GM3.1L no spark

Wow. Dref and EST activity were zero while cranking? I mean, you're
only looking for ~0.3 volts on EST below 400 rpms and ~3 volts above
400 rpms on a DVOM. But the 12 volts on the Tan / Black bypass wire
seems bad, very bad. There may be a corroded spot between two
terminals in the ECM connector. You may have a short to power in the
harnass or I guess the ECM could have failed. Maybe you could tell me
-- could the ECM have a zener diode in the 5 volt reference circuit
that could fail and pass full battery voltage? That seems weird. But
if you have an adequate crank sensor input, then during sub 400 rpm
operation (or if you pull apart the bypass wire connector in the
passenger footboard area) you should get base spark control to the
coils. Suspect the ICM if you have proper powers and grounds -and- you
fix the 12 volt bypass problem.

I still think your crank sensor should be good enough to trigger the
ICM. Did you get the 0.7 volt reading previously with your meter in
the A/C volts setting? Just checking.

Try this article I just found.
http://www.asashop.org/autoinc/may97/techtips.htm It's written better
than I could dream of writing on this subject. Don't let the title of
code 42 detract from its relevance here. Its pretty detailed.
Toyota MDT in MO

I think you're dead on. Excellent diagnosis. I've got a 95 Lumina with
a 3.1 and could possibly do some ref checks, however I think my
readings below 400 rpm would be erroneous.
With 12vdc on the ignition bypass circuit chances are good that he
might of fried his "known good ," control module also.
I might be worth disconnecting the wht wire, till it is known that it
will be putting out a proper signal before and after the 400 rpm.
Let me know if I can't oblidge in taking some ref measurements.

JP

Found a relacement ECM, had just enough daylight to slap it in and
check for spark, no dice. Did not have time to check out what was
happening, so the next day went out to do some checking and the hood
latch was stuck (frozen) closed. got a couple of warmer days ahead so
I'll do some checking when I can get the damn thing open. Gonna verify
wiring from the module to the ECM, but I'm convinced both once working
modules are now junk from the battery voltage sent to the bypass line.

Should fire right up after the module is replaced. I just may pop one
of the toasty modules open to see if any of the parts are identifiable,
maybe try to repair one just to see if it can be done.

Findings will be posted when they're....uh....found.



gobroncos wrote:
> I think you're dead on. Excellent diagnosis. I've got a 95 Lumina
with
> a 3.1 and could possibly do some ref checks, however I think my
> readings below 400 rpm would be erroneous.
> With 12vdc on the ignition bypass circuit chances are good that he
> might of fried his "known good ," control module also.
> I might be worth disconnecting the wht wire, till it is known that it
> will be putting out a proper signal before and after the 400 rpm.
> Let me know if I can't oblidge in taking some ref measurements.
>
> JP

any luck yet?1

Well kids.....got it fired up today. Kinda lost interest in it for a
while but decided to fool with it some more. After taking care of the
rest of the problems, a new crank sensor did the trick in bringing it
back to life. I already had the sensor, so it didn't really cost me
anything. Just the will to wiggle under the damn thing and get it done.

Next project.....get the heater blower diverter to do its job. Right
now its just stuck....not in any particular position. Service
information on this in the Haynes type manuals is, well, non existant.
gobroncos wrote:
> any luck yet?1

boardjunkie wrote:
> Well kids.....got it fired up today. Kinda lost interest in it for a
> while but decided to fool with it some more. After taking care of the
> rest of the problems, a new crank sensor did the trick in bringing it
> back to life. I already had the sensor, so it didn't really cost me
> anything. Just the will to wiggle under the damn thing and get it
done.
>
> Next project.....get the heater blower diverter to do its job. Right
> now its just stuck....not in any particular position. Service
> information on this in the Haynes type manuals is, well, non
existant.
> gobroncos wrote:
> > any luck yet?1

Did you find out anything about the 12 volts on the bypass wire?
Toyota MDT in MO

Comboverfish wrote:
> Did you find out anything about the 12 volts on the bypass wire?
> Toyota MDT in MO

That 12v on the est bypass line was due to the ecm gone south. So I
ended up replacing that, the ig. module, and the crank sensor.
My guess is that the cr sensor tested good by itself, but was actually
failing when loaded by the module's input. Maybe a shorted turn in the
coil somewhere.