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Thread: 93 mighty max, no spark, possible ECU failure

  1. #1

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    1993 Mitsubishi Mighty Max
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    4G64

    93 mighty max, no spark, possible ECU failure

    Hi all,
    I'm working on a 93 mighty max, 4g64, with a no spark condition. I'm fairly sure the ECU is the problem, but thought I'd run it past some experts, see what y'all have to say.
    Like I said, no spark condition. Coil power supply is fine, and I can make the coil wire spark manually grounding/ungrounding the signal side of the coil.
    Distributor seems to be working. Both signals are present manually spinning the dist when removed from vehicle. I also swapped the dist for one out of a 94 that I had lying around, no change.
    The rotor spins when engine is cranked. Timing belt is present.
    Checking for signal coming into the ignition control/power transistor, I find no + coming from the ECU to that unit.
    Wiring seems to check out. The dist signal wires at the ECU show decent continuity. I hadn't backtraced the signal wire to the ignition control/power transistor, but that grounds out with the key off - I believe that shows continuity to the ECU there. No break to ground on any of these wires.
    The main ECU relay is clicking, and also swapped that around with one out of a 94 for giggles. Don't think that's the issue..
    So, to my question, are there any other sensors or relays that could shut off spark at the ECU? For instance, is there any security system that could shut down spark? Or, anything like an ASD relay on these? Any other things you might think to check? Or, is it pretty certain that the ECU is at fault?
    Thanks for your time and any suggestions.

  2. #2

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    1985 Mitsubishi L200
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    Hi and welcome to mightyram. There isn't a security system that would prevent spark. If you didn't have the clutch engaged or for an auto the shift in park or neutral, it wouldn't turn over. If the engine is turning over and you don't have a signal out but the distributor and coil are checking ok, crack open the ECU and look for bad capacitors. You'll know when you see them. They'll be swollen and may even have leaked onto the board. Have you tried running a diagnostic on the ECU to see if it's throwing any error codes? Probably the best place to start...
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  3. #3

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    1993 Mitsubishi Mighty Max
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    Right on, thanks for the welcome and fine suggestions.
    Unfortunately, I didn't 'get' where the diagnostic port was before I started, and didn't grab the codes. I know the procedure now, just spaced doing it last time I was up there. At this point, I kind of trust the physical testing more than I trust the ECU, and it's ~25 miles to run up and plug it back in.
    I'll probably open up the ECU and look for any obvious faults. One of the screws was being nasty with me when I first tried to open it and didn't have a drill handy, and hadn't got back around to that. Kind of the same bottom line though - if I crack it open and don't see anything bad, my diagnosis is pretty much the same as it was before. To my mind at least. But, like I said, will probably open her up for fun in the morning, before sending it off for repair.
    Mostly trying to make sure I'm not overlooking something simple at this point. I've seen some funny hidden reset switches and stuff on other projects..
    Thanks again

  4. #4

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    It is possible someone fitted a coil kill switch - good luck with finding it. Most additions to wiring will be obvious when you start pulling things apart. I had a weatherproof kill switch hidden under the front bumper on one of my cars that dead shorted the coil (learned it wasn't good way to disable the ignition later but it nerfed the engine so in that aspect, it worked) Now if I remember seeing this on MR50 correctly, the 93 had a different ECU in it than other models and there was also another difference with the engine that caused grief. One of the pros on here will chime in if I've got it wrong and also fill in the details.
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  5. #5

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    Did get a chance to crack open the ECU today. Didn't see any swollen capacitors, but it did have a bit of green funk on the circuit board/does look like something has been slightly leaking for a while.
    Any advice regarding cleaning the board?
    I should be near the vehicle tomorrow, will run a diagnostic and update if anything odd comes up.

  6. #6

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    And will look again for any modifications of that nature. Looked before, but will look again. Thanks

  7. #7

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    Use a cotton swap and some isopropyl alcohol or even vinegar to dissolve the sulphation on the board. If it's around a capacitor - it has leaked. If you can use a soldering iron and can read the value off the side of the capacitor, you can fix this yourself. We got a thread on it here -

    http://www.mightyram50.net/vbulletin...ECMs-and-ECU-s
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