage when cranking.

The injector model cannot cause the FETs to draw more current from the injectors than the 15A they can handle.

[GintsK]

Don't worry: these settings can't kill FETs. But if you want good vision about various injectors, you can download A E M tuning software and under setup->injectors->primaries->batt.offset wizard find good graphs.
As I understand rampup time is very small time when needle travels from closed to fully opened. Opening time is time between start of pulse and rampup start - much longer time. According to Andreys documentation in russian, sum of both settings (opening and rampup) is linear graph anyway.

Are you fire all injectors simultaneous? May be that is a cause? I have more than 20 VEMS installed and no fried FETs without a clear reason.

Gints

[Sprocket]

Thanks for the re assurance Rob.

I have All sunday now to get to the bottom of this, and if that means pulling every wire out of the loom, then so be it.

I know what you are saying. These FETs can draw 15amp continuous, just seems odd that the 10 amp fuse isnt blowing. I might try a 7 amp fuse or something in there tomorrow.

It isnt the config, its not the injectors, that only leaves the hardware and the wiring. anything else?

[Sprocket]

Don't worry: these settings can't kill FETs. But if you want good vision about various injectors, you can download A E M tuning software and under setup->injectors->primaries->batt.offset wizard find good graphs.
As I understand rampup time is very small time when needle travels from closed to fully opened. Opening time is time between start of pulse and rampup start - much longer time. According to Andreys documentation in russian, sum of both settings (opening and rampup) is linear graph anyway.

Are you fire all injectors simultaneous? May be that is a cause? I have more than 20 VEMS installed and no fried FETs without a clear reason.

Gints

Running sequencial injection.

This is a replacement board, and has not been tested in action, this is its first outing. I never had any FETs fail on the last board. Thats why I thought it was perhaps the excess heat from desoldering, but its smoked one of the new replacement ones already.

going to have to order a rack of replacements

[GintsK]

Hm. Why alternate banks from h[0] is 0 only. With such setting only simultaneous firing is possible.

[Bat]

Hi,
The zero is from my settings on the left. I'm running Tbi
Cheers,
Gavin