HEI IGNITION MODULES FAIL

I can't keep ignition modules in this thing.  Sometimes they last a week, sometimes 5 or 6 weeks.  They fail the same everytime.  When I go to start the car, either from overnight, or at the end of the work day, no spark.  The car runs great otherwise, no misses.  The motor is a 455 fresh build with about 6,000 miles on it.  HEI is a coil in cap by DUI.  Battery, alternator, under-hood wiring is all fresh with the rebuild.  I seem to have good ground.  The battery negative is connected directly to the block, and the distributor hold-down is metal-to metal contact with the distributor base.  I tested the ignition voltage with the car off, key on, got 12 volts at the distributor.  When I cranked it, it went to 10 volts.  Not sure it should do that.  Haven't tested it running.  When I installed the engine, I cut off the resistor wire at the firewall block, and connected the new ignition wire to it, and to the distributor.  Should it be replaced all the way back to the ignition switch?  I use the dielectric grease that comes with the new modules.  Electrical is my least favorite thing...Someone please advise.  Thanks.  

Soulds like you have done everything right. A bad coil with too much resitance or producing too much voltage can burn out modules and vise versa. Replaceing an old coil with a new (especially a performance coil) will usualy burn out the module in 50-500 miles. Once the module is change, you should be good. I suspect there is something wrong with your coil. Swap it out. I like to keep old working factory coils around to test when I or a customer has module problems.

Original GM HEI coils produce a full 40,000 volts same as the best aftermarket coils made. For a decade, I raced with an HEI set up running 10:1 compression, running 11's and also 10's with NOS. Every aftermarket coil I tried was crap compaired to an original GM part.  

The HEI is a new DUI 50,000 volt unit.  Could it have a bad coil?  Can it be tested?  If so, how?  

You might consider going with some sort of CD ignition box like an MSD 6AL, Mallory HyFire, etc. You can bypass the module completely and chuck it, and you get multiple sparks at lower rpms plus a rev limiter. I had a bad module in my HEI, put a Hyfire 6AL on it & it works great. If you don't mind a sort of off-brand unit, MSD has a sort of lower-line box called a Streetfire (I think), has all the same features and sells for about $135 at Summit or Jegs.  

Yea, I might have to consider that.  I went with the single unit coil-in-cap drop-in HEI to keep it simple under the hood, but so far, it hasn't been reliable.  If I can figure out what is burning modules, I'd like to keep it that way.  

You can easily have a bad new coil. Almost every single aftermarket part I have bought that is made in the USA has been crap. From March billet pulleys, MSD, Willwood brake kit, Butler heads, AI2 block, SSBC break valve, Accel dual sync dist, Moroso vacume pump, Melling oil pumps, power steering pump, and more that I have bought have all had failures or are machined way off spec.  Anything new for performance parts has about a 15% reject rate. The industry is about one thing only. Marketing. Quality is their last concern. I know this from inside sources as well. Indy is a prime example of complete *** for your money as well.

Find yourself a working used GM coil and run it to see if you burn out another module. Also measure the resistance of the GM coil compared to the aftermarket coil. As I said I have race in the 10's with 30 year old GM coils. Eventually I went to a MSD Digital 6 box (it took 3 before I got one that worked correctly) with a Blaster coil and new dist. Performance improvement for $1000 investment. Zero gain at the track.  

When removing the resistor wire you should do it by removing the pin at the firewall plug in the under hood harness and remove the wire completely and recrimp a new one in. This is a fun chore. There is no resistor wire past that firewall plug on the inside harness all of it is in the under hood harness. But I agree the coil is probably the problem. Some the shop manuals will state the spec primary and secondary resistant required. I have sucessfully use Accel HEI distributors in a couple of GM cars with their in cap hot rod coil and never had a failure. The coil and modules should be a matched set up like the OEM or correct after market.  

ok... lets step back....

pull the coil out of the cap...   sand the paint or coatings off the corner of the coil frame where the black wire with the ring terminal gets a screw through it...

sand the bottom corner of the coil also....  make sure the folded metal ground strap is installed...    it goes to the center wire of the 3 wire harness up from the distributer housing..    the screw that holds the strain relief and the condenser ground the coil to the housing..     if this ground wire is loose...  the folded metal strap under the coil  is left out... the voltage will build up in the frame of the coil... until it arcs over and blows out the module...   when the ground strap is left out... it will usually make a small noise... like a 12 gauge shot gun going off under the hood..

on the power wire ....  install a relay...    use the existing wiring to switch terminal 85...  pin 86 to ground...     pin 30 with 10 gauge wire with a 4 inch 14 gauge fusible link on it.. to a terminal block that has enough power from the battery...  and pin 87 in 10 gauge to the hei bat terminal...    

this sends full battery voltage to the HEI...    when you have reduced voltage.. the current goes up...  makes the module hotter...

i agree that it could be the coil...   but it could be the other things also..

i would also try a replacement capacitor and harness... 12 bucks from AZ  part number duralast  U518

here is a picture of it..

http://contentinfo.autozone.com/znetcs/product-info/en/US/wl2/U518/image/8/

see the screw sticking up from the capacitor harness... that is the one that grounds the coil...


one more thing...    do you have a battery cut off switch????    do you shut off the battery before the engine comes to a stop????    if so, you might be creating a voltage spike in the charging system...  when the battery is removed from the charging system and the system is still charging.. there is no place for the electrons to go... so they build up as voltage..  it can go way over 14 volts.. sometimes upwards of 80 volts..   same thing happens when someone pulls off the battery cable to test an alternator..    an instant high voltage spike damages the electronics...

you also might try a different brand of ignition module...

electronics stores have white transistor thermal paste...  works just a little bit better than dielectric tune up grease....

i also hope that you were not using silglide.. as a replacement.. that is not for thermal transfer ...  

just to keep things straight...

the coil windings ground through a black wire with a ring terminal.. this goes under the screw.. the ground through the coil laminations, to the ground strap to the coil.. to the center wire on the capacitor harness...  to the ground screw that holds the capacitor down...    through the housing.. the hold down... the engine block.. the cylinder heads... to the spark plugs... up the wires...  to the rotor.. up through the middle and into the coil...

some racers have started installing ground wires from the back of the heads to the distributer housing...   gives the high voltage a shorter path to return to the coil...

reduces the length of the circuit...    lets you really pour on the sealer.... some people have used enough sealer that only one head would fire the spark plugs...


one could shorten it even more by tapping into the center wire of the cap harness..  

Thanks guys,

Sounds like a lot of great experience here.  I've now got a lot more things to consider.  I've been a subscriber to HPP for many years, and it was great source for building my motor and restoring my 71 formula.  This the first time I've used the forum.  Thanks for all the help.