I'd like to play "stump the HEI experts" here. I have a '82 454 that died while running fine going down the highway. I had the coil go bad about a year ago, so in this recent instance, I immediately replaced the coil, (with no improvement, engine will not start) . I have 2 spark testers , one is a Harbor Freight conventional tester, I'll refer to it as the Harbor Freight tester. I also have the correct HEI spark tester (resembles a spark plug with no ground electrode and has a alligator clip on the ground), I'll call it the HEI tester. I get spark at the plugs and the carbon button (coil secondary) inside the distributor cap with the Harbor Freight tester, and nothing with the HEI tester. I put a volt meter on the Batt input to the distributor, and get 12V with the key on, drops to ~10V when starter cranks. I put a trouble light between the Batt and Tach inputs to the distributor, and get a blinking light when the engine cranks. This would seem to indicate to me that the coil is bad, but I replaced it. Anything I'm missing here? Its got me stumped. I'd appreciate any insight. Thanks!

 

Can you post a picture of that engine and distributor?
82 corvette had computer controlled distributor and transmission, are you using that?

 

I don't know what your "trouble light" is, but a resistive load or bulb will correctly pulse/flash if connected between the Bat and Tach terminals while cranking, if the coil is good, open, or just partially shorted. A fully shorted coil would prevent a light from flashing much.

 

Thanks Mike, - I don't have a pic handy. (The unit is off-site from my house) . I'll post one asap. In full disclosure, its not a Corvette, its a P-30 Chassis. Its a carbureted 454,the distributor is mounted in the rear, it has a vacuum advance, ignition coil mounted in the distributor cap with a kidney shaped ICM module, it has a pick-up coil mounted inside the distributor that connects to the ICM. I've read so many thoughtful ignition diagnosis' here, I thought a working-class Chevrolet problem might be a easy troubleshooting for someone in the gallery.....

 

Its a simple Harbor Freight 12V test light. When across the Batt and Tach terminals it blinks from bright, to slightly dim and back when the engine is cranking (telling me the ICM is working correctly)?

 

Take the 4 pin module out and have it tested or just replace it........it is very important to smear a nice even thin coat of heat sink compound on the back of the module when you install it. The module gets warm when running and the base of the distributor is its "cooling fin".
Other than that....just make sure your spring and button are ok........or you can just R&R a new HEI for $100 from the hundreds of suppliers out there......
I don't mess with HEI's anymore unless by request......I can stab a distributor and time the engine in less time than it takes to remove the module and have it tested.

Jebby

 

Points distributors work great for me. Good luck with the "off shore" hei modules!

 

FYI: The "New" HEI replacement parts can be "hit or miss" at best.

 

Most likely a defective module...(which have have sustained some significant "stress" from the coil failure). Just replace it as a 'good maintenance thing to do'.

 

Yes and No. The blinking indicates that the module is (correctly) switching your test light (and the coil) on and off during the dwell periods. That's obviously good. But what your test light won't tell you is if the module internal power transistor is damaged (and breaking down/shorting due to the coil primary winding flyback voltage spike at the end of dwell). A damaged power transistor will cause a loss of secondary voltage capability or energy transferred to the plug (ie: a weak spark).

 

Buy 2 modules. Install one and put the other in the glove compartment. The best $35 investment you can make.

 

In cap coil

Engine compartment

ICM output during engine crank

another scope trigger ICM output

Thanks for all of the feedback. The consensus seems to be the module is bad. The vehicle is off site and I got a chance to work on it today. I didn't want to start ripping parts out w/o some more evidence that they were bad, so I bought a cheap mini-scope from the orient and snapped a couple of photos of waveforms to the coil that I captured while the engine was cranking (also a few pics of the distributor and the engine) The waveform isn't what I expected (I expected a square wave). I don't know if the module is actually bad or I'm just not triggering with the scope to get a good image. I grounded the scope to the Batt lead, and put the scope probe to the Tach lead. (I slapped the scope on a working '87 GM HEI for a reference and that waveform made more sense, although it over shot-ground). Any thoughts of what I'm seeing here? Thanks.

 

Couple of quick thoughts on the scope traces:The basic waveform shapes look correct. The voltage levels, though, look a bit odd to me. Can you move your ground lead to the distributor body (or IIRC the ground lead on the cap) and take another picture showing the ground reference point on the screen? I'm trying to figure out if it's a shitty power transistor or ground connection in the module, or a resistive ground path in the wiring. A separate picture (and voltage measurement) of the peak of the flyback voltage (TACH signal) would be helpful too (to see if the power transistor is breaking down at too low of a voltage).

 

I don't mean to dismiss your diagnostic methods. Truth be told I'm a little bit in awe. Diagnosing ignition problems with an oscilloscope is beyond my skill level. Nonetheless, bad ignition modules are such a common problem with HEI distributors that replacing the module is the first thing I do with an HEI that's acting up. In my experience it's rare that anything else goes wrong with them. And don't forget the heat sink compound.

 

No worries. We all have our own methods or philosophy when working on our toys. The OP supplied some scope traces for examination, and I'm happy to give an opinion on them.

