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Electronics Need help wiring something up? Thinking of adding an electrical component to your car? Need help troubleshooting that wiring glitch?

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Old 06-29-2001, 04:23 PM   #1
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Diagnosing a bad ECM.....

Anyone know how I can test to see if my ECM is good without swapping in a new one. How can I test the existing unit? My friend jolted himself on the altenator while finishing up a header install and the car won't start. I've tried every possible fix except the computer. Any help here would be appreciated.


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Old 07-01-2001, 01:02 AM   #2
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Did you try to run for codes? If the ecm is bad it should still give you a code--don't know what it would be off hand tho.
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Old 07-01-2001, 06:51 AM   #3
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I did check for codes, but all I get is the usual code twelve that you're supposed to get. If the car doesn't even run, would it even have a chance to recognize something is wrong and set a code?


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Old 07-01-2001, 11:00 AM   #4
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Does the car crank (starter turn over the engine)?

Are all the fuses good? Don't forget the fuel pump fuse (on my '91 it is located directly behind the battery in its separate fuse holder for a 20 ampere fuse).

I think most will say that if you're getting the normal code (12), there really isn't an easy way to check the ECM unless you swap it with a known good unit.

I don't know what you mean by "jolted" when your friend was by the alternator (was he shocked or did ground contact the output stud?). That in itself would not normally damage anything, much less to the ECM, since the output terminal (stud) is just connected to the battery positive terminal. If anything, I'd first suspect the fusible link between the alternator and battery positive (not all models have this), but this wire has nothing to do with the actual starting of the engine... It'd be like just shorting the battery positive terminal to ground for a short moment.
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Old 07-01-2001, 06:20 PM   #5
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The car cranks, and it's getting spark and fuel to the cylinders, the gas is fresh, and the plug routing is correct and the fuses are good. It's almost like the timing is way off somehow, or the coil wire is off, but the distributor was never removed and the coil wire is tight and getting spark to the cylinders. I'm stumped as to what else it could be.... any insight?

I believe my friend was tightening the connection on the output terminal when he was "jolted", but I don't know if he was actually shocked. So more than likely he grounded it I guess.


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Old 07-01-2001, 06:36 PM   #6
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Dave,

Well, you've stumpted me. As I said, I cannot see how just grounding the alternator output (battery positive) for just a millisecond would keep the engine from starting. With a good spark, good fuses and the ECM showing a normal code (12), something must have changed since the headers were installed... I cannot even think of any wire that could have been accidentally removed from such an installation that would keep it from running.

I have no doubt that there are plenty of others here that can contribute an intelligent guess though.

[This message has been edited by Stuart Moss (edited July 01, 2001).]
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Old 07-05-2001, 02:33 PM   #7
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UPDATE: I unplugged the timing connector (the one you unplug to set the base timing) and tried to start the car, and for a brief second, it sounded as if the car was about to start, it sounded as if it started to suck air, but it just didn't catch and still doesn't start. In desperation, I also went back to my original PROM and tried to start the car with it in place, but nothing.

While we were installing the headers, my friend manually turned the flywheel by hand while the tranny was in neutral. The serpentine belt was off at this point. I don't know how, but could this have affected the timing in any way? Everything mechanical inside the engine should still be in sync, timing wise, unless the timing chain was broken, correct? Any ideas would be greatly appreciated.... I'm very frustrated and very p!ssed, not having driven my car for 8 months now.


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Old 07-05-2001, 02:33 PM
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1999, altima, bad, diagnose, diagnosing, ecm, ecu, faulty, sign, signs, swap, symptoms, tpi, trans, unit
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