Overheated and blew injector driver

mhiscott · Post by **mhiscott** » Tue Aug 01, 2006 1:02 pm

I'm running a v2.2 board on my toyota 4AGZE with stock 365cc low imp injectors. I was driving the car for about a month with no WOT runs due to a nasty tach spike and reset issue. After fixing these issues, yesterday I went for a run on the hiway to tune the car a bit. After giving high throttle for about 20 min or so, the car shut off and left me stranded at the side of the hiway.
I was checking some connections around the MS due to the fact that the case was SUPER hot, and I shook the ecu and heard a rattle inside. Upon opening the case, I noticed the injector driver chip had melted off the nylon nut/bolt holding it to the circuit board and created enough heat to cleanly unsolder itself from the board. Upon trying to reconnect the injector driver to the board without solder (in the middle of nowhere on the side of the road) it blew up and myself and a friend took the ecu, left the car on the side of the hiway and went home to replace the burnt chip. Upon replacing the injector driver, the car started fine again. So from what I can tell, the injector driver chip stays cool unless you give it a few bursts at WOT or close to WOT. At that point it will heat up quite dramatically. I am running the starting setting described in the mega manual for low imp injectors which are :
PWM Time Threshold = 1.0 millisecond
PWM Current Limit (%) = 75%

I have not adjusted these values furthur however. Any ideas?

-Mike

mhiscott · Post by **mhiscott** » Tue Aug 01, 2006 1:09 pm

My injectors are 2 ohm btw.

Firefox · Post by **Firefox** » Wed Aug 02, 2006 6:32 am

got a Flybackboard?

mhiscott · Post by **mhiscott** » Wed Aug 02, 2006 10:31 am

Nope, no flyback board installed, just a stock v2.2 board. I guess I could try resistors in line with the injectors to see if it makes any difference. I dunno what impedance to use tho, also which wattage...

RNash · Post by **RNash** » Wed Aug 02, 2006 2:14 pm

Have you read the injector section of manual?

http://www.megamanual.com/v22manual/minj.htm

"....To eliminate PWM altogether, use a 5 to 8 ohm resistor (with a 20 to 25 watt rating) in series with each injector. "

mhiscott · Post by **mhiscott** » Wed Aug 02, 2006 2:59 pm

I read the injector section many times, but I'm not sure as to which end of the given impedance range to run. 5-8 ohms is a decently wide variation, so I'd like to get an idea of which would be closer. Does the added resistor couple with the injector's resistance to total what an equivilant high impedance injector would behave like? Ie, would my 2 ohm injectors, with an 8 ohm resistor in series with each resistor appear as a 10 ohm high impedance injector to the megasquirt? In which case I could turn off PWM all together. Would this option be as good as the other possibility of building a flyback board?
Thanks,
Mike

mhiscott · Post by **mhiscott** » Wed Aug 02, 2006 3:08 pm

I see the built in calculator for injector resistors, but I don't know the "Hold current (amps) usually ~1 or 2 amps" value to enter.

RNash · Post by **RNash** » Thu Aug 03, 2006 10:35 am

The flyback board is definitely the best way to go.
But if you want to use resistors, they will work, but you may need to experiment.
You are correct on the total ohms. Just add the injector and resistor.
I would start with 1.5 amps hold current. The spec's I saw for your injectors show them at 2.9 ohms. So that gives you a resistor of 6.4 ohms.
I would use a 7 ohm 25 watt for starters.
If you use resistors you would set your PWM at 100% and time threshold to 25.4 ms.

mhiscott · Post by **mhiscott** » Thu Aug 03, 2006 1:26 pm

Would upgrading to a v3.0 net me the same benefits of ordering in a flyback board? I want to alleviate this problem the best way I can.
-Mike

RNash · Post by **RNash** » Fri Aug 04, 2006 11:42 am

http://www.megamanual.com/ms2/pcb.htm

"PWM Flyback On-Board: The flyback and synchronous rectifier (flyback damper) circuits are now on the V3 board, so no external flyback board is required as was the case with version V2.2 MegaSquirt. This is a build option, so if you are using high-impedance injectors and no PWM current limiting these components can be omitted to reduce assembly cost."