[Gmecm] Re: Big cap HEI on 7747

[Carl Rumberger]

Mark/Darryl/Andrew:

See the two added response sections below.

Carl

----- Original Message ----- 
From: <dgilbert78 at juno.com>
To: <gmecm at diy-efi.org>
Sent: Monday, June 05, 2006 9:38 PM
Subject: Re: [Gmecm] Re: Big cap HEI on 7747


> EST timing. I once had a small cap problem that I fought for awhile with
> erratic timing. I benched the dist in a vise, connected a few wires and
> played with it a lot. used the timing light to watch the dist. Found that
> had to much end play in the shaft. Could fire the dist by simply moving
> the shaft up and down.

Normal rotation strengthens and weakens the magnetic field, generating a
oscillating voltage in the pick-up windings. Anything that changes the field
strength can generate a signal to trigger the module. Moving the shaft up
and down moves the trigger wheel in and out of the pick-up coil,
strengthening and weakening the magnetic field. Thus it would be possible to
trigger the module by moving the shaft up and down.

The real question is does that occur in normal operation in the engine? The
answer is no. Under normal operation the camshaft drives the dist. gear,
distributor, and oil pump at a relatively constant speed. The combination of
the gears, being cut at an angle, and the power required to drive the oil
pump, generates a considerable upwards thrust. This thrust on the
distributor gear, forces it(and the shaft) up against the distributor's
tanged washer and the trust shim. It stays in this up position as long as
there is rotation and load from the oil pump. It is normal for GM
distributors to have .050" - .080" end play with no adverse effect on
timing.

Now if the camshaft rotation oscillates(drive problems or cam walk) or the
oil pump load varies(binding or cavitation) then this could lead to a up and
down movement of the shaft and spark timing problems, but of course the
distributor didn't create the condition. And though you may be able to mask
the symptoms by removing end play, the real fix lays elsewhere.

>Watch out for cracked magnet material, makes extra
> poles i think. Shimmed shaft endplay so would not cause spark when shaft
> moved vertically . It is truly amazing just how slow you can rotate the
> shaft and still produce a spark. I think the internal coil module has 1
> less pin, but is basically the same module. In bypass the ECM has no
> control, it is all internal. If jumping timing it is the ECM telling the
> module to fire when not in bypass. If you have a knock module it could be
> detecting knocks and retarding the spark at random. Could be false
> knocks. An O scope at times like this works wonders. I have a complete
> break out box i made for the 1227747, I can scope any wire or measure any
> voltage if i need to work on something that is giving me problems. I had
> a small cap and augured the top of the cap out at each tower so I could
> watch the spark inside the cap when running, that showed me a lot,
> uncovered a timing error on my part. I do not know if you can do that on
> the big cap with the coil in top (i assume you have the coil in top) Good
> luck
> Darryl...
> On Mon, 5 Jun 2006 21:11:51 -0700 "Mark Romans" <romans at starstream.net>
> writes:
> > Hi DarrylAndrew:
> >
> > We switched the wires on the module and it seems to have killed
> > the module.  What's weird is that your description is so exactly
> > what the
> > symtoms are.

I appear to have gotten in on this series late and am not exactly sure what
the symptoms are but there seems to be a concern about possible reverse
polarity of the pick-up coil. The effects of reversed polarity are unstable
triggering at low RPM, timing shift of near 40 degrees(see other response
below)(of course this could be offset by changing/correcting the initial
timing), incorrect rotor fazing(this can cause significant problems), and
timing shift with RPM. Reversing the polarity as an experiment shouldn't do
any damage to the module unless the shift in rotor fazing caused such an
increase in required spark voltage that the coil secondary windings arced
through the insulation to the primary windings. Of course the primary
windings are connected to the module and the resulting voltage spikes could
possibly do it damage, though the likelihood is pretty low.

The low tech way to check for correct polarity is to disconnect the timing
connector(put the ignition in bypass) and check the timing with a timing
light. Turn the engine off and align the crank timing marks to where they
were observed with the timing light. Remove the dist. cap and rotor so as to
view the alignment of the pick-up coil inner rotating points and the outer
stationary points. If the point are aligned the polarity is correct. If the
rotating points are near midway between the stationary points the polarity
is reversed.

Is the distributor commercially rebuilt(the housing will be abrasively
blasted). If so, I have seen several rebuilt distributors where the pick-up
coil was incorrectly assembled with the magnet upside down. This reverses
the magnetic polarity which in turn reverses the output polarity.

> > Keep in mind this is a big cap HEI with an internal module.
> >
> > The timing varies by huge amounts and greatly by engine rpm.
> >
> > It idles fine in EST bypass.
> >
> > Everything has been replaced.  One more thing is that this is an EST
> > HEI
> > modified to fit
> > a 350 Buick running a 747 ecm.
> >
> > Mark
> >
> >
> >
> >
> > ----- Original Message ----- 
> > From: "Andrew Gibson" <andrewsharyn at yahoo.com>
> > To: <gmecm at diy-efi.org>
> > Sent: Saturday, June 03, 2006 5:09 PM
> > Subject: [Gmecm] Re: Big cap HEI on 7747
> >
> >
> > >I wasn't talking about the 4 wires from the module to the ecm. I
> > was
> > >talking about the two wires from the ignition pickup coil inside
> > the
> > >distributor to the module. If the wires are switched the timing
> > pickup is
> > >coming off the curved section of the waveform instead of the
> > vertical slope
> > >in the center. This can offset the timing by as much as 40 degress
> > and will
> > >vary greatly by engine rpm. (See the piggyback igintion module
> > discussion I
> > >started a couple of months ago.) The wires should be white and
> > green (at
> > >least off the pickup coil) I don't remember which is positive and
> > which is
> > >negative.
> > >
> > >  Also since you are obviously having problems and must have
> > extended the
> > > pickup coil wires to have a external module: If you need the
> > module more
> > > than 1.5" away (The length of  stock pickup coil wires) the
> > spliced in
> > > wires need to be twisted for noise reduction. (If you peel the
> > insulation
> > > off a piece of network cable you will get the idea)