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I'm beginning to get fustrated with GM HEI ignitions. A few months ago, my 1979 L48 base and nearly stock vette left me in the middle of US27 at night! Diagnosed a no spark situation. After taking the distributor cap off and looking around, I put it back together and it started and got me home. After figuring it was the Standard brand ignition module, that after it had cooled off started working again. So I ordered a Davis Unified Ignitions (DUI), coil, cap, rotor, harness and module, but them in and 150 miles later the car failed again on US27, but this time I was able to coast into a Walgreens parking lot. Yep the module had failed. Put the original previously failed Standard brand module in and got home. Called Summit, they sent another DUI module and told me to trash the failed one. I installed the new DUI module, also drilled and tapped the distributor housing for a ground wire to the intake manifold, even though the distributor showed a good ground to the battery. See picture of the opened up failed DUI module below. Today after nearly 1000 miles this time on the busy Tennessee 62 hwy leaving Oak Ridge, it failed again. I had bought a Delphi module from somewhere and had it as a spare to my spares. Again I got home.
Here's what I noted today: The distrubutor metal gets very hot!!! Hot enought to burn you. So am wondering if the termal paste that I have always added is allowing the very hot distrubor to cook the little module?? Some hotrod friends and a mechanic that works for the US post office GM vehicles says NOT to use the paste. I'm actually considering moving the module out of the distributor to maybe a fender well or even into the cab of the car and mounting it on a heat sink. Have any of you folks run into this and what was the solution?
I've included lots of pictures that might help others.
Last edited by jsbowman6; Apr 11, 2026 at 07:42 PM.
Dunno why you're having too many module failures; perhaps it is heat.
It can only help if you DO use a heat sink. GM did so beginning with the Vortec era. ALL L35 V6 and L30 and L31 V8 have em.
GM P/N 10474610
Yes your dist housing gets hot as a firecracker, and it's even hotter cooped up under cap.
The bolt pattern won't match your 4-pin module; just drill another hole in heat sink. I've done this remote module mount on a few circle track cars. It does help to run the module cooler. Mount heat sink to a bracket atop motor (like OE Vortec) or on firewall. Give it some air flow. But, ya still Require the heat transfer paste.
Either trigger aka pickup coil and or ignition coil or Both may've been damaged: one failure keeps feeding the other failure? Check your thin pair of trigger wires to ensure they're neither damaged or rubbing sides of housing as advance plate articulates. Too common they rub thru their insulation and short against housing.
*** Oh, and those folk who say don't use heat transfer paste are mistaken; and probably never members of teams of degreed engineers who GM spent millions on to develop HEI.
Clean that old paste "residue" away and replace with New genuine heat transfer paste (but Do Not substitute dielectric grease).
Last edited by Rebelyell; Apr 12, 2026 at 05:29 AM.
Thanks folks for the feedback. Today I'm putting the car on the lift and pulling the spark plugs to check the gaps (making stuff up but maybe if the gap is too big, it could cause heat?). Pulling the distributor and checking bearings, might be causing heat. If the shaft and bearings look ok, might change the magnetic pickup, not sure how that could cause issue, but I have one on hand.
IF I decide to replace the distributor, is there a brand that is reliable for a cruising car? Not racing, not hot roding, just cruising around. Thanks
I'd try to find an OEM 4-pin ignition module before doing anything else drastic.
Here are some other things to check, suggested by Grok. Specifically, I'd ask DUI for instructions to measure their coil.
And you have a 79, but are you 100% sure that the distributor is getting full, clean voltage?
Originally Posted by Grok
The most common reasons for repeated 4-pin GM HEI ignition module failures (even when heat sink compound is properly applied) boil down to electrical stress on the module, not just heat. The module switches high current to the coil, so anything that makes it work harder, spikes its voltage, or disrupts its ground/reference signals will kill it quickly—often in weeks or months instead of years. Here are the top likely culprits, ranked by how frequently they show up in GM forums and repair discussions:
1. Faulty Ignition Coil (Very Common Culprit)A failing or mismatched coil is one of the biggest repeat-offenders. It can cause excessive primary current draw, internal shorts, or high-voltage back-feeding/spikes directly into the module.
Check this first: Measure primary resistance across the coil's + and - terminals (key off, coil disconnected). It should typically be 0.3–1.0 ohms for a stock HEI coil (exact spec varies slightly by year/engine—consult your service manual). Too low (e.g., near a dead short) overheats the module fast. Also test secondary resistance (~6,000–10,000 ohms).
Look for visible damage, arcing, or a cracked tower.
Many mechanics report that swapping in a known-good OEM-style coil (not a cheap no-name) stops the module-killing cycle. trifive.com
2. Poor or Missing GroundsHEI modules are extremely sensitive to grounding issues. A bad ground forces the module to carry extra current or creates voltage spikes.
Key spots to inspect:
The black ground wire/pin on the module itself.
