[Gmecm] Re: Big cap HEI on 7747

[Mark Romans]

Thanks Carl!  This one is really starting to drive everyone crazy!
Mark
----- Original Message ----- 
From: "Carl Rumberger" <perfdyn at cwnet.com>
To: <gmecm at diy-efi.org>
Sent: Wednesday, June 07, 2006 11:12 PM
Subject: Re: [Gmecm] Re: Big cap HEI on 7747


> 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)
>
>
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