I was hanging with you guys up until post #12 than you went and broke out the gizmos and started talking waveform stuff.

 

So ... Since the point of all this is to try to get the car to run correctly - I can see a couple of possibilities.


1) Run a "jumper" ground wire from the distributor to a good chassis ground or Batt negative.

2) Pull the Ignition module out of the distributor and do whatever you can to "improve" it's connection to ground.

3) Replace the Ignition Module (and don't go super cheap).

4) Replace the coil with a new one (one that doesn't have a super low resistance).

5) Say "Screw It" - get a replacement HEI distributor - with module and coil - and install it.

 

OK folks, a quick refresh, what I am posting here is from the subject 454 HEI vehicle that died while rolling down the interstate, and I replaced the coil (a defective coil was replaced a year before), has weak spark but no HEI spark, will blink a test light from Batt to Tach on the coil primary. Now that I have a fairly stable scope set-up (with a 10X shielded probe) , I applied the scope to the Tach wire on the coil (using the Batt wire as the ground reference. You can see what I got from the attached pics(while engine is cranking). This looks to me like it should be firing the coil? The only remotely odd thing (to me) is that the square wave below ground doesn't have as clean a trailing edge as the previous examples that I've posted. This may be just a artifact of this particular HEI ignition model, or the battery voltage drop while cranking. Its still pulsing the coil like I would think it should. Now the thing that I really find strange, keeping in mind what Purp said in the previous post, I checked the ground on the coil core, and there was none (the core was floating, and thus the black wire from the coil would also be floating) . So I sanded the black paint off the core where the ground connections are made connected the ICM ground and the coil ground to clean connections on the core. I didn't observe any difference except for slightly cleaner waveforms in the attached pics. I ran a ground jumper from the neg terminal of the battery to the distributor, no difference. Ran the same ground jumper to the coil core no difference. What puzzles me is how the thing ever ran at all given those ground connections on the coil were essentially floating??? I'd suspect the coil, but its new? Comments welcome.


Tach wire @ Coil, Batt wire is ground 10V /div

Tach wire with chassis (coil core) as ground . 10V /Div


Tach wire @ coil, scope ground on Batt. 10V / div

 

So - I may be about to show my ignorance (wouldn't be the first time) - but when you were checking to see if there was a ground from the coil neg side - was the ignition on ???

The Ignition Module uses transistors to switch the coil to ground and to break that circuit. I don't know if the Ignition module needs to be "powered" to work properly ??? Just a though...

And IMHO - Just because the coil is new is not a Guarantee it works. A now working "New" Ignition Coil would not be a first...

 

Thanks- The ignition was "off". I first thought the coil was grounded because I put the ohm meter between the yellow ground wire that comes from the ICM and connects to the coil core via a mounting screw, and the chassis , it showed continuity. Then I placed the meter between the same yellow wire on the coil core and the black wire (that originates from the coil and connects to the coil core via another mounting screw) , it showed open. I sanded the paint off the core where each wire attaches to the core, then I had continuity from the ICM ground (yellow wire) to the black wire from the coil and to the chassis. ( I still can't conceive how that coil could ever fire with those 2 wires essentially placed on non-conductive paint) .

It is always wise to consider that new components can be defective, and I would not bet my lunch on this "new" coil being pristine. However, it ohms out like a good coil, and I'm generally not unlucky enough to get a bad component out of the box (although I understand that it can happen) . Given I don't understand how it (or the one before it) could of ever worked with with 2 flimsy ground connections on paint (unless the mounting screws connect to a ground within the distributor cap) , perhaps I've damaged it somehow. (every piece of documentation I've ever seen on those coils cautions that they need 2 robust grounds or they won't work ).

I'm going to give it some more thought, but unless you or the gallery can see something wrong with these waveforms, I'm going to consider buying another new coil ...... Thanks again

 

I purchased a $13 replacement coil on line (3rd coil I've bought in 14 months) . It didn't ohm-out any differently than the previous 2. I installed it (this time sanding the black paint off the 2 corners where the grounds connect) . I connected the HEI tester plug to the carbon-button inside the cap, got no spark. I then connected the Harbor Freight (flashlight-bulb w/ inductor) tester in the same configuration and it lit up fine (I even disconnected it from ground and got it to light faintly) . I then reached for the nearest rusty lawnmower and took the rusty spark plug out of it, put it in this configuration and got bright spark, I then took my pliers and expanded the gap significantly and still had a robust spark jump the gap. (again, still no HEI spark and the engine wont fire). [I recently verified the the HEI plug tester (plug with alligator clip and no ground electrode) works on another vehicle. ] I took a good ohm-meter and got consistent readings between ICM ground/chassis ground/ distributor ground/and battery ground of <1 ohm. At this point I'm stumped. I suppose via the process of elimination, its time to consider looking closer @ the ICM and or the pick up coil (this goes against my instincts as the ICM waveforms I've posted should fire that coil (and they are, just not hot enough). Am I'm missing anything here?

