[Diy_efi] 928/749

Thu Aug 16 20:45:18 UTC 2007

A correctly built ignition coil will never - even at open circuit - give 
so high voltage that it could cause a full internal insulation 
breakdown. However, so-called partial discharges might occur.
Many interesting things can be learnt by studying the ignition voltage 
by oscilloscope. Of course, either a proper high voltage probe or a 
capacitive clamp on the ignition cable is needed.

But another problem is that the primary peak voltage also rises too high 
  during no load conditions and this can damage the switch transistor in 
the ignition module. This is particularly true for GM HEI ignition 
modules that do not have any overvoltage protection for that transistor. 
Those systems rely on the fact that their original ignition coils never 
give more than 350-400 V across the primary. But almost all other 
ignition modules (except suspect aftermarket parts) either have a zener 
diode chain from collector to base of the switch transistor, or a switch 
transistor with zener diodes incorporated on the chip.

Best regards

Torbj?rn Forsman

Kevin Murray wrote:
> Hi Bill, I understand your frustration as I've been there once before. In my 
> case it turned out to be a bad ignition coil. Firing the plugs in open air 
> is much easier than when under compression. When the impedance at the plug 
> is too high (in the cylinder under compression) the coil energy will 
> discharge internally. To make things worse, this situation wears the coil 
> out faster.
> I'll admit I'm not familiar with Porche ignition systems but perhaps this 
> will give you some direction.
> 
> Kevin Murray
> 
> ----- Original Message ----- 
> From: "Bill Shaw" <b.shaw at comcast.net>
> To: <diy_efi at diy-efi.org>
> Sent: Wednesday, August 15, 2007 11:23 PM
> Subject: [Diy_efi] 928/749
> 
> 
> I've been chasing a long series of problems with my 928 since I
> installed the new motor this spring .  Right now it won't start,  it's
> having some weird electrical problem.
> 
> A little background...
> I ran it in a similar configuration for 3 years... Porsche 4.7L v8,
> Vortech supercharger, GM ignition module, sensors, throttle body, and
> 749 ecu.  Ran great,  had loads of fun flogging the crap out of it on
> all the road courses in the northeast.  Last fall it ate 2 spark plug
> tips and scored 2 cylinders at Lime Rock.  New motor was in the works so
> I finished it up and got it in the car.  The new one is a 5.0L Porsche
> v8, '87 big dish pistons,  16 valve ported & extrude honed heads all
> adding up to about 8.5:1 cr,   bigger cams, bigger throttle body, bigger
> exhaust.  It ran pretty good for about 2 days,  good enough to set me
> back in my seat and put a grin on my face,  then it all went to he!! again.
> 
> I've swapped ECUs, ignition modules, plugs, cap & rotor, been through
> all the plug wires with an ohmmeter, wrung out the wiring harness from
> the sensors to the ECU.  I'm getting fuel,  I can hear all the injectors
> clicking,  and have 43 lbs of fuel pressure.   The REF signal from the
> distributor makes it through the ignition module to the ecu and the ecu
> is putting out EST pulses, verified by o'scope.  #1 is firing at TDC
> with bypass unplugged.  Everything is great,  it just won't start.
> Tonight we checked the cam timing and retensioned the timing belt.  I've
> been through all the basics I can think of, all is good but no fire in
> the hole. I've reverted to my last known good bin while doing all this
> testing.
> 
> The only weird thing I can put my finger on is with spark.  I move my
> inductive timing light from wire to wire checking for spark on all 8.
> I'll find one not firing (it changes some times),  pull the wire off the
> spark plug,  insert another spark plug in the wire and ground the tip on
> the block and it will fire,  the timing light will light up.  Move the
> wire back to the plug in the block and it won't fire.  Pull the plug
> from the block,  put it in the plug wire and ground the electrode on the
> block and it will fire again.  WTF???  This is driving me nuts.  I've
> pulled one plug wire off 3 times to check resistance, wiggled both ends
> and the whole wire looking for an intermittent break,  everything is
> great,  3.1k ohms per spec.
> 
> I'm running out of ideas.  Anyone have a thought??
> 
> Thanks,
> 
> Bill
> 928s 5.0L Vortech/749
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