Author Topic: No Spark all of a sudden...  (Read 1131 times)

Offline fiddler

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No Spark all of a sudden...
« on: July 11, 2020, 02:35:24 PM »
I have a 1974 chevy C10 stepside.  So I replaced my vacuum advance unit with an accel adjustable.  The truck was running perfectly and I was road testing and tweaking in the timing.  I had a bit of pre ignition so I came to the house and backed off the adjustable advance unit a bit more, and headed down the driveway for another road test.  I got about 200 feet and I totally lost ignition.  I have been through everything in the distributor that I touched and I can't find anything wrong.  This is HEI ignition with a high output ACCEL coil.  I have 12 volts to the coil, and the coil shows continuity.  With no points where does one check next?  Could the Capacitor short and cause a no output condition??  Any help appreciated.  The good news is I have a long driveway so I am not in the street.  The bad news is the truck is blocking the driveway so I can't get my car out for parts or testing.....   

Thanks much...
Ricky

Offline bd

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Re: No Spark all of a sudden...
« Reply #1 on: July 11, 2020, 04:33:13 PM »
If the ignition is completely dead with a verified 12-volt supply, the ICM or PUC may have failed.  An intermittent open may have developed in the pickup coil or its leads may be chaffed and grounding.  Check the pickup coil continuity wire-to-wire and wire-to-ground while you ever-so-gently wiggle its wires.  Have an assistant crank the engine while you measure coil voltage with your meter set to the 5 AC volt scale.  Remove the rotor and flip it upside down.  Is there any evidence of ashy discoloration or texture around the center axis, below the distributor cap carbon button?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline VileZambonie

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Re: No Spark all of a sudden...
« Reply #2 on: July 11, 2020, 06:53:09 PM »
If it's a high output coil with a stock GM HEI you probably sent a lightning bolt through the module. Do the checks bd said but I would recommend a new high performance distributor as opposed to putting high performance parts in a stock HEI.
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline fiddler

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Re: No Spark all of a sudden...
« Reply #3 on: July 11, 2020, 09:53:07 PM »
Turned out to be the ICM.  I am looking more for reliability than extra performance.  If I replace the distributor with a high performance unit are they as reliable as the stock units?  Maybe in my case it would be better to go back to a stock coil.  Of course if I can get the reliability in a high performance distributor that would be more fun and I would be going that route...   
Ricky

Offline fiddler

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Re: No Spark all of a sudden...
« Reply #4 on: July 11, 2020, 10:25:36 PM »
So do you guys think the replacement of the vacuum advance unit could have caused the "frying" of the ICM?  Are they sensitive to ESD?  (Not a good thing for automobile ignition I think).  Has anybody seen a schematic of these things and how they work.  Nobody in my neck of the woods can test one.  Looks like right now this block on the block diagram would say.. "the magic happens here".  I only got to it by checking everything else, then replacing the one thing I could not check.  And thanks for the diagrams on the site.  It was great to be able to test the coil and the pickup. 
Ricky

Offline VileZambonie

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Re: No Spark all of a sudden...
« Reply #5 on: July 12, 2020, 07:10:46 AM »
Yes they are ESD sensitive. The stock HEI with mismatched components usually leads to premature failures, rotor/carbon button, cap melting, ICM failure and so on. MSD distributors have proven to be awesome to me. Very reliable and provide major improvement in drivability and performance.
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10