 

Excuse me for butting in, but what is that yellow wire to the coil laminations? There should be 3 wires to the coil, Yellow, & red that go to the connector, and the black, which goes to the coil laminations.

 

Butt-ins welcome. The yellow wire that goes to the laminations (what I call the core) is a ground from the ICM (the ignition module) via the same connector as the Batt (red) and Tach (also yellow) wire. I believe the black wire (which originates from the coil) is the ground for the secondary of the coil. Its my understanding the black wire gets it's ground via the laminations which are tied to ground through the yellow wire which is also attached to the laminations.

 

sc4ram - Have you checked the plug wire resistances ????

 

OK let's go to square 1. Coil resistance should be .5 ohms on the primary. and about 9k on the secondary. One other item to check. the resistance for the pick up should be about 900 phms.
Also check the color of the leads on the coil with the leads on the pick up. If you have a yellow and red lead on the coil, then the pick up should have a yellow and green lead. The colors signifies the polarity of the coil and the pick up, and they must match. If the pick up has a white and green lead, then the coil must have a red and white lead. If all of the above checks out, then it's either the coil arcing internally, or the module not triggering properly. The HEI system is pretty simple, and assuming that you didn't change any wiring, it has to be a component, especially if it was running before, Pep Boys or Autozone will load test that module for free.

 

Purple, I have not measured plug wire resistances. I hadn't thought to, since the engine quit while running fine going down the road. (I'm a novice here, but it wouldn't occur to me that the spark plug wires could open up their resistances simultaneously enough to shut down the engine ). I understand your thought process here, which is why I did the spark tests described in yesterday's post via the cap carbon button (with the cap removed), not any of the motor's plug wires. However, thinking along those lines, the test I conducted yesterday from the carbon button was with a old conventional plug wire (which measures ~33K Ohms) . I also ohmED the carbon button while I had it out yesterday , it measured ~38K ohm (so in theory the coil would have to light that HEI plug though a 70K Ohm resistance. ) I don't know what what these resistances should be , any idea? I'm thinking out loud I should order a new carbon button.......

 

Just measured one.....5k ohms. Have you checked you rotor? Sometimes they arc through the bottom to the dist. shaft.

 

Thanks MrVette, I'll order a new button and eliminate that from the suspects. I've examined the cap, and don't see any obvious signs of arcing. That was another reason I did the spark test from the button and not at the plugs. I'm slowly working towards your thinking to suspect the pick-up or the ignition module. (I didn't want to disturb them as per my earlier photos the module output pulses look ok to me) . Once I clear the carbon button out of the mix, i'll ohm the pick-up and get the module tested.
PS I just re-read your post, I examined the rotor too, and held it up to the sunlight, I didn't see anything wrong with it or any holes (again, spark testing from the button would eliminate that from the chain) . It seemed to have a high resistance from the tempered center piece that touches the button and the rotor point. You don't happen to have a resistance reading on a good rotor? thx

 

Those measurements are in line with what Carbon Core resistance wire should be.

There are three kinds of plug wires - Solid conductor, Suppression, and Spiral Wound. The solid conductor are good only for racing - and on non-computer controlled cars - they tend to create a lot of RF energy - which can screw with computers. The suppression wires like you have are what OEM's have been putting on cars for years now. They work. The Spiral Wound wires use a conductor that is wound around the wire core - so a 2 foot plug wire may have 25 feet of wire in it - but winding around the core tends to null out the RF - so the can be used on computer controlled engines, and typically have resistance of about 50 ohms per foot. They tend to work "better" then the carbon core wires - but are more expensive.

I'd say - the issue is probably NOT your plug wires. The resistance of the "button" in the cap sounds a little high - but not crazy.

