[Diy_efi] Bosch ignition coil

Tom Visel five10man
Fri Mar 24 01:51:03 UTC 2006

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.


David Allen wrote:

>  OK someone told me "1" is negative (to ignition module) and 15 is + (from
>ignition key-switched power relay).  Hope this is right.
>  I'm trying to find a coil that will hold up more then 6 months.  I've had
>terrible luck with the parts store knockoffs of the GM original coil - they
>all "arc out" between the coil body and metal core frame.  It gets a little
>white spot there and after dark you can see it arcing.  Then the car gets a
>stalling issue.
>  I've relocated the coil to the fenderwall where it is cooler and tried
>different brands and styles of coils and they all fail about the same.  The
>ignition module has been replaced with a factory Delco from the dealer.
>  This Bosch coil has the same primary DC resistance as the OEM coil but the
>secondary has a little more DC resistance.  Since there is more to a coil
>than DC resistance, I'll have to see how it performs.  It definately cranks
>up easier than the old "arc'd out" coil.
>  Hopefully this Bosch unit will hold up better.
>>Anyone know off top of your head which terminal is + and - on a Bosch coil
>>from an 84 BMW 325.
>>It's got "1" and "15"
>>I looked breifly and can't find a diagram which shows the coil terminal
>Diy_efi mailing list
>Diy_efi at diy-efi.org

More information about the Diy_efi mailing list