The ground strap or wire under the distributor cap (often forgotten during cap/rotor changes).
Engine-to-chassis ground straps and battery negative cable.
Clean, tight connections—corrosion or loose mounts are silent killers. Bad grounds can fry a module "in a heartbeat," per multiple long-time HEI owners. facebook.com
3. Low or Unstable Voltage Supply to the DistributorHEI modules hate anything below ~12V. Low voltage makes them work harder (higher current draw) and fail prematurely.
Test: With key on/engine running, measure voltage at the module's power terminal (usually the pink or red wire). It should be a solid 12–14V.
Common causes:
Original resistance (ballast) wire still in the circuit (many 1970s–80s GM cars had one).
Weak alternator, bad battery, or poor connections.
Voltage drops in the ignition switch or harness. If voltage sags under load (e.g., during cranking or at higher RPM), the module suffers. corvetteforum.com
4. Bad Pickup Coil (Magnetic Stator) or Worn Distributor ShaftThe pickup coil sends the trigger signal to the module. If it's weak, shorted, or the reluctor wheel is out of alignment (due to worn distributor bushings/shaft play), it can send erratic signals that overstress the module.
Test: Measure pickup coil resistance—typically 800–910 ohms. Check for shorts to ground.
Spin the distributor by hand and watch for excessive shaft play (should be minimal). Worn bushings are common on higher-mileage units and let the reluctor rub or misalign. This is a frequent "hidden" cause of repeated module deaths. trifive.com
5. Wiring/Connection Problems in the Distributor HarnessLoose pins, pinched wires, or bad crimps in the 3- or 4-wire connector to the module/coil can cause intermittent high current or arcing.
Tug-test the wires—they shouldn't pull out easily.
Common issues: Factory-crimped terminals failing, wires getting pinched under the cap, or the tach terminal being accidentally grounded (instant module toast). elcaminocentral.com
Quick Diagnostic Steps to Narrow It Down
Replace the coil with a quality unit and re-test (easiest first step).
Verify full battery voltage at the distributor and solid grounds.
Test the pickup coil resistance and check distributor shaft play.
Use a known-good (preferably AC Delco or MSD) module for the next replacement—cheap offshore ones fail faster.
Consider adding a heavy-gauge power wire directly from the battery (with fuse) as a temporary test if voltage issues are suspected.
Other less-common factors include fouled plugs (extra load), aftermarket ignition boxes wired incorrectly, or contamination inside the cap, but the above cover 90% of "keeps eating modules" cases. If you've already ruled out the basics, pull the distributor and have the pickup coil and shaft inspected. A good HEI should last 50,000+ miles with proper support. Let me know what year/make/model/engine you're working with if you want more specific specs or diagrams!
Last edited by Bikespace; Apr 12, 2026 at 10:15 AM.
As an aside, if you do choose to do something drastic, you can delete the ignition function of your distributor by converting to a reluctor and an array of LS-style coils-on-plug.
You can get the reluctor and cam sensor parts here: https://www.eficonnection.com/
And then use an MSD 6014 (or similar) to fire the LS coils (which you can probably find used for nearly free).
You'll still need the distributor shaft to turn your oil pump, but no more fried cheap Chinese HEI modules. And it might not cost much more than buying two DUI distributors.
This just in. A friend of mine noticed this on the new rotor button. It has been arcing on the contact. I need to measure it, but as I recall the shaft has very little side play, but moves up a fair amount, enough to hear a kur-chunk each time. He's thinking causing arcing and heat. Another piece of the puzzle- the 1st time it failed with the Standard brand module, I pulled the cap off, saw evidence of arcing, rebent the spring contact up and it started. I assumed the module had over heated. So when I figure out a reliable distributor brand, I'll most likely replace it.
No surprise. DUI has went south. I ordered a new one a few years back for an 87 C4. It was dead in the water and the whole thing looked like it was fabricated with a hammer and chisel. While many will disagree imo the old ICM's of today are garbage.Been stranded too many times. I do hear good things about the MSD ICM but for the money I went with the MSD ULTRA this time. It's interchangeable with the old style 6AL which are cheap used and I carry for a spare and diagnosis.It fits nicely and is vented through the cowl area.
This just in. A friend of mine noticed this on the new rotor button. It has been arcing on the contact. I need to measure it, but as I recall the shaft has very little side play, but moves up a fair amount, enough to hear a kur-chunk each time. He's thinking causing arcing and heat. Another piece of the puzzle- the 1st time it failed with the Standard brand module, I pulled the cap off, saw evidence of arcing, rebent the spring contact up and it started. I assumed the module had over heated. So when I figure out a reliable distributor brand, I'll most likely replace it.
That's unfortunate, but at least you likely know what is wrong.
