1990 lumina 3.1 will only run if connected to diagnostic computer or with alternator unhooked

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mushroomwick@hotmail.com - 25 Apr 2007 16:58 GMT

I am stumped and running out of money. My 1990 lumina 3.1L only
runs properly if hooked to a diagnostic computer or with the
alternator unhooked. Of course the computer is showing nothing wrong
but as soon as you disconnect it the car dies. It starts everytime but
won't idle or stay running under any condition. Ether in the intake
makes no difference. Everything has been replaced in the fuel system.
New fuel pump, fuel filter, fuel injectors, even a new computer and
battery. However I don't feel any of those things is the problem since
by simply unhooking the alternator it will run perfectly as long as
the battery has life left or pluging in the computer has the same
effect.
can someone help me please?

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Raymond Sirois - 26 Apr 2007 04:20 GMT

> I am stumped and running out of money. My 1990 lumina 3.1L only
>runs properly if hooked to a diagnostic computer or with the
[quoted text clipped - 8 lines]
>effect.
>can someone help me please?

Okay, just call this craziness, but.... Ever think of having the
alternater checked?

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mushroomwick@hotmail.com - 26 Apr 2007 15:36 GMT

> On 25 Apr 2007 08:58:00 -0700, mushroomw...@hotmail.com, in an
>
[quoted text clipped - 7 lines]
> SysOp: The Lost Chord BBShttp://thelostchord.dns2go.com:6080
> telnet://thelostchord.dns2go.com:6023

Alternater has been checked and is ok. The original problem is the
same problem it has now. The trouble started one morning all of a
sudden. I thought it was a fuel problem at first. It seems its not. I
expect its electrical in nature, but have no clue really.

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C. E. White - 26 Apr 2007 16:00 GMT

> Alternater has been checked and is ok. The original problem is the
> same problem it has now. The trouble started one morning all of a
> sudden. I thought it was a fuel problem at first. It seems its not.
> I
> expect its electrical in nature, but have no clue really.

Alternators are cheap. I'd try a replacement. I am guessing (only a
guess) the alternator is creating some sort of electrical interference
that is causing the PCM to fail. Plugging in the diagnostic computer
adds an additional load to the circuit that is reducing the effects of
the interference. The alternator could still test as good and be
creating the interference.

Ed

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aarcuda69062 - 27 Apr 2007 00:21 GMT

In article
<1177516680.108527.308080@t39g2000prd.googlegroups.com>,

> I am stumped and running out of money. My 1990 lumina 3.1L only
> runs properly if hooked to a diagnostic computer or with the
[quoted text clipped - 8 lines]
> effect.
> can someone help me please?

It's entirely possible that the diagnostic computer is completing
a ground when it is plugged in, and/or disabling the alternator
is allowing a ground path that wouldn't normally exist.

You need to test the voltage drop for the entire EFI system,
sounds complicated but is actually very easy to do.

1) turn the ignition on, engine not running.
2)go to the ALDL (where you plug in the diagnostic computer),
install a jumper between cavity "A" and "B" (same as extracting
flash codes).
3) connect a volt meter set to read DC volts between the jumper
installed in step 2 and the battery negative, + lead to the
jumper, - lead to the battery negative.
4) read voltage drop on the meter.

Excessive AC voltage from the alternator could also be a problem,
to test; set volt meter to AC volts, connect + lead to output
stud on the alternator, connect - lead to battery negative, start
the engine, the AC voltage should not exceed 100 millivolts.

Try a redundant ground to the ECM. 1990 was the dark days for GM
ECMs, lots of shoddy build and plagued with problems.

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mushroomwick@hotmail.com - 27 Apr 2007 21:03 GMT

> In article
> <1177516680.108527.308...@t39g2000prd.googlegroups.com>,
[quoted text clipped - 36 lines]
> Try a redundant ground to the ECM. 1990 was the dark days for GM
> ECMs, lots of shoddy build and plagued with problems.

Ok I've had a new alternator hooked up to the car. It didn't make a
difference. The tech i've got working on it noted that his diagnostic
computer wouldn't work after being attached to my car until he
disassembled it and did what he called a Hard Reset. whatever that
means. he's afraid to hook it up again for fear of destroying his
computer which he says he has many thousands invested. he did say
there is a problem with the injectors syncing in time. He feels its a
wire somewhere grounding out and will start the search for it. In the
meantime i'm all ears for any ideas.
I will pass on the jumper idea. Any plan to isolate the problem will
not be ignored.

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