[DeTomaso] Ignition Coil Failures

Thu Sep 4 20:53:09 EDT 2008

Doug, Asa and Larry

Thank you for sharing your thoughts and suggestions.  I appreciate the 
insights.

Doug, What makes the Jacobs coil superior?

JT
----- Original Message ----- 
From: "Doug Braun" <doug at silicondesigns.com>
To: "John Taphorn" <jtaphorn at kingwoodcable.com>
Cc: "DeTomaso Forum" <detomaso at realbig.com>
Sent: Thursday, September 04, 2008 4:13 PM
Subject: RE: [DeTomaso] Ignition Coil Failures

> John,
>
> The MSD tech is correct about too much resistance in the secondary circuit
> possibly causing your coil failures but I doubt very seriously that your
> head to battery ground resistance is the culprit.  A simple Ohms check
> between one of the plug threads in each head to the negative battery post
> would reassure you but I'd be very surprised if you found more than a few
> Ohms for a reading.  This test would of course require long enough test
> wires for the meter to reach both points.
>
> Cylinder head to ground resistance is insignificant in comparison to the
> typical 100-300 Ohms in a resistor plug and the 1000 to 15,000 Ohms in a
> typical radio noise suppression type plug wire.  What stresses a coil is a
> secondary resistance well above 30,000 Ohms, very wide plug gaps (.055 or
> wider), or a break inside an ignition wire.  High secondary resistance 
> leads
> to electrical arcing within the coil which results in shorted windings.
>
> I'd recommend you do an Ohms check on each of your plug wires and your 
> coil
> to distributor cap center wire.  Each of the 9 wires should measure no 
> more
> than 1000 Ohms per inch.  If any exceed this spec, replace them.
>
> I'd also recommend you do an Ohms check on each of your spark plugs and
> replace any that read open or well above the others.  This measurement
> should be made on each spark plug between its bullet tip that engages the
> plug wire terminal and its small ignition tip surrounded by porcelain (not
> the ground electrode near the tip.  You should see less than 300 Ohms.
>
> When I was putting my ignition system together 15 years ago, I'd heard of
> other owners having MSD blaster failures so I chose to use a better 
> quality
> coil which has been running fine on mine for 15 years now.  Like the MSD
> Blaster it's specifically designed for use with capacitive discharge
> ignitions but it is very high quality.  An updated version of the coil I'm
> using is available here:
>
> http://store.summitracing.com/partdetail.asp?autofilter=1&part=JAC%2D380876&
> N=700+115&autoview=sku
>
> Good Luck,
>
> Doug Braun
> blue 73L #550
>
> -----Original Message-----
> From: detomaso-bounces at realbig.com
> [mailto:detomaso-bounces at realbig.com]On Behalf Of John Taphorn
> Sent: Wednesday, September 03, 2008 7:01 PM
> To: detomaso at realbig.com
> Subject: [DeTomaso] Ignition Coil Failures
>
>
> The Space City Chapter recently helped me diagnose an RPM related 
> vibration
> in my engine.  I noticed the vibration after installing the new roller cam
> in my engine.  It started around 2400RPM and grew worse until about 
> 3000RPM
> and seemed to trail off after that.  Fortunately, It sounded like a 
> misfire
> to Dan Mixon and this helped us diagnose it; although others, including 
> me,
> at first thought it mechanical related.
>
> My initial thought was that it was an unbalance caused by the double disc
> clutch pack. It is a big unit that I always thought would be difficult to
> fully balance and could only assume the shop had done it correctly.
> Accelerating a longer story, it ended up that it was the MSD Blaster 2
> ignition coil.  This is the second MSD coil I have had fail.  Admittedly, 
> it
> may be my fault.  On more than one occasion I have inadvertently left the
> ignition switch on without the engine running.  Also, my start routine
> includes turning the ignition switch to "on" and allowing about 30 seconds
> for the Accusump to prelube the engine before starting the motor.  I may 
> be
> frying the coil with these actions.
>
> I understand that on a conventional point ignition, if the points happened
> to be closed, turning the ignition "on" without starting the car could 
> burn
> the points and overheat the coil.  However, I didn't think that to be the
> case with the MSD mag distributor and 6AL.  Perhaps I am wrong, and I have
> been cooking the coils.
>
> MSD has a Forum at the website that has a troubleshooting section.  Not
> surprisingly, they describe their coils as sturdier built and more 
> reliable
> than competitor's coils.  The technician says that the reason coils fail 
> are
> generally because of too much resistance in the secondary circuit.  This
> includes bad sparkplug wires with too much resistance, plugs and
> inadequately grounded heads. They elaborate on the importance of grounding
> the heads to each other and the block and then directly to the battery, 
> not
> the chassis.  This theme seems to surface often when they review failure
> causes of other ignition components as well. I would think the head bolts
> would adequately ground the heads to the block.  However, I doubt many
> Panteras ground the heads or block directly to the battery due to their
> distant proximity.
>
> I am already employing an additional ground strap from the starter bolt to
> another chassis stud.  Yet, now I am considering running an ~ 6 gauge 
> cable
> from the heads to the stud in the front trunk where the battery grounds. 
> It
> would be relatively easy to do.
>
> I am curious of other's thoughts on the importance of grounding the engine
> to the battery and whether they have addressed it?  Anyone else with coil
> issues?
>
> JT
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