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That X#$&^*@~* '75 still won't start--the saga continues


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I posted our sad tale last week--the '75 with the stock ignition system that has a no-spark situation.   We know it's ignition related as there's gas visible in the carb and the starter turns the engine over just fine; just no spark.  It was running a few weeks ago on a 120 mile round trip.  Despite extensive diagnosis and testing, there's still  no spark anywhere between the coil and the spark plugs, even though the coil is getting its 12 volts at the + terminal.
 
Here's a recapitulation of our original observations and tests, followed by additional tests done as a result of your suggestions:
 

'75 2002, stock ignition system (no ad-ons).  Only deviation from stock is a Bosch blue coil.  

 

Symptoms:

  • no spark at all, using a new spark plug connected to #1 plug wire and resting on the valve cover.  We also tried disconnecting the HT coil lead at the distributor end and holding that end to the valve cover while cranking.  No spark there either.
  • Sufficient voltage at the coil while cranking (10.8 v); 12.84 v with ignition switch in the on position.
  • with ignition on, popping the points open with a screwdriver elicited no spark

 

What's been checked:

  • points, plugs, cap, rotor, condenser and plug wires are new
  • checked the carbon button on the cap; it's OK (new cap)
  • Point gap is correct at .016
  • coil was tested for resistance and is within specs
  • +12 volts at the coil's low tension terminal both with the ign on and in the start mode.
  • tried a second Bosch (black) coil; no change 
  • tried a Bosch blue coil; no change 
  • Swapped out condenser with another new one--no difference
  • replaced the resistance wire in the wiring harness with a regular wire (properly soldered in) since we're using a Blue coil with its own internal resistance
  • wiring at the coil is correct
  • checked the dizzy-to-coil low tension wire for continuity; it's good
  • checked the HT lead from coil to dizzy; it's good (and new)

 

Additional, more detailed tests/observations since the original posting above:

  • tested the entire ground path from the battery to the engine block to the distributor and coil; it's complete, as is the ground path to the body from the battery.
  • Ignition switch is good, as a voltmeter on the coil + terminal reads over 12 volts when the ignition key is turned to "on," and the coil continues to get voltage when the key is turned to start. 
  • fixed and movable points are insulated from each other when the points are open, and the insulated (movable) points do not ground to the dizzy body.
  • positive and negative sides of the condenser (both were tested) are also insulated from each other
  • The rotor tests OK--continuity between the center contact and the one at the tip
  • The cap's center terminal shows continuity between the carbon button and the HT lead socket.
  • A jumper wire run between the coil's - terminal and the distributor's LT terminal made no difference
  • There's continuity between the point plate and the distributor body--the little braided ground strap is intact, so the electrical part of the distributor (points, condenser, cap and rotor) all test good.  No missing grounds or positive-to-ground leaks. 
  • The coil HT lead tests good (and is new)
  • Tried 3 different coils, all of which tested within specs
  • Per Rob Siegel's suggestion, we couldn't get a spark at the points with the ignition on by twisting the rotor shaft just enough to open and close the points
  • Our test spark plug, pressed against the valve cover and connected first to #1 plug wire, then #2 is brand new.  
  • We disconnected the three black wires on the coil - terminal, one at a time, to pursue the possibility of a shorted out tach or the emission system control box grounding the circuit.  No difference.  

 

We're really out of ideas now, as everything that causes the ignition system to function has been tested.  Short of having three bad coils (after testing within specs) or two new condensers--different brands--both bad out of the box, we're stumped.  

 

mike

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Maybe he started a new thread about it.   😋😁

He's high on Ohms...

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It could be 2 defective condensers, parts quality of even previously trusted brands is questionable now.

 

I'm thinking defective points, since it's the only thing you haven't tried multiple different ones.

 

I think I would try what Rob Siegel said and swap out the distributor for a known good one.

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Hmmm... how about testing just the coil by running a jumper wire from negative terminal that can be then grounded and disconnected manually? That should make a spark on the coil wire. 

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I really liked Toby’s suggestion from the last thread, it doesn’t sound like you tried that ?

 

Im leaning towards non conductive points. 
 

If at all possible, I would recommend swapping parts with known, functioning parts as opposed to different unknown parts. 

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Any of those new parts could be defective, may want to try swapping some of the new parts you installed with  another new part just to confirm functionality or at least put a meter on them to make sure the current paths are good.  I’ve had a number of brand new parts not work over the years.

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4 hours ago, 2002iii said:

I'm thinking defective points, since it's the only thing you haven't tried multiple different ones.

 

 

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Voltage drop test battery cables, starter engaged. Look ,feel for heat at connections, starter also. Just thinking back on weird, slow,no start cars I have dealt with over the years 🤔

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Okay, since all the good ideas have been tried here’s some odd ones:

 

I don’t recall if you ever mentioned if the car was manual or automatic. If it’s an auto - It couldn’t be the extra Starting Relay could it?

 

Could a short over in the Diagnostic plug on the drivers fender have this effect? Tach, starter, volt regulator all tie in there.


As others have mentioned - try new points (or a PerTronix). Check the new plug wires all have continuity.

 

Ask your friend what was done to the car (everything & anything, no matter how minor) since the car did the 120 mile drive. Something changed, or something failed by itself during non-use, which is pretty rare.

 

Tom

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7 minutes ago, visionaut said:

f it’s an auto - It couldn’t be the extra Starting Relay could it?

Tom, the auto neutral safety switch cuts power to the starter motor so you'd get a no crank symptom, it doesn't interfere with the spark at all.  

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Posted (edited)
1 hour ago, Son of Marty said:

Tom, the auto neutral safety switch cuts power to the starter motor so you'd get a no crank symptom, it doesn't interfere with the spark at all.  

 
Yeah. I was just glancing over the wiring diagrams, and noted this extra Auto-only Starting Relay, which connects to a Reversing Light Switch w/Starter Inhibitor Lock instead of the normal Reverse Light Switch….  Throwing ideas out there, since we’re stumped.

 

If this car has that extra ignition diagnostic connector on the valve cover, that thing is tied directly into both the Dizzy & Coil wiring…

74E3D732-E6BB-4FAE-85F1-B55CBADF4FAD.jpeg

Edited by visionaut
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Posted (edited)
11 hours ago, Tommy said:

Hmmm... how about testing just the coil by running a jumper wire from negative terminal that can be then grounded and disconnected manually? That should make a spark on the coil wire. 

Think its time to look at the primary and secondary circuits separately.

 What Tommy suggests would test the secondary circuit up to the coil outlet and eliminate it from consideration.

I betting you will get a spark if you try it but dont zap yourself.

Something is preventing the apparently energized primary field from collapsing across the secondary and inducing an ignition pulse, I think.  That would have to be points or condenser related. Try a third condenser on it.. Why not?

Possible interface between a diagnostic port and distributor? Fascinating ! Had no idea there was a second port on some cars.

Edited by tech71
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