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Author Topic: testing HEI coil  (Read 8337 times)

Offline mhamilton

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testing HEI coil
« on: August 16, 2011, 03:37:59 PM »
Okay, I need a sanity check here... this should be a very simple test, yet I am at a loss.  First, electronics is my business, so I am very familiar with digital multimeters and transformers, etc.  I'm trying to check a coil-in-cap HEI coil for a street rod that has no spark.  I have multiple factory manuals from GM with the diagnostic tree for the HEI.  

I measure the primary resistance, between Bat and Tach terminals, get 0.3 ohm.  Okay, that's good.  Then, it shows in every schematic that the secondary is measured between the carbon button and the Tach terminal and then the button to ground, should be 6k to 30k ohm.  I get infinite (open).  The first 2 coils I figured were just bad.  The 3rd & 4th I was puzzled.  Then I pulled the coil from a working car's HEI and can not get a measurement on the secondary.  I've tried every combination of connections between the button contact and coil wires.  Tried 3 different DMMs.  Nothing.  I can measure the carbon button (150 ohm) and the pickup coil (800 ohm), but no secondary.  

What in the world is going on here? I am completely lost with this.  I remember checking HEI coils before.  I don't think I used a VTVM to do it, I'm just lost!
« Last Edit: August 16, 2011, 03:39:42 PM by mhamilton »

Offline kev2

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Re: testing HEI coil
« Reply #1 on: August 16, 2011, 04:25:15 PM »
    How bout we try looking at this another way-   tell us the issue yur having AND maybe that will suggest another test?


I am posting this - bUT I guess yu already have it

 
Connect an ohmmeter between the TACH and BAT terminals in the distributor cap. The primary coil resistance should be less than one ohm (zero or nearly zero).
To check the coil secondary resistance, connect an ohmmeter between the rotor button and the BAT terminal. Then connect the ohmmeter between the ground terminal and the rotor button. The resistance in both cases should be between 6000 and 30,000 ohms.
Replace the coil only if the readings in Step 1 and 2 are infinite.
NOTE: These resistance checks will not disclose shorted coil windings. This condition can be detected only with scope analysis or a suitably designed coil tester. If these instruments are unavailable, replace the coil with a known good coil as a final coil test.


Offline billr

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Re: testing HEI coil
« Reply #2 on: August 16, 2011, 04:40:25 PM »
I agree with kev2, it would be more fun for us if we had the whole story.  However, here is a long-shot:  isn't that rotor button spring-loaded?  Maybe the actual contact between the button and the coil is open, but small enough gap so that "good" coil will operate; this is the secondary, after all, the spark can jump there as well as between the rotor and cap terminals.  Are these coils by themselves, separated from the triggering electronics?  If so, and you are supposed to have a resistance reading from the button to "bat" and button to "ground", then try reading resistance from bat to ground...

Offline mhamilton

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Re: testing HEI coil
« Reply #3 on: August 16, 2011, 05:12:19 PM »
Well, the long story is not really any help, I don't think.  It's a street rod with a Chevy 350 and a HEI. Cranks but no spark.  Have done all the usual checks, I just can not for the life of me understand why I can not measure the secondary winding on *5* different coils, some that are known to be good and working.

I've eliminated the spring and button from the equation, I had the coils out of the cap and on the bench.  Was going right to the brass contact. 

Now the GM manual I have says the tach terminal to the button is the secondary, not the battery terminal to button.  That is going by this picture:

Anyone have an actual schematic? Not necessary, as I mentinoed, I have tried measuring from tach and bat to the button, and all other combinations. 

Offline NickD

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Re: testing HEI coil
« Reply #4 on: August 16, 2011, 05:16:17 PM »
More than likely would measure infinity between the rotor and the cap spark plug terminals, if you could figure out how to get a probe in there, but it still works.

Not that difficult to pull the coil out of the cap, connect it to the module, and hit it with 12V and a pulse generator, and that center high voltage contact point to a spark gap.  Lets you check the coil output for a 3/8" spark over the entire speed range to be sure the coil is functioning properly.

Resistance readings are next to worthless, you won't get that hot spark in particular with an open primary, but not bad to check the resistance of the secondary.  Can still get a hot spark even with an open circuit, but that gap would increase to the point where you won't.  Will find corrosion on the high voltage contact, that can be cleaned up.

With my long history with the HEI, seem to just replace the shaft bushings, in particular the top one that just seem to get enough oil.  It would if you turn your engine upsidedown.  And all those terminals always need a good cleaning including the base of the distributor so you are getting a good ground.  Oh and those pigtail leads for the pickup coil always need to be soldered properly.