If you don't want to spearhead the coil-on-plug swap, I've had good luck with an OEM GM 93440806 distributor. I bought it as a ZZ4 crate engine take-off (NOS, never run, on a shelf for 10 years). It was $100- when I bought it in 2018 or so. And it's $500- now on Amazon! Crazy. Mine is now in its second car, and doing well. You'll need an adjustable vacuum advance, and some shims, to really dial it in, along with a @lars -style tune.
FWIW: GM HEI has no bearings; only two (upper and lower) bushings, which are replaceable, but may require reaming to align/fit.
Older bushings are an Unlikely source of any significant heat.
Some 15-20 years ago I also snagged a brand-new Takeout ZZ4 HEI.
From TPiS' clearance sale; also a steal at about $100.
Last edited by Rebelyell; Apr 12, 2026 at 12:40 PM.
When I've had a module fail in the past, I've noticed excessive brown like dust from the rotor arching through to the advance weights and Im sure some archs were hitting the module. My bad for not changing the rotor often enough.
Also wanted to add that when all my modules failed the engine had reduced power and a missed when pressing the accelerator. Like the module couldn't fire the increased fuel or cylinder pressure but the car always ran.
The only times the car would stop running was when the pickup coil failed.
Buy a good brand of Module. Makes all the difference. IIRC NAPA & Standard are good ones.
Use computer heat paste heat sink compound, I like the silver containing stuff. The more heat transfer, the better.
I heard you say your engine is basically all stock. You would be way ahead by cooling it off and getting rid of GMs strongly retarded spark timing curve which heats the engine up, on purpose, for a lousy 70s attempt at emissions.
That timing curve does not play well with modern gasoline, which has a way different vapor pressure than 1970s gasoline.
The modern stuff is blended for high pressure fuel injection, not carbs.
A new performance timing curve will help a lot to cooling the engine down, and tolerating modern gas better.
Search for Lars Timing Curve on here.
Basically full manifold on the vacuum advance can, not ported like OEM. Vac can limited to 12*
Then 12* initial and 35* at 3000rpm mechanical.
Exhaust manifold temps can drop a 100* or so.
I found my distributor had 0.085” of vertical play and the magnetic pickups magnet was broken in 3 places. The DUI stuff just created a light show underneath the cap and I’m thinking zapped the modules. Summit was kind enough to refund/exchange the DUI stuff. What I installed was a Summit HEI refresh kit. This included cap, coil, module, rotor, weights, springs and an adjustable vacuum advance. I put the medium springs in. I also put in a Standard brand magnetic pickup and reduced the vertical play to 0.013” with Moroso shims. I needed to make some bushings to take some of the slop out of the mechanical advance weights. The mech advance total (w/o vac adv) was 31 degrees at 2800 rpm and the curve looked pretty good The car runs much better and has lasted about 300 miles so far with a clean cap and rotor. The gas mileage has improved from 10.5 mpg to 14.5 mpg. The timing chain needs attention, it has about 8 degrees of slack. The new vacuum advance seems to pickup early at 1”Hg and is full in at 5”Hg using a mini vac with a meter. It also adds 21 degrees of timing all in. That seems a little much.
Where should the vacuum advance start and how much timing should it add? How do I limit the vacuum advance?
I found my distributor had 0.085” of vertical play and the magnetic pickups magnet was broken in 3 places. The DUI stuff just created a light show underneath the cap and I’m thinking zapped the modules. Summit was kind enough to refund/exchange the DUI stuff. What I installed was a Summit HEI refresh kit. This included cap, coil, module, rotor, weights, springs and an adjustable vacuum advance. I put the medium springs in. I also put in a Standard brand magnetic pickup and reduced the vertical play to 0.013” with Moroso shims. I needed to make some bushings to take some of the slop out of the mechanical advance weights. The mech advance total (w/o vac adv) was 31 degrees at 2800 rpm and the curve looked pretty good The car runs much better and has lasted about 300 miles so far with a clean cap and rotor. The gas mileage has improved from 10.5 mpg to 14.5 mpg. The timing chain needs attention, it has about 8 degrees of slack. The new vacuum advance seems to pickup early at 1”Hg and is full in at 5”Hg using a mini vac with a meter. It also adds 21 degrees of timing all in. That seems a little much.
Where should the vacuum advance start and how much timing should it add? How do I limit the vacuum advance?
Wow! Thanks for following up. I'm glad Summit took care of you. DUI was the go-to distributor for years on this Forum. No longer, it seems.
What kind of adjustable vacuum advance did you get? I added an Accel 31035 to my Delco distributor. It has an internal adjustment to limit the total vacuum timing (I set it to 12 degrees), but a fixed range where the vacuum advance kicks in (it works with my engine, but may not work with a large cam).
@lars ' instructions include some alternate ways to set the max vacuum advance, including a permanent mod, or an add-on mod.
Is the 13 degree for vacuum advance number, the distributor degrees or the harmonic balancer? The vacuum can I put on is showing 22 degrees at the harmonic balancer, but that would be 11 degrees at the distributor.......I think.
DUI ignition is DOA
