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Old 03-11-2011, 12:46 PM
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HEI gradual coil failure - troubleshooting

Hi all,

Looking for some advice, please.

1975 GMC 454, stock HEI ignition, timing 14 deg. initial, 38 total.
8.5 comp., Edelbrock performer 2.0 intake, Edelbrock carb.

MSD plug wires (spiral)

Plugs gapped at .045

Symptoms:

Gradual power drop off, at 2800 RPM plus it turns to "spark blow out".
Limp home, replace HEI coil (tried Echlin, AC Delco, etc.)

Replacement process:
Check cap, check rotor, check bushing. Notice signs of carbon track- regardless of using AC Delco cap and rotor , MSD "high performance cap" rotor, or Standard's "Blue Streak" cap and rotor.

After the third coil replacement, the "pickup coils" tested bad on the DVM and appeared to delaminate. Replaced the entire distributor after the third coil with a basic MSD "street fire" plain HEI. (No MSD box.)

If tracked, I've replaced the carbon tracked components.

(NOTE: all rotors have had "metal screws" not nylon.

After parts replacement, truck runs great!

Then two weeks later, same symptoms.

I've checked all 12 volt grounds and cleaned them with emery to "brighten" the ground points.

Ground points are:

Engine block to frame horn on front clip (original GM ground point)
Battery to engine alternator mount (original GM mount point)
Frame from GM ground point to body

I'm suspecting the coil is "burning through" the insultation on its windings within its secondary due to some high voltage grounding issue? Inside the cap? Could this explain the "limping" and poor drivability with weak spark-- obviously, I think...

The plug wire "test" OK with my Fluke DVM.

Obviously, the "buy a new coil" routine every two to three weeks is getting stupid.

Any advice would be great.

Thanks,

Matt

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Old 03-11-2011, 01:18 PM
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Quote:
Originally Posted by msilveira
Hi all,

Looking for some advice, please.

1975 GMC 454, stock HEI ignition, timing 14 deg. initial, 38 total.
8.5 comp., Edelbrock performer 2.0 intake, Edelbrock carb.

MSD plug wires (spiral)

Plugs gapped at .045

Symptoms:

Gradual power drop off, at 2800 RPM plus it turns to "spark blow out".
Limp home, replace HEI coil (tried Echlin, AC Delco, etc.)

Replacement process:
Check cap, check rotor, check bushing. Notice signs of carbon track- regardless of using AC Delco cap and rotor , MSD "high performance cap" rotor, or Standard's "Blue Streak" cap and rotor.

After the third coil replacement, the "pickup coils" tested bad on the DVM and appeared to delaminate. Replaced the entire distributor after the third coil with a basic MSD "street fire" plain HEI. (No MSD box.)

If tracked, I've replaced the carbon tracked components.

(NOTE: all rotors have had "metal screws" not nylon.

After parts replacement, truck runs great!

Then two weeks later, same symptoms.

I've checked all 12 volt grounds and cleaned them with emery to "brighten" the ground points.

Ground points are:

Engine block to frame horn on front clip (original GM ground point)
Battery to engine alternator mount (original GM mount point)
Frame from GM ground point to body

I'm suspecting the coil is "burning through" the insultation on its windings within its secondary due to some high voltage grounding issue? Inside the cap? Could this explain the "limping" and poor drivability with weak spark-- obviously, I think...

The plug wire "test" OK with my Fluke DVM.

Obviously, the "buy a new coil" routine every two to three weeks is getting stupid.

Any advice would be great.

Thanks,

Matt
I have not seen a coil give this kind of trouble they usually work or not. When hot they quit then some of the time after they cool will work again. The first thing I would change is the module as they have been the problem for me and for others that have posted trouble with fire questions on the board. OH remember new DON'T mean good.The work or not also applies to the module.Good luck.

