[Diy_efi] Bosch ignition coil

David Allen davida1
Fri Mar 24 05:12:22 UTC 2006

  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.

----- Original Message ----- 
From: "Tom Visel" <five10man at commspeed.net>
To: <diy_efi at diy-efi.org>
Sent: Thursday, March 23, 2006 5:51 PM
Subject: Re: [Diy_efi] Bosch ignition coil

> 15 would be ISO standard for key-switched power, and 1 would be the coil
> negative.  There is a whole set of standardized pin / circuit numbers
> called DIN 72552 that define this stuff - just like pin 30 on an ISO
> relay is the switch common contact and 87 is the normally open contact.
> For a run-down on the most commonly seen circuit numbers, see
> http://en.wikipedia.org/wiki/DIN_72552 .
> Since you're describing a metal frame for the coil, are you installing
> this on an HEI system?  If that's the case, higher secondary resistance
> is not a big deal, but it does mean less total spark output is
> available.  Not a big deal in general, but might bite you if you run
> lean, high RPM, or under boost.
> If your vehicle is eating coils via arc-through, you have either (1)
> been buying some really crappy flea-market coils or (2) a secondary
> ignition problem.  My money is on #2.  The reason for electrons jumping
> through the coil housing to the frame is that they found it easier to do
> that than to travel through your wires, cap and rotor, and then jump
> your spark plug gap.  This might be due to a really bad coil design.
> More likely, it is a secondary ignition problem.
> If there is high resistance in the wires, or a wide gap in the plugs, or
> a very lean mixture (or poor mixture motion due to bad chamber mods,) or
> a big crusty gap to jump in the distributor, this will create a higher
> spark demand.  In other words, it will take more voltage to push through
> the stacked-up resistances in the system and then jump the plug gap.
> The coil will provide this higher voltage, up to the limits of its and
> the module's ability to do so.  **Here's the tricky part**  If there is
> a poorly insulated spot in the system - like in the coil housing, for
> instance - it will not stand up to this increased voltage.  The
> electrons will "leak" out at the weak spot, you will hear a snapping
> sound, and your engine will have a misfire.  The coil may be fine for
> normal operations, but failing when put under the stress of an unusual
> situation.  Once this starts happening, it becomes an easier and easier
> path for the spark energy to follow, until only the wires/plugs that are
> absolutely perfect see any spark at all.
> Another thing to be on the lookout for is a dying ignition module.  I
> know you've replaced it already, but the new one has had to deal with
> the increased spark demand for ____ [only you know how long.]  All of
> the increased energy that the coil has had to put out - the higher
> voltage that caused its untimely demise - has also traveled through the
> module's power transistors.  As these transistors switch the increased
> current on and off, they build up heat, leading to their early death.
> Unless you have next-to-zero hours on that module with the current
> ignition system (coil, plugs, wires,) I wouldn't trust it for a road
> trip.  Of course, your appetite for adventure (and your wife/sig-o's)
> may vary.
> TomV

More information about the Diy_efi mailing list