Offline kev2

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Re: testing HEI coil
« Reply #5 on: August 16, 2011, 05:46:01 PM »
  
   coil testing later- we can KNOW that even the good coil tests the same as the others....lets press on looking for another cause of no spark...

  sorry if it sounds stupid easy - sure you did it but got to start somewhere.

  the dist shaft IS rotating?

 carbon button spring and rubber washer are correctly instlled?

  there is 12v at coil and the ICM,  not only in RUN but in START?

 that metal ground strap is there- coil frame to terminal at plug in?

 pick up coil - wires are secure, not open or grounded?
« Last Edit: August 16, 2011, 05:51:34 PM by kev2 »

Offline NickD

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Re: testing HEI coil
« Reply #6 on: August 16, 2011, 05:51:52 PM »
Can also put an old fashion dwell meter on the tach terminal, should show 30* for a V-8.  Can't adjust the dwell, but lets you know the primary circuit is working.

Offline kev2

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Re: testing HEI coil
« Reply #7 on: August 16, 2011, 05:52:50 PM »



1.  Test for power at the pink BAT terminal.  You should have battery voltage w/ the key in the start and RUN positions.



2. Connect the ground side of your test lamp to the battery POSITIVE cable.  Probe the TACH terminal on the dist. cap while a helper attempts to start the engine.  The test lamp should blink repeatedly as the engine cranks.  No blink= bad module or pickup coil.  Further testing is required to pinpoint the problem. Blink but no spark = bad ignition coil.



3.  Remove the cap & rotor.  Remove the green & white leads from the module.  Connect your ohmmeter to the green & white leads.  You should have approx. 800-1500 ohms depending on the ambient temperature.  Open circuit (infinite ohms) = bad pickup coil.



Wiggle the green & white leads as you test.  Ohm reading should remain constant if the leads are good.  If the reading varies as the leads are wiggled, the pickup coil is bad.  You'll often find broken pickup coil leads this way.



4.  DVOM (meter) still connected to green & white leads.  Set your DVOM to AC VOLTS.  Have a helper crank the engine as you watch the AC VOLTS reading.  A good pickup coil will produce about 3V AC when cranking.  Less than approx. 2V AC indicates a bad pickup coil.

Offline billr

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Re: testing HEI coil
« Reply #8 on: August 16, 2011, 06:42:41 PM »
Mhamilton, since you have access to a coil that works on another vehicle, how about trying that in the street rod?

Offline mhamilton

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Re: testing HEI coil
« Reply #9 on: August 16, 2011, 08:06:32 PM »
Quote
More than likely would measure infinity between the rotor and the cap spark plug terminals, if you could figure out how to get a probe in there, but it still works.

Not that difficult to pull the coil out of the cap, connect it to the module, and hit it with 12V and a pulse generator, and that center high voltage contact point to a spark gap.  Lets you check the coil output for a 3/8" spark over the entire speed range to be sure the coil is functioning properly.

Well, okay I certainly understand the spark jumping the gap between the rotor and the cap, but I didn't think the same applied to the secondary winding of the coil.  If I had a function generator I would certainly try that... actually, was thinking of building a little PWM generator for testing components.  Seems like there's nothing left on a new car that isn't PWM.  Anyway, that's another subject...

Quote
Can also put an old fashion dwell meter on the tach terminal, should show 30* for a V-8.  Can't adjust the dwell, but lets you know the primary circuit is working.

Quote
2. Connect the ground side of your test lamp to the battery POSITIVE cable.  Probe the TACH terminal on the dist. cap while a helper attempts to start the engine.  The test lamp should blink repeatedly as the engine cranks.  No blink= bad module or pickup coil.  Further testing is required to pinpoint the problem. Blink but no spark = bad ignition coil.

Those are both very good ideas, thank you very much.  I didn't think of those, but sounds like that will be the simplest way to pinpoint the issue.

Definitely have voltage at the B+ terminal when cranking, 11.5+ volts.  Have 12.5 volts in run, new battery.  GM manuals says over 7v when cranking is good, so think that's okay. 

Quote
Mhamilton, since you have access to a coil that works on another vehicle, how about trying that in the street rod?

That is another really good suggestion.  I did think of swapping coils into the working car's HEI, but didn't have that much time. Will definitely try that to go through all those old half-dozen coils I have sitting around.

Unfortunately this is another one of those times where I'm 150 miles away from the car at the moment, otherwise I'd run out to the garage and check right now.  Thank you all for the help, I really appreciate it.  This is exactly why I come here, always good to get ideas different than mine!  :)

Offline NickD

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Re: testing HEI coil
« Reply #10 on: August 16, 2011, 08:44:50 PM »
What vehicle is this HEI off of?  Been several changes over the years.

