Mechanics

MIGHTY-MAX V6 ECM TEST - CODE READER

1991 Mitsubishi Mighty Max • 6 cylinder 4WD Manual • 145,000 miles

1991 Mitsubishi ECM/ECU test, or Code reader.

Hi, I have a 1991 Mitsubishi Mighty-Max 4x4 truck with 3.0 V6, 145k miles. Is there anyway to test the ECM myself, or a trustworthy place that will test it for me (I'm afraid they'll try to sell me one whether mine works or not). Also is there a way to read the codes, I've heard that this model is too old for the standard readers. I once saw that someone was able to make their own code reader using just a single L.E.D. Light from a computer, he hooked it up to the reader socket and then counted the number of light blinks, then used that to check the code in some index.

My truck won't start anymore, I've never had any trouble with it till now, and I'm afraid it's going to be an expensive electrical fix, so I'd like to test as much as I can before investing in expensive parts that can't be returned.

Thanks for your time. - Tom
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TurboTom
April 13, 2011.




I doubt the code reader is going to be any help with your problem anyway.

All "crank, no start" conditions are approached in the same way. Every engine requires certain functions to be able to run. Some of these functions rely on specific components to work and some components are part of more than one function so it is important to see the whole picture to be able to conclude anything about what may have failed. Also, these functions can ONLY be tested during the failure. Any other time and they will simply test good because the problem isn't present at the moment.
If you approach this in any other way, you are merely guessing and that only serves to replace unnecessary parts and wastes money.

Every engine requires spark, fuel and compression to run. That's what we have to look for.

These are the basics that need to be tested and will give us the info required to isolate a cause.

1) Test for spark at the plug end of the wire using a spark tester. If none found, check for power supply on the + terminal of the coil with the key on.

2) Test for injector pulse using a small bulb called a noid light. If none found, check for power supply at one side of the injector with the key on.

3) Use a fuel pressure gauge to test for correct fuel pressure, also noticing if the pressure holds when key is shut off.

4) If all of these things check good, then you would need to do a complete compression test.

Once you have determined which of these functions has dropped out,
you will know which system is having the problem.

Wrenchtech
Apr 13, 2011.
Thanks Wrenchtech, I didn't want to go into too much detail too soon, as I could write many pages about this if needed. I have worked on many different cars since the mid 70's, so I understand quite a bit about how a car works. I started the diag. By checking the easy and obvious first, but I may now have to go through a more thorough checklist to be sure I did everything correctly and didn't miss anything, there are a lot of sensors and servos I didn't bother with, and also the neutral switch.
I also have the two volume factory shop manual for this truck, so I have most the tech info I should ever need about this truck.

Here is a little info about how it failed.
The truck never had any problems in it's 145k miles (other than going through alternators every 5 years, bearings usually wore out). The first time I had trouble with it was a very cold winter day, about 0 degrees, I started the truck and was warming it up for about 10 minutes. I was about to leave for work when I remembered something I needed in my house, so I shut the truck off so I could unlock the house, I came back less than a minute later and the truck wouldn't start. Everything seems fine with the truck, but it just wouldn't start/run. I initially checked fuel and spark, but wasn't getting any spark out of the coil. It sat for about a week while I did some diagnostic and had some electrical parts checked/replaced (new distrib cap/rotor, plugs + wires, coil).
A friend of mine came over and got it started by just jiggling the key and working the ignition, so we thought it was just a bad ignition switch, and after replacing that it worked for a couple weeks but eventually continued to have the same problems. It worked some of the time, very erratic, sometimes it would work for a couple weeks or more with no problems, other times it was a crap shoot if it would start anytime. It seemed if I could get it started once in the morning it would start ok for the rest of the day, but then it became worse, it began shutting off while I was driving (something it had never done before), and then it got to the point it just wouldn't start at all. One trick that seemed to work for a while, was to just leave the ignition on with the dash lights on for a while (2-20 min.) Until you heard a few fast clicks (sounded like the fuel pump kicking on, or some other servo switches, or both?), After that noise the truck would start with no problems (when it would start it ran very well). But eventually that trick wouldn't work anymore. I've had a lot of the electrical tested or replaced, but they can't test the distributor or computer, and they're both on the pricey side to replace.
A friend of mine knew a certified master mechanic, and although he never actually looked at the truck, he was able to check his special database about problems with this truck and engine, and from our description of the problem he felt strongly that it was the computer that was bad, and that it was probably burned out by a bad throttle sensor unit(or crank position sensor?), But I never saw any smoke or smelled anything burning or too hot. It seems to me to be just some sort of switch or connection that just clicks on, and then the truck will start. The mechanic was moving out of state that week, so we were never able to find the time to have him look it over so he could be sure.
I'm sort of a jack of all trades, and having grown up in Michigan I've always done all my own car repairs. I've done a bit of research on this problem myself, and the computer seems a likely suspect, but I've also heard several good arguments that it could be the distributor, or maybe even a loose wire somewhere (aaaargh!). I know intermittent electrical problems can be the hardest thing to fix on a car, that's why I was hoping I could read the error codes somehow, as that would be a good place to start looking.
So I'd like to have the ECM tested first to be sure that is the problem, before I paid to have it replaced or rebuilt (then test the distributor). But I'm very skeptical about having a company test it that also sells/rebuilds them (can I trust them?).

The ECM is a MD162826, just in case that helps.

Thanks, - Tom
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TurboTom
Apr 14, 2011.

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