[Diy_efi] 2.0 diss module with the 1227749

[Shannen Durphey]

Original DIS types would take some research.  Both the hall effect and the
vr styles were released around 1986.  The 2.0l system that I've been
working with was released in '87.

"cheat DIS" is completely possible.  Some methods might be just as tricky
as fixing the code issues.

First, let me say that the problems I've encountered are specific to one
situation:  Trying to use the 7749 and distributor based code on an engine
designed with a DIS module.  There is an alternative in my case.  I could
try to install a distributor in the block to run the dizzy code.  That
would likely require a mechanical solution to two parts occupying the same
space at the same time.  Now, for people building a completely custom DIS
intallation, the code issue can be overcome with hardware, in the grand old
"I can't get into the ecm" tradition.  This is for the 4 cyl DIS module as
used in the GM 2.0l engine, and this discussion applies to 4 cylinder
engines.

There are drawings on the ftp site that show when the module generates
ignition pulses in this engine.  During startup the DIS module fires the
spark plugs at TDC for easy engine starting.  At this time it is using only
internal logic to generate spark at the correct time.  After the bypass
voltage is applied, the module fires based on timing of the EST signal. EST
signals are calculated and generated within the ecm.

Assume that a distributor delivers a ref pulse 180 crank degrees BTDC.  We
can say this, because in a 4 cylinder each spark event is 90  crankshaft
deg. before the next.  If the time between pulses were measured, and an EST
pulse delivered a certain amount of time after the ref. pulse came in, then
a spark would be generated at the distributor at 180 deg - (wait time)
BTDC.  In other words, if we get a pulse at 180 deg, and wait for 150 deg,
then make a spark, we have delivered a spark at 30 deg BTDC.  The cool
thing about a distributor is that it doesn't matter where the crank is when
we deliver a spark pulse to the module, because the position of the rotor
is going to direct spark to the closest cylinder.

DIS REF pulses are delivered 60 crankshaft deg. btdc.  The crank pos.
sensor is mounted in the block so it is directly above a specific notch in
the crankshaft when the piston is at TDC.  It is this notch that the module
generates REF pulses from when the engine is running.  The DIS module also
has to keep track of which cylinder fires next, unlike the dizzy module. 
It does this by counting the notches that pass the cps.  If the synch notch
fails to pass the sensor, the module will stop delivering spark to the
engine regardless of bypass line state or est signal.  If trying to use the
DIS system with the 7749, issues in the code appear when trying to _not_
wait for greater than 90 camshaft deg before delivering spark. 

A custom dis installation might be able to overcome this issues without
ever approaching the code.  The easiest approach might be to install a
distributor base to generate REF pulses outside the dis module.  Yes, the
module operates in OEM state with a 60 deg window between the point when
REF is generated and the next cps notch passes the sensor.  If you set your
maximum advance to 60deg BTDC and minimum advance to zero deg. you should
never see spark outside this window.  I'd expect it would be almost a
non-issue to ground the dis generated ref signal through a resistor and use
the dizzy based module to provide signals at the correct time to the ecm. 
Bypass and est would work the same way as they usually do.

This would also work if a distributor cannot be physically installed.  A
crank sensor and wheel can be fabricated to generate 2 pulses per
revolution, each at TDC for the 1+4 and 2+3 cylinder pairs.  A traditional
HEI module can be installed and connected to the ecm REF line.  The CPS can
be close to TDC without being correct, as the offset can be measured and
accounted for by adjusting "ref angle" within the calibration.

Other methods of trying to fool the DIS module by adjusting the cps signal
timing may cause issues.  It is possible that the dis module is designed to
select the "next" cylinder pair even before it receives
the est signal to fire the current cylinder pair.  But it's entirely likely
that the window of opportunity is limited to 60 deg BTDC plus a small
amount ATDC.  In this case there might be complications if the notch timing
were modified.

Shannen


William Shurvinton wrote:
> 
> I was working off a 93 transport as it is the only GM thing I own, and the
> only thing I have a full GM service manual for.
> 
> Interesting that they 'evolved'. Does mean that 'cheat DIS' using HEI
> modules and older pickups is entirely possible.
> 
> Shame I'm out of HEI modules. I get em in, they go. That's the problem with
> being 3000 miles from the nearest boneyard
> ----- Original Message -----
> From: Garfield Willis <garwillis at msn.com>
> 
> I don't know about very modern versions, but the original GM DIS systems
> used VR sensors, not hall. And the wheel is part of the crankshaft
> 
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