Offline al daniels

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Re: testing HEI coil
« Reply #11 on: August 16, 2011, 10:19:27 PM »
ignition module.

Offline al daniels

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Re: testing HEI coil
« Reply #12 on: August 16, 2011, 10:21:18 PM »
check the pickup coil.if its ok ignition module is your likely culprit.dont think ive ever had a coil fail on one of these.

Offline NickD

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Re: testing HEI coil
« Reply #13 on: August 17, 2011, 05:15:36 AM »
This topic is jarring old memories when it was told the only way to check out an ignition system was with a scope even with some outrageous statements, via the spark line, could tell if cylinder #4 had low compression or a leaky intake valve.  Was started I believe in 1948 by Dumont with the introduction of an ignition scope and followed with the likes of Sun, Snap-on, Allen, and Marquette that basically purchased a 20" off the shelve black and white TV set for the display mounted in a huge console.  These were purchased by dealers mostly to impress their customer base with a huge complex console mechanics didn't like to use has there were alligator clips everywhere to make connections.

The high voltage pickup was a capacitive clamp that clipped unto the coils high voltage lead, but developed huge problems for them back in 72 when the HEI first came out.  Really no access to the secondary circuit in the V-8 distributor.  The 4 and 6 cylinder versions still used an outboard coil.  But then a clamp on plate was developed, forgot by who and was patented in an attempt to read these signals that clamped unto the top of the distributor.  But these weren't very useful with the HEI as these point replacement electronics, just did that, barely replaced the points.

A key change as a opposed to a conventional point ignition system was the elimination of the so-called condenser, in parallel with the points.  When the points opened, this condenser was in series with the primary of the autotransformer that would resonate the coil and that condenser could actually be adjusted to obtain the longest spark line as if it was too large or small, like tuning a radio, wouldn't get the correct resonant frequency.

Further complicated by the fact, no one had an idea of what the capacitance of that condenser was, or how to adjust it, these condensers were never mark with basic parameters like capacitance and break down voltage.  But could be done with mylar capacitors, but those didn't have screw tabs to tie them down.

With electronic ignition system, a bipolar transistor was use that only conducts in one direction, placing a capacitor across that would reverse bias that transistor and burn it up in an instant.  So a 60 V breakdown zener had to be used, and goodbye condenser and that longer spark line.  With electronic, would only see a single spike, in this light, ignition points are far superior in that both positive and negative alternations were generated and the spark would last even up to a millisecond.  Amplitude only told you that the plug was shorted due to carbon built up or had electrode gap wear with a seemingly good much higher voltage.

Recall joking about these consoles, paying up to $20,000 back then to check a 50 cent spark plug.  But the public caught on, and this phase of ignition testing died.  Plugs were routinely changed anyway.

The key to getting energy in an ignition systems deals strictly with the size of the magnetic core used in the coil and to have an ample supply of current to magnetically saturate that core over the speed range.  Conventional coils to limit this current is to use a high resistance primary that was nothing more than a waste of energy and damage to the coil as it would overheat.  HEI does use a very low impedance primary that is pulsed by a monostable circuit feeding a controlled current output stage that is sized just to saturate the core of the coil.  If you leave the ignition switch on, draws very little current as opposed to other electronic ignition systems or conventional points if closed that can burn up the coil if the ignition switch is left on.

If you want to bench test an HEI, would have to mount 8 spark plugs to a plate with each distributor lead connected to each one, have a 12 volt supply, and a means to turn the shaft.  My variable speed lathe does a good job at that.  Even turning the shaft by hand will show a firing sequence.  One of the main killers of electronic ignition is an open spark plug wire, all that energy has to be absorbed by that zener diode, can never be made large enough.  It will short out killing that pulse.

Offline mhamilton

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Re: testing HEI coil
« Reply #14 on: August 17, 2011, 12:27:10 PM »
Quote
What vehicle is this HEI off of?  Been several changes over the years.

Not sure, it's the same as what's in my '80 Chevy, 5 pin module and coil in cap.

Quote
check the pickup coil.if its ok ignition module is your likely culprit.dont think ive ever had a coil fail on one of these.

The module was replaced once before I got there to look at it, but it still had no spark.  The coil was replaced once in the past due to a no-spark condition.  Plug wires are being replaced this time around as well.  But I think the test light or dwell meter test will help find the problem in no time.
« Last Edit: August 17, 2011, 12:27:53 PM by mhamilton »

 

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