[Diy_efi] Ignition/coil

Bill Washington bill.washington
Fri Mar 24 07:15:18 UTC 2006

	Most coils are 9V fed thru a ballast resistor during normal running to drop the 
12V(nominal 14V really) to about 9V. During cranking the ballast resistor is 
shorted out to provide extra 'oomph' because the battery voltage sags during 
	If you go looking for this 'Ballast' resistor in most cars you will not find it 
because the actual wiring is sized (small) to provide the resistance, and during 
cranking a lower resistance path is connected in parallel either by the ignition 
switch or by a relay.
	In a ballast resistor system, if the supply side has been modified (ie the 
resistive wire bypassed) or the relay has failed with the contacts permanently 
closed you will have lots of spark but a short coil life....
	If the car was designed for a 12V coil, ie it does not have a ballast resistor 
and a 9V coil is fitted the result is the same - lots of spark, short coil life.

	Some years ago an "auto electrician" without my knowledge fitted a Bosch GT40R 
(9V) coil in place of my GT40 (12V) coil when a workshop had trouble starting my 
car (I had told them that they would not be able to start it and to call me, but 
they "knew" better) - My car had a 12V coil standard - and no ballast resistor - 
the coil only lasted a few weeks!!! and I had to replace it - the workshop 
refused to accept any responsibility - needless to say they have never seen any 
of my cars again!!!!

Just some thoughts
Best Regards

Re: [Diy_efi] Bosch ignition coil
"David Allen" <davida1 at hiwaay.net>
Thu, 23 Mar 2006 21:12:22 -0800
<diy_efi at diy-efi.org>

   Thanks for the info on DIN 72552.
   As for the iginition system in general; it's a remote-coil HEI.  The
resistance of the coil-to-cap lead is 12K, and the plug leads range from 10K
to 13K depending on length. The cap and rotor had evidence of cap/ rotor
interference but the contacts were clean and the rotor tip wasn't broken
off.  I replaced both as a set.  The plugs are Bosch Platinum (original; not
multi-tip etc..) gapped at .045". Engine is a 305 running about 10:1
compression normally aspirated.  There haven't been performance issues
betwen coil failures.
   With each coil I replaced the cap and rotor shortly after and tested the
wires, suspecting the high secondary resistance issue you describe. I've
never had a module outright fail on this car. The new one on there now was
purchased as a replacement for one I had to "borrow" to get my other car
   I think the coil quality may be the problem.  The last couple times the
coil has died I have been on the road and had to locate a local parts place
to buy a coil. The choices have been limited.  Will see how this one holds
   Thanks for the ideas. I do have a great apetite for adventure and no wife
to worry about so I will let the module stay for now!  :-)
   I plan to measure the peak primary voltage and see how this is affected by
changing to non-platinum plugs.  I remember a thread where someone was
concerned they were damaging plug wires by use of platinum spark plugs. It
sounded far-fetched but you never know.

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