Release the Magick Smoke!

[Zeke's Garage]

Hello all,
I'm deep into my learning curve on my GM TBI install on my '67 Chevy inline 250. I purchased the MS II as a kit along with the JimStim, and the assembly went along without a hitch, despite this being my first time building anything this involved.
I have all my hardware and wiring installed, and have actually had the truck idling on the MS for a few minutes. But here's the problem... I was checking some cranking and warm-up enrichment values while the MS (MS II V3 board) was plugged into the JimStim here at work. I left it powered on while I went out for lunch, and when I came back, no lights or power. I disconnected the stim, and it powered up fine. Reconnected it to the MS and the lights on the stim flash then fade out like a dying battery. (I'm running it with a known-good 12v 1000ma wall wart). I then stick my nose to the MS unit and smell that something has fried.

I've looked over the board, but can't see anything physically cooked, but figure it must be in the power circuit. So now where do I start to troubleshoot so that I can find the bad component, and more than that... how can I see what I've done during assembly that might have caused a failure in the board.

Thanks!

Shawn

[Matt Cramer]

To check for a short in the Megasquirt PCB, power up the Megasquirt on a Stimulator or on the car and check the following points for voltage with a multimeter.

You should find the same voltage as the battery voltage on the following points: S12, S12C, the center legs of Q9 and Q12, the left (non-banded) end of D3, the left leg of U5, and the left (banded) end of D9.

You should have 5 volts at the following points: S5, the two +5V holes in the proto area, the right (non-banded) end of D9, the right leg of U5, the left (banded) end of D19, and pins 1, 20, and 31 of the CPU.

Let me know where you find the correct voltages, and where you find the wrong ones.

[Zeke's Garage]

Hey Matt,
Thanks for the reply. All the 12v points spike to about 3v, then drop down to about .8v and no 5v at all. Here's the results:

12v side

S12, - .83v
S12C, - 1.4v and falling
The center legs of:
Q9 - .8v
Q12 - .8v
The left (non-banded) end of D3 - .8v
The left leg of U5 - 1.4v and falling
The left (banded) end of D9 - 1.4v and falling

5V side

S5 - 0.0V
The two +5V holes in the proto area, - 0.0V
The right (non-banded) end of D9 - 0.0V
The right leg of U5 - 0.0V
The left (banded) end of D19 - 0.0V
CPU pins:
1 - 0.0V
20 - 0.0V
31 - 0.0V

[Matt Cramer]

Try desoldering U5 and repeat the check - this will tell if the fault is on the 5V or 12V plane.

[Zeke's Garage]

Matt,
After de-soldering U5, upon power-up I get smoke from what appears to be C13, C14, R19 area(C14 gets super hot) . I didn't do any of the voltage checks beyond that for fear of burning up even more stuff. Or should I just let it burn up and troubleshoot from there.

Thanks

Shawn

[Matt Cramer]

I would first de-solder those capacitors and then repeat the test.

[Zeke's Garage]

Ok, I've removed C13,C14 and R19 since the smoke was coming from that little area, (And also U5 is removed) and re-ran the tests:

Power supply is 12v wall wart outputting 12.42v at no load

12v side

S12, - 10.46v
S12C, - 9.8v
The center legs of:
Q9 - 10.46v
Q12 - 10.46v
The left (non-banded) end of D3 - 10.5v
The left leg of U5 - 9.9v
The left (banded) end of D9 - 9.9v

5V side

S5 - 0.09V
The two +5V holes in the proto area, - 0.09V
The right (non-banded) end of D9 - 0.0V
The right leg of U5 - 0.09V
The left (banded) end of D19 - 0.09V
CPU pins:
1 - 0.0V
20 - 0.0V
31 - 0.0V

[Matt Cramer]

Ok, now try putting the voltage regulator back in and see if the 5 volt side gets power again. If it is, one of the components you removed in the previous step was toast.

[Zeke's Garage]

Matt,
Finally got my head out of my broken Subaru engine and replaced the U5 regulator and now have .499 volts on all 5v pins, as well as still having proper voltage on the 12v side.

So it looks like I just need to replace C13,C14 and R19 and I should be good to go?

I'm just wondering if this indicates something else wrong with my construction, or installation, or just a bad component? Again this failed while plugged into the stimulator for about 90 minutes, but I wouldn't think that should cause any issue should it?

Again, thanks for the help, and I hope to be able to use the Facebook 17~18% discount on a wideband controller instead of another MS II kit.


Shawn
Zeke's Garage

[Zeke's Garage]

Sorry, the previous post should say 4.99v.

Thanks

[Matt Cramer]

Yep, I'd replace those parts.

[Zeke's Garage]

Just wanted to post a reply, that I replaced the three components in question, and everything seems to be working normally.

Now on to the start-up and tuning...

Thanks!

Shawn