 

fyi- I received a new button (distributor brush) via what I presumed was the standard replacement part number (DR454) , interestingly its not carbon, it appears to be brass (or some kind of plating) and it measures 0 ohms. I doubt this is the problem, but its quite a difference from whats currently in the vehicle. I had another idea, before I start pulling off connectors and messing with the pick up coil and module, I have a hand held tachometer, I'll connect it to the tach wire and see if the module will register some rpm on the tach during cranking, that should give me another read (along with the wave-forms pictured earlier) on how the module piece is performing. ...

 

I spent the afternoon back in the Chevy HEI school of hard-knocks. I used my high-impedence multi-meter and wrote down the measurements this time. No progress, but this is the info I collected. I have a old hand-held tachometer and connected it to the tack wire. It reads ~60 RPM while cranking. (I would think this is further evidence that the ICM is working ok) . I went back to measuring grounds. With the ignition switch set to off, I measured the ICM ground (yellow wire) to chassis ground and got .8 ohms. I then turned the ignition switch on, and got 2 ohms. When I cranked the engine the ground measured ~55 ohms. The rotor brush (button) measured 3.5K ohm. I replaced it with a brush that measured 0 ohms. I measured the rotor from button to rotor-tip, got 1 ohm. I again failed to get HEI spark (at the rotor brush/button, see pick of the test set-up) . I then measured the pick up coil (yellow and green wire) , it read 850 ohms. I also included a pic of the advance mechanism. The picture of the gunk on the advance looks more red than it actually is (I've read that red dust denotes internal arcing ) , it didn't look as red as some pics I've seen. So given the only failed component

Test set up for HEI ignition, (HEI test spark plug terminal held against rotor brush/button with old spark plug boot, jumper from alligator clip to ground) it wouldn't fire btw, but a conventional plug would.

Picture of advance weights/springs, ICM on left with pick-up coil connector bottom left.
reported on these message boards that cut-off the engine while running is the ICM (ignition module) , I pulled it off and will take it to AZ tomorrow (I don't think its the problem, but I'm running out of things that qualify) . If any of this raises any obvious red flags, let me know . Thanks!

 

From my personal experience, if the car just dies, it's the module. My old distributor would eat them constantly, I ended up just buying a new dizzy off Jegs and have had no issues since.

 

I removed the module and replaced it with a 6 AL digital MSD. . haven't had an issue yet.

 

Well, I hope this post illustrates that a Chevy nightmare can eventually have a happy ending. (and it should also suggest that one may want to pay more attention to their older and wiser HEI aficionados in this forum) . The Cliff Notes story here is I installed a new ICM which resulted with plenty of HEI spark at the rotor brush , once I reinstalled the cap, the engine started right up . For those that want to read on, I'll digress below:

I was hesitant to to remove the ICM as it appeared to me to be working properly (via oscilloscope waveforms and test-light blinking on the coil primary) I wanted to avoid simply shotgunning with replacement components unless I was desperate. Given I was running out of suspected trouble points, and that there were many instances reported of the ICM failing while the engine was running fine cruising down the road (with was my failure experience), I had nothing to loose by getting the vehicle's ICM tested yesterday. The ICM failed on the tester @ AZ. The tester performs a chain of tests and signals that they passed or failed by lighting a series of green lights or red lights respectively, with my module, it got towards the end and failed the test marked "energy". (I can only guess what this is, but my guess would be the tester makes the module either fire a coil or observes if the output performs properly on a coil-type load) . The AZ guy ran the test 3 times and it repeated. Interestingly when finished, the module was too hot to touch (I've read accounts of where guys with rough running vehicles took their module in for a test, it passes and when they re-install it the engine won't run at all, Id guess exercising a vintage module w/o a heat sink isn't good for its longevity) . I was still skeptical that the module was the issue, but given most of the faulty tests I've read about determined a bad ICM to be good (not vice-versa) , and that its a common fail mode while running, since I had 2 data points now I installed a new module (a AC Delco replacement btw) with successful engine start. I'm still confused why the bad module gave me a (what I thought) was a good coil pulse. I've attached the pics of what a good ICM output looks like, the only major difference I can see from the bad one is that the dwell (the negative going square wave) is about half the width of the same wave on the "bad" module. I also removed the new 1 ohm rotor brush (button) and replaced it with the original ~

ICM output wave-forms from new ICM while engine cranking (ignition working properly)

ICM output wave-forms, new ICM, engine running
3K ohm brush. I'll make sure modules are on my redundant parts list for road trips. I'll donate these pics to the forum archives here. I appreciate everyone's inputs, if you have more, I'd love to read them. Thanks!

 

Glad you got it resolved !!!