 

Engine cranking, Probe on Tach wire to distributor, ground reference @ chasis. 20V /division verticle, 50uS / division horizontal. Ground is the point of the yellow arrow on the left/center of the screen.

Same as previous photo except .1ms /division horizontal.


Always grateful for the feedback. I probed the tach wire at the distributor and grounded the scope input to the chassis. ( I also grounded to the distributor ground, with no discernible difference) When I first cranked the engine it fired (but was dead during the remaining duration of ignition waveform capture) . Both photos are with the engine cranking , I believe the scope is triggering on the first input it sees. With the ignition on, the scope shows the tach wire with the DC battery voltage, when cranking, this DC voltage drops (The scope is saying its ~7V, the photo shows that is the level where the ignition pulse starts from.) Ground is the point of the yellow arrow @ the left center of the screen. The pulses themselves are what I would expect. However I'm surprised the scope says they peak @ ~80V. (Is that what these HEI coils require to fire?) and it overshoots ground by the same magnitude. I don't know what a properly functioning ICM output is supposed to look like?

Observations, comments welcome. Thanks!

 

Personally - that second set of scope traces don't make a lot of sense to me.

The voltage at the neg side of the coil should be either +12V or zero (depending on if the coil module has "switched" the ground connection on or off). I can understand the higher voltage recorded as, and right after the neg side of the coil is switched "off ground" - and the current flow ceases and the magnetic field collapses. BUT - as you said - I would expect more of a square wave....

The first set of traces show the neg side coil voltage to be hovering at 5 - 6 VDC. I'm wondering if you could have a bad ground - or it could mean the module is bad, and has an internal resistance - thus not really letting enough current flow for the coil to "saturate", and therefore not collapse in the appropriate manner.

 

I'm with Purple above. The waveform doesn't make sense. But if I look at the 'scope screen upside down, the shape of the waveform looks more "normal" (leads swapped?). The labels also look odd. At 50 uS time scale, there's only about 75 uS of dwell shown on the screen. That wouldn't make any spark if that was real, but you indicated that the engine is just at cranking speed during this, so this doesn't make sense. The volts per division don't make sense either.

Ideally, the picture you should see is a flat line at 12v, then dropping to about 1 volt at the start of dwell, then a ramping/angling shape up (due to the voltage drop across the module ground path), and leveling to about 6 volts, then the sharp flyback voltage up to around a hundred volts on the Tach/C- during plug firing, and then back to the flat line 12v between dwells.

Can you recheck your connection points and disconnect the tach harness wire if you have a dashboard Tach? Also, if you can help us understand why the time/volts labels don't make sense when looking at the scope picture that would help.

 

I posted this earlier, I tried to paste a waveform pic and it triggered a few pages of computer characters, it appears the webmaster took it down (thank you )

Thanks for the observations, I'm glad that you had many of the same questions I had. Your thoughts on the scope ground are pertinent.

Part of my problem is that I don't have a baseline waveform picture to compare my observations to. (You would think with the gazillions of coil-in-cap GM HEI ignitions built in the 70s and 80s that there would be a reference set of waveforms somewhere) However, at the risk of inducing oranges to this apples discussion, I do have a '87 Chevy 2.8L V-6 with a GM HEI ignition . (however it does have a 4 input separately mounted coil) . I slapped my "econo-scope" on its tach wire (see attached pic) , (speaking of grounding, i happened to attach the scope to the Batt wire and got a >80V pulse) . This scope doesn't have a probe, it has 2 alligator clips wired to the scope's BNC connector. I suspect the wire is in some cases acting like a antenna and picking up spurious wave-forms (probably from the coil secondary) . For the pic, I used a shielded 10x probe (not the alligator clip cable) . For a baseline, I found where somebody had posted a baseline waveform pic of this ignition (however they used a old analog scope that probably has external triggering) . My scope has no external triggering and seems to only trigger on the first pulse it sees (and thus limits what I can see on the output of the ignition module). You can compare my waveform vs the "baseline" waveform. Your descriptions of what the waveform should look like largely resemble this photo. I think the exaggerated ringing below ground on my pic is a function of my cheap scope. What I plan to do is take the my new scope set up with shielded 10x probe, and analyze what the tach signal looks like on the disabled vehicle.

This is my scope photo of a later HEI example on a 2.8 V-6 with a separately mounted coil using a 10x shielded probe (thus the volts / Div would be 2V (not .2V)

This is the basline waveform (for the detached coil HEI in my 2.8 Chevy) that I found on line.

 

This is my Chevy V6 "test vehicle" HEI Tach signal with the scope grounded to Batt (sorry I'm not able to rotate these pics upright)

Baseline waveform from the internet.


I think I have a baseline to work with now with my Chevy V-6 HEI test vehicle, the top pic is my econo-scope trace of the tach signal with the car running, scope grounded to the Batt.(10x probe, multiply V/div x 10) The second pic is from the internet of what I believe the ICM output waveform is supposed to look like if your scope could get a comprehensive view of the series of pulses. I'm thinking the top pic is the best its going to get with my el-cheapo scope. Now that I am able to capture a quasi-sensible waveform with my hardware, I will take this set up to the disabled 454 HEI and see what that ignition module output looks like. If something looks out of wack here, let me know . Thanks!