OLDROD
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Old 03-11-2011, 04:35 PM
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The coil failed in my 350 but I didn't find it untill I tried another mudule with no success. I didn't screw around with trying to fix the thing, just bought one of these 49 dollar job's from KMA performance with 65K V coil. No more problem.
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Old 03-11-2011, 07:21 PM
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I don't know what you mean by "spark blow out" exactly, but I'll reply to what I think you mean as a weak spark misfire.
Use your DVM on DC volts. If you can get someone to help or if you can make sure that you can get the meters negative probe to stay on the batteries negative terminal (not the clamp!) and probe the distributor body with the pos. lead with the engine running and the lights on. The dizzy gets its ground from the block(duh). If its not getting a good ground, you should see a voltage reading on the DVM. Could be your battery wiring or alternator mounting need cleaning?
If you do get a voltage, even a very small one, I would try to re-wire the battery. First, I would run 2 wires to the negative battery terminal. One wire to the engine block(alt. mount ok after cleaning it's block attachments, manifold/waterpump) and one wire to the frame. Then another wire from the firewall to the engine block around the bellhousing area(preferably braided flex).
If you didn't get a voltage (as in a bad ground) measure the volts on the distributors plug. With the engine off(duh) I think you can back-probe the dizzy plug with a paperclip unwound so that it will stay inside the connector(contacting the 12V terminal of course) on one end and grab the top of the dizzy where its written "BAT" with the other end like a mini C-clamp. When I say "back-probe" I mean to get to the plugs electrical contact from the wire side of the connector(duh). Of course you won't have to use a paperclip if your DVM leads are skinny enough to get inside the back of the connector along with its wire. With the engine running you should get more than 12volts if I'm not mistaken. Sorry for being so long winded, but you said "any advice".
As far as modules go , it works or it doesn't. There is no such thing as a "weak module" that I know of. IMO
ssmonty
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Old 03-11-2011, 08:31 PM
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The common denominator in the issue would seem to be the plug wires.
They could be the cause.If they have to high resistance the can increase the secondary voltage stressing the coil,burning the secondary winding out.
Check with DVOM.Just a thought.
Is the primary winding burning out? or is the secondary burning out?
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Old 03-11-2011, 09:58 PM
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Matt mentions the wires being MSD Spirals, which are very
low resistance wires. However if the terminals aren't secure
and the spark has to jump a gap in the wires, then the coil
could indeed heat up and fail. I would also check the wire
resistance. Anything under 6K ohms per foot of wire is OK.
If a wire is found with infinite resistance, then you've found
the problem.

What about using a ballast resistor? Are these HEI coils oil-filled
or the E-type? If they're all failing due to heat, then a
resistor will certainly keep them cooler.
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Old 03-11-2011, 10:20 PM
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Matt, I just thought of something else, concerning those
spiral core wires. It's possible they're producing what's called
"flyback", which is basically a partial spark return to the coil
as the sparkplug fires. No I'm not crazy, (although the wife
says I am sometimes).

It happened to me once when I installed solid core wires on
a Jeep that had electronic ignition. Resistance wires are designed
to absorb the excess spark and prevent the flyback effect,
but when using solid cores, or low resistance wires the effect
is enhanced. Especially with HEI ignitions.

The cure is simple. Don't replace the plug wires, just install a
resistance wire between the distributor and coil. You can
use the stock original wire for this. Or any wire with about
6K ohms resistance.
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Old 03-12-2011, 06:07 AM
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HEI's need 12V power you can't use a Resistor or Resistor wire. If you do they wont operate properly = weak or no spark.
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Old 03-12-2011, 11:29 AM
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Quote:
Originally Posted by SSedan64
HEI's need 12V power you can't use a Resistor or Resistor wire. If you do they wont operate properly = weak or no spark.
That is what I was thinking, as the voltage goes lower the current will have to go higher to try to fire the coil. This will cause additional heat and ruin components. I would check the feed voltage when the symptoms appear. Also as mentioned above, check the distributor ground.
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Old 03-12-2011, 03:32 PM
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Hi everyone

Thanks for the insight. (I found the problem-- it was my fault).



I went back and double checked everything.

I found that my MSD wires had some issues with improper resistance.
(Due to a torn retina last year, I have a hard time seeing the fine "M" and "K" marks sometimes on my Fluke.)

The MSD wires were "OK" when I had the MSD 6M, but for a "stock" HEI configuration the resistance was improper. (About 170 ohms).

I've replaced the ignition wires with more "conventional" type resistance wire and the ignition issues have "settled down."

Lesson:

When replacing the MSD system, and going back to "stock" go "all the way" and replace all components with types that are designed for the conventional system.


Thanks,
Matt
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