Car won't start anymore... no RPM on stim

[mach4]

I've been working through the details on my Megasquirt installation (MS-II, ver 3 board) on an air-cooled VW engine. On background, the conversion is using Mexican intake with a Ford Escort throttle body, GM sensors, coil negative post for tach input. Megasquirt build went well and everything tested fine on the stim and upon installation in the car started right up. I had originally built the board with solenoid for idle control, but later did the mods for PWM to control the stock Ford IAV. The car started and idled fine after the mods.

I'm in the early stages of tuning, and on one of my first logs noticed excessive voltage followed by a reset. I was in the process of getting the car started to troubleshoot that problem when it stopped abruptly after running for a minute or so and would not restart.

Pulled the Megasquirt and put it back on the stim and noticed that I could not get any RPM indication in Tuner Studio. As I turn the RPM pot the INJ LEDs begin to flash, the FP LED comes on and all three of the LEDs on the MS flash in sync with the injector LEDs. It appears that there are two speeds that they flash, at first (very low, like cranking) about 3 per sec then 6-7 per second. Tuner Studio immediately goes off line until the RPM pot is brought back to zero. The fuel pump LED glows substantially brighter but flashes in sync with the other LEDs too.

All inputs work fine - CLT, IAT, TPS, MAP

I washed the board in Isopropyl alcohol, checked all the solder joints, checked the most recent mods and went though TunerStudio resetting all the parameters.

It seems like something has shorted somewhere that has caused all to flash in sync.

I obviously could have damage due to the high voltage, but I could use some guidance of what to check at this point and where to start looking to find the problem.

[mach4]

Here is a quick follow-up video to show the problem I'm experiencing with my Megasquirt. It's easier to show than to explain.

Any suggestions on what to check?

[downwardspiral]

I had a high current short which blew d19. In the words of the great Matt Cramer:

"To check for a short in the Megasquirt PCB, power up the Megasquirt on a Stimulator and check the following points for voltage with a multimeter. Wait until the "overheat" issue surfaces to do this.

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.

I'll bet D19 is blown - if it is, you can tell because it will be hot to the touch."

[mach4]

Thanks for the suggestions, downwardspiral.

Here are the results:

Battery voltage was found on S12, S12C, D3, U5 and D9. (Q9 and Q12 were not installed since I'm using high impedance injectors)

5 volts was found on S5, 5v proto holes, D9, U5, D19 and CPU 1,20,31.

D19 is cold

So it doesn't appear that I have an "overheat" problem and all the test points check good

I discovered one potential problem, and that relates to D2 where I installed a 22v Zener diode reversed as called for for in coil negative operation. I jumpered this for stim operation as indicated, and did not unjumper when I installed it in the car. I wonder if I could have blown something by running it with this still jumpered.

[Matt Cramer]

While jumpering D2 shouldn't blow anything in and of itself (it would simply let through too much noise), it may be that your opto input circuit is in fact blown. I've found R12 seems to be the most frequently damaged part there - you can check its resistance with a multimeter and see if it's in spec.

[mach4]

R12 checks out at 390 ohm

Would it be worth it at this point to just replace the opto input chip U3 to rule that out? Any other components as part of that circuit that should also be swapped out?

Thanks

[mach4]

More data -

I just switched the jumpers on the board to go VR rather than coil negative terminal for tech input. This should, as I understand it, use a completely different set of components to handle the tach input circuit.

If the problem goes away, it's pretty conclusive that it's in the opto circuitry somewhere and if it stays, it's somewhere else.

Plugging in the stimulator gave the same exact results as with the coil negative I was using.

Any other ideas based on this new data?

[mach4]

Well, gave up trying to fix it myself. Thanks to everyone that had ideas - really appreciate the support of the community.

Sometimes you just have to bite the bullet and turn it over to the pros.

I'll post the diagnosis when it gets back.

[trakkies]

R12 checks out at 390 ohm

Would it be worth it at this point to just replace the opto input chip U3 to rule that out? Any other components as part of that circuit that should also be swapped out?

Thanks

IMHO, not a good idea to simply swap components at random. The opto coupler is a simple device which electrically isolates one circuit from another while transmitting data across it. With a stim connected and working you need to check the signal on the opto input and output. A 'scope is best, but there are other ways. A DVM that measures frequency. Even an LED and suitable resistor - details in the manual fault finding section.
If you set the rpm on the stim low so you can see its LED flashing, you'll get the same speed flashing on the opto input and output.

[mach4]

IMHO, not a good idea to simply swap components at random.

Thanks trakkies. That's the conclusion I came to and not having the electronics knowledge or proper tools, I decided that it was best to just have a pro fix it so I sent it in yesterday.

[br1070]

Hi Mach4,

Did you ever get your problem solved? Who did you send your MS too after all. I'm having issues as well and may have to go that route.

Thanks,
Brad
512-557-6162