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Old 09-06-2011, 04:01 PM   #1
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Default D10 blown, I need help

To all the Megasquirt gurus out there, I need your help please!!


I'm driving a MS1 standalone 1990 turbo for over a year now.

**** happens to me lately, I blew a headgasket 2 weeks ago.

While trying to figured out how comes I was'nt able to make my COPs works, I found the "Even better spark circuit" from Joe Perez

and because I'm tired of the intake manifold backfire, I decided to do it.


When I opened my MS, I remembered that I used "wire wrap" wires to do the jumpers on the back, I thought maybe this was
why the COPs were not working but the OEM coils trougth the ignitor were working. (maybe the signal was not strong enough or I don't know?!)

So I changed the little wires for 22awg cables and replaced them all.



Then I did the "Spark output inverted Yes" circuit and went to bed.



This morning I plugged the MS in the car, unplugged the ignitor, and put the car to "ON" to be able the burn the new code for the spark output.

I did'nt even had the time to start megatune then white smoke were coming out off the MS shell, so I put the car to OFF.

When I opened the MS, I saw D10 was blown (totally, split in half)



I took a look at my new wires, I had two cables that the isolation was melted over random pins on the board.

The first I saw was the jumper we put between the banded end of D9 and a resistor to D1 was stucked into the pin 16 of U6 (pin 16 is Vcc, maybe bad thing happens because of this...)

The second one was the cable going from IAC2B to D16 for the "spark output mod", the isolator was burned over a pin of C11 (but did'nt seems to be perforated)


I replaced D10, lifted every cables from the board to avoid contacts and I tried a second time, same thing happen, D10 blew.


Then I removed Joe Perez circuit and went back to Brain "spark output inverted NO" circuit, replaced D10 and changed the faulty cables, looked at some connections but everything was looking alright.

I tried a third time, nothing, I cannot even power up the MS.



I looked at the troubleshooting section of the megamanual, U5 is ok, R37 and R38 are ok, nothing is burned except the lead under C11 (Somehow, C11 seems to be fine).


Now, I'm really confused, I don't know what bring D10 to blow, but now, even if it's replaced, I cannot power up my MS.


I need your help, thank you!
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Old 09-06-2011, 04:04 PM   #2
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D10 is on the 5V power circuit... you have short between 12v and ground.

c11 is on the crank signal input...maybe that's where the 12v is coming from? bad jumper? check r13. check between c11 and the opto, and check between c11 and optoout.
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Old 09-06-2011, 06:06 PM   #3
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-I have continuity between C11 and optoout
-On the other pin of C11 I have continuity with ground and with 5V
-I have nothing between C11 and optoin
-R13 is fine


I'm now looking to see where 5V is shorted to ground

*edit*

maybe U5 is blown?

*edit #2*
even with U5 out I still continuity between 5V and ground, still looking
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Old 09-07-2011, 12:09 PM   #4
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The most common cause of continuity from 5 volts to ground is a blown D19.
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Old 09-07-2011, 02:02 PM   #5
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Thanks Matt, I'm gonna look at this diode, I hope it's the one!
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Old 09-08-2011, 02:07 PM   #6
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If D19 is blown (1N4734A, 5.6V 1W Zener) can I replace it with a 1N4733A (5.1V 1W Zener)? I have a bunch of these but I don't have any 1N4734A on the hand.

Thanks
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Old 09-08-2011, 02:13 PM   #7
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Obviously, I took of D19 and put a 1N4733A in place to test it and finally, there is no more continuity between ground and 5V.

Now, I don't want to powerer it up until someone tell me it's safe to do so with a 5.1V zener instead of a 5.6V one.

Thank you all!
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Old 09-08-2011, 02:18 PM   #8
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it'll be close. it appears that would be the diode that protects from any flyback on the VCC. so if it ever spikes above 5.1v it'll clamp to ground. doesnt leave a lot of room for tolerances...
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Old 09-08-2011, 02:22 PM   #9
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Ok, can I just replaced it with a regular diode then (like a 1N4004)? I think I must order a 5.6V and wait...
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Old 09-08-2011, 02:27 PM   #10
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Honestly, I dunno.

Last edited by Braineack; 09-08-2011 at 02:43 PM.
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Old 09-08-2011, 02:34 PM   #11
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lol, Thanks alot Scott!

If there is a 5.6V zener in this spot instead of a regular diode is for a good reason. I'm not gonna change anything then :P
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Old 09-09-2011, 10:43 AM   #12
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Quote:
Originally Posted by absRTP View Post
Obviously, I took of D19 and put a 1N4733A in place to test it and finally, there is no more continuity between ground and 5V.

Now, I don't want to powerer it up until someone tell me it's safe to do so with a 5.1V zener instead of a 5.6V one.

Thank you all!
It isn't safe, at least not for that diode (the rest of the MS would be fine). If that zener diode's tolerances are a little off, you could have a 4.9 volt zener in there. Going the other way, to a 6.2 volt zener, is perfectly acceptable.
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Old 09-09-2011, 10:52 AM   #13
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Thanks Matt, now I'm pretty sure this will fix my problems!
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Old 09-12-2011, 04:44 PM   #14
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I changed D19, everythings work great now!
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