[Diy_efi] More 4.3L TBI Timing Issues.

Tue Dec 13 00:41:38 UTC 2005

> -----Original Message-----
> From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]On
> Behalf Of David Allen
> Sent: Monday, December 12, 2005 10:56 AM
> To: diy_efi at diy-efi.org
> Subject: Re: [Diy_efi] More 4.3L TBI Timing Issues.
>
>
> > First, a quick run-down on the vehicle...
> > '81 Malibu running a '85 Monte Carlo TBI 4.3 V-6, T-5 5-speed,
> and an '88
> > S-10 4.3L ECM.
> >
> > I replaced the ignition module, which took care of the
> stuttering/sputtering
> > problem.  I'm still having some timing issues, though.  The car has lots
> of
> > spark-knock, even under light acceleration, and the ignition
> advance still
> > seems to be "all or nothing".  The engine revs smoothly, which is
> > surprising.
>
> Okay. Interesting
>
> > When I disconnect the connector to set the base timing, the
> engine RPM and
> > timing do not change, and the CEL does not come on.  Unplugging that
> should
> > set a code 42, right?
>
> It should set a code;  UNLESS this has been disabled in the .bin
> calibration.

As far as I know this is a bone-stock ECM.

>
> > Unplugging the ESC module sets a code 43 after about
> > 5 seconds.  Running the engine with the diagnostic terminal
> grounded does
> > not change the timing, either.
>
> Does the CEL light flash when the engine runs with this terminal grounded?

Yes, it does.  It's not a steady flash, but I didn't pay close attention to
it.  Somehting like a long flash, long pause, two long flashes, long pause,
long flash, etc.  I can check again to make sure if it will help.

>
> > Could the ESC module be causing this
> > problem?  If it could be, how can I test it before I drop $70 on a
> > replacement?  I would also assume that I would get an ESC
> module to match
> > the ECM, right?
>
>   ESC could cause retarded timing, but I think there is an
> underlying issue
> in the ignition system and ECM wiring.  The ignition system has 2 modes of
> operation, one is controlled by the ECM and the other is not.
>   When the engine is first started, the ignition module fires all by its
> self, with no timing information from the ECM. It is just firing when the
> trigger wheel teeth pass by each other; or at a fixed advance under high
> revs.

Sounds like the mode I'm stuck in...

>   The second mode is when the ignition module tells the ECM when
> the trigger
> wheel teeth pass each other, then waits for a signal to be
> returned from the
> ECM to fire.
>   The 2 modes are selected by the "bypass" wire between the
> ignition module
> and ECM. If this wire is disconnected or grounded out the ignition system
> will be stuck in the startup mode with no ECM control.  What is
> strange is,
> the ECM looks to make sure this circuit is connected and sets a code if it
> is not.
>   If there is a broken wire that is also shorted out to another wire it
> could give the ECM a signal making it beleive the ignition bypass
> is working
> when in fact it is not reaching the module.
>   The ignition control signals are 5 volt and the distributor module has a
> ground reference wire (ref low) between the ECM and dist. Any bad
> grounds or
> stray signals could wreak havoc on it.

Thank you very much for this information.  This helps my understanding of
this system greatly.  I knew it was somehow controlled by the ECM, but was
not sure exactly how it worked.  I'm printing this out to look over while
troubleshooting this monster.

>   The history you give is interesting.  Something strange is
> going on in the
> wiring. With the original ECM showing a code 12 that indicates it isn't
> regestering that the engine is running.
>   There are 2 orange wires in the harness at the ECM, one is memory power,
> the other is diagnostic data. I mixed those two up once. I don't remember
> the exact symptoms but it caused problems.

I thought I had those mixed up when I fist made those final connections in
the dash harness.  I double checked them once I saw the initial problems
with the first ECM.  I'll check them again just to be sure.

>   When you swapped transmissions, was there any wiring on the
> orignal trans
> that came from the Monte Carlo? What about any grounds on the old trans?

There was only the 3rd gear indicator wire (ECM wiring shows 4th gear
indicator, but the car was only a 3 speed auto), TCC lock-up wire, and
possibly a ground wire, going to the tranny.  That plug is just tied above
the tranny now, unused.  Speaking of transmissions and switches, what role
does the Park/Neutral switch play in the operation of the ECM?  Since I have
a manual tranny now, the switch stays in "Park".  Should I move it so the
ECM thinks it's in "Drive", or does it even matter?

>   How is the car's body ground?

The body ground was good before, but I will go over all of the grounds again
to make sure.

>   Wiring problems can be hard to diagnose.  I had a problem with
> the TPS on
> my Century.  It would run HORRIBLY rich and set codes,
> intermittently.  Got
> worse and worse.  Finally figure it out when I could rock the
> engine by hand
> (in park) and make it die. The harness had a bad area from
> bending/ rubbing.
> The tape I had wrapped it in hadharneded and then cracked. This
> one cracked
> area bent easily and the wires fatigued. It had run great for 2
> years before
> this showed up.
>   Have you tested the 4 ignition module wires between the ECM and
> module for shorts, opens, and resistance?

Not yet.  That's in my plans for tomorrow night when I get home from work.
I'm going to check the pick-up coil in the distributer as well, since I will
be under the hood testing wires anyway.

> Something should certainly change when the
> set timing connector is unplugged. Unplug the ECM and 4-wire connector at
> the module.  Measure resistance between all of the 4 wires; each of the 4
> wires and ground; and each of the 4 wires and the other wires in the ECM
> harness at the ECM plug. There sould be no continuity.
>   Now ohm out each wire from the module connector to its corresponding pin
> at the ECM connector. Each should have a good solid connection.

That will be part of tomorrow night's adventures as well.  I'm sure this is
something simple...

>   Have you connected a scan-tool to the ECM to see what the real-time data
> from the ALDL is showing?

I have a copy of WinALDL, but the PC I have it on is experiencing technical
difficulties of its own.  Kinda hard to drive with a PC sitting on a
portable workbench anyway.  I ordered a lap top 'puter, which should be in
soon.  It's first mission will be to run WinALDL and help me troubleshoot
this car.  Unless of course I find the trouble before it gets here.

>   Good luck! I know these problems can be frustrating!
> David

Frustrating is a very light term for it...

Just for grins, I plugged in the 2.8L ECM last night, too.  Exact same
timing problems, in addition to the funny idle.
BTW, when I was looking for the initial problem at the beginning of this
swap, I looked at the pin-outs of the S-10 ECM and the Monte Carlo ECM.  The
only difference that I can remember (besides things like the cannister purge
solenoid) was the ground circuits for the MAP and TPS.  They're switched.
The S-10 grounds the  MAP on pin A-11 and the TPS on pin D-2 while the Monte
Carlo is just the opposite.  Both ground circuits are good...I checked them
at least twice.  That got me thinking...I wonder if the problem could be in
a circuit that the Monte has, but the S-10 does not use.  The wiring is
still there, and could be shorting out into a circuit that the S-10 does
use...  Something else to think about...

Thanks again for your help.  I'll post back with any results I get tomorrow
night.

Dave
>
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