[DeTomaso] Ignition Coil Failures

Thu Sep 4 17:13:28 EDT 2008

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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