Inverse ignition smokes IGBT?

TPI 85 Blazer · Post by **TPI 85 Blazer** » Sun Jul 16, 2006 6:06 pm

IF I choose the wrong polarity of spark drive in Megatune does this smoke the IGBT?

I was working on getting some base ignition timing with a new MS-II 3.0 install (engine hasn't run yet) when I realized that I hadn't put the ignition jumpers on the PCB. I installed them and turned on power to invert the ignition setting with Megatune (since it requires a reset). It had been set to "Normal", but I changed it to "Inverted". When I turned on power again and cranked the starter, I was greeted by the horrible smell and tons of smoke from Q16. I have checked out all of the wiring and can't find anything wrong.

Having the wrong polarity is only a problem when MS is trying to spark, right? i.e. I can have ignition on, but no current flows through the IGBT. as soon as the MS sees that the engine is cranking it starts turning on the transistor, but for much too long of a dwell time. Am I thinking of this the right way? You select the opposite spark output in Megatune because the IGBT circuit is inverting, right? I got that setting from here: http://www.megamanual.com/ms2/vb921.htm Is there any chance that the newer version of code has in inverter built in? I am running 2.35.

I have verified that the IGN output is not shorted to ground. I plan to replace the IGBT, but I want to make sure this won't happen again.

Here's the setup: It's a TPI top end swapped onto a Goodwrench 350. The coil is in the cap of the HEI, which has been locked down so that it has no mechanical or vacuum advance. MS drives the coil directly and uses the VR coil in the base for pickup.

Thanks for the help

TPI 85 Blazer · Post by **TPI 85 Blazer** » Sun Jul 16, 2006 8:04 pm

BTW, what's up with the 330 Ohm resistor? Is this supposed to go between the uP and the VB921ZV or do I not need it at all?

FoundSoul · Post by **FoundSoul** » Mon Jul 17, 2006 6:30 am

TPI 85 Blazer wrote:IF I choose the wrong polarity of spark drive in Megatune does this smoke the IGBT?

Definitely-- it's important to have this right before turning over the car. It can burn the VB921 and/or the coil if it's set incorrectly.

TPI 85 Blazer · Post by **TPI 85 Blazer** » Mon Jul 17, 2006 12:12 pm

OK, I'm still confused. The coil is about 3 Ohms DC, so even if the VB921 is on with 100% duty, that should only be about 4A or so. Add in the harness, connectors, etc. and the current is even less. The VB921 is rated for 40W, but even if its output transistor is dropping 1.2V, it should only be seeing half of that. It is supposed to limit "coil current" to 8A, but if it tries to do this with a very high duty ratio, it can go well over its power rating (because it's only designed to do it for short durations). To blow this violently, it almost has to have battery voltage across it, which requires a wiring error or a shorted coil.

TPI 85 Blazer · Post by **TPI 85 Blazer** » Wed Jul 19, 2006 12:15 pm

I should have some more VB921s today, so I will fix the ECU, place a resistor in series with the coil and get a scope to see what is really happening.

TPI 85 Blazer · Post by **TPI 85 Blazer** » Wed Jul 19, 2006 7:44 pm

The ECU seems to be fine now with the new VB921. I verified operation on the bench with an ignition coil and everything works fine up to 3900 rpm or so, where the tach signal goes crazy whenever the ignition coil is hooked up. I will deal with that later if it turns out to be a problem in the vehicle.

Also, I was wrong about the coil resistance. The "stock-type" coil is about 1.1 Ohms and the MSD coil that is in the vehicle is 0.7 (as far as I can tell, it is still good). Sure enough, without the ignition inversed, it tries to power the coil all the time when you aren't cranking (Doh!). So I must have smoked it while trying to invert the signal and just didn't smell it right away. So if the IC does it's job limits the current to 8A, it will dissipate close to 100W

At least it all makes sense to me now. I should really have a fuse on the ignition coil. It looks like 4ms of dwell produces less than 3A in the coil. So even at high rpm, the average current through the coil would be maybe 2A. Does this sound right?

I also verified with the scope (and Excel) that the dwell time is correct and the right delay exists between VR events and spark events based on the advance.

Tomorrow I will try to get spark in the vehicle.

Post by **jsmcortina** » Thu Jul 20, 2006 2:22 am

You are on the right track. You can crudely set the dwell by monitoring the temperature of the VB921 - if it gets warm/hot then your dwell is too long. If you get misfires then your dwell is likely too short.

If you have a scope you can monitor the current through (R43?) and set the optimal dwell time.

James

TPI 85 Blazer · Post by **TPI 85 Blazer** » Sat Jul 22, 2006 1:14 pm

I have successfully started the engine a couple of times, but it is inconsistent. I am having trouble setting the timing, because the spark output seems erratic. Looking at the current through the VB 921 shows that sometimes it is giving nice spark signals and sometimes it turns on for too long. It will turn on for like 100ms at a time. Then when the engine stalls, it turns on for about 800ms before shutting off. I can hear the ignition coil click at the same time.

It almost seems like it wants one signal to start charging the coil and another signal to spark. When the engine stops it "times out"

It is set to:
Input ignition capture = Rising Edge
Cranking trigger = Trigger rise
Coil Charging = standard coil charge
Spark Output = going high (inverted)

I guess I will look at the VR pickup circuit and make sure it is working, but if there was a problem, wouldn't I just miss sparks?

TPI 85 Blazer · Post by **TPI 85 Blazer** » Sat Jul 22, 2006 2:48 pm

Thanks for the reply Mike.

I was thinking of reversing the VR lines, but haven't tried it yet. It would explain the inconsistent timing, but not the unusually long dwell times. It shouldn't ever be waiting for another trigger to turn off the ignition coil, but it is. I switched input ignition capture to "falling edge" and the cranking trigger to "trigger return" and that seemed to help (it is missing far fewer spark events and the timing is fairly consistent). It still is not doing what it should. The IGBT signal is only turning off when VRout has a rising edge (+dwell time). When the engine stalls, there are no more rising edges because these correspond to zero crossings from the VR sensor. It should only turn on for the calculated dwell time.

I don't know if it thinks I have Ford EDIS or what. There are plenty of things that could make the spark happen at the wrong time, but regardless it should only turn on the coil for the calculated dwell time.

TPI 85 Blazer · Post by **TPI 85 Blazer** » Sat Jul 22, 2006 3:34 pm

The key here was to select "Calculated" for Cranking Trigger. That keeps it from holding the coil on and doing all of the dumb stuff I described above.

I swapped the VR coil and I am getting good, consistent timing now, but I think I could swap it back and reverse the "Ignition Input Capture" setting. Or maybe I should move the jumper from VR to VR INV.

The VB921 is not getting hot anymore either, so I think I am on to getting the fuel right

TPI 85 Blazer · Post by **TPI 85 Blazer** » Sat Jul 22, 2006 7:47 pm

OK, with those changes, the spark is stable. My MT advance readings are now solid with my timing light. The dwell times are matching on the scope and the MT display. I got the truck running tonight and had it idling for a couple of hours working on tuning stuff.

Maybe tomorrow I can drive it

Does anybody get one of these to run without an oscilloscope?

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Inverse ignition smokes IGBT?

Inverse ignition smokes IGBT?

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