[Diy_efi] Bosch ignition coil

Fri Mar 24 06:51:32 UTC 2006

I have noted that the GM HEI ignition modules can very easily get 
damaged when anything other than original ignition coils are used.
They differ from all other ignition modules in that they have no 
overvoltage protection for the power transistor, they rely on the fact 
that the original coils seem to have an inherently limited voltage 
kickback. With a GM coil, the voltage peak across the primary will never 
exceed about 350 V at any secondary load (open circuit, some hundred pf 
capacitive load or what may be the worst case). With a coil that is 
intended for another brand of ignition system (for example, Bosch or 
Chrysler), the HEI module will blow out the first time an ignition cable 
comes loose.

When measuring ignition coils to see which ones can replace each other, 
not only the winding resistances are critical.
The inductance of both windings should also be measured, the primary 
with open as well as shorted secondary winding.
The secondary winding often has a inherent resonance at about 5-10 kHz, 
so all inductance and turns ratio measurements should be done at a lower 
frequency. 1 kHz is usually ok for most measurements.

I found out all this some ten years ago when i was busy with development 
of electric fence energizers and the customer wanted me to check out if 
it would be possible to use a standard automotive ignition coil instead 
of the expensive custom designed pulse transformers they were using. The 
answer was definitely NO.

Best regards

Torbj?rn Forsman

David Allen wrote:
>   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
> running.
>   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
> up.
>   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.
> Later,
> David
> 
> 
> 
> ----- 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
>>
> 
> 
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