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yellowcat yellowcat
New User | Posts: 14 | Joined: 09/13
Posted: 09/06/13
01:15 PM

Hey all, need advice from a GM Hei distributor expert please.  I have a '79 GMC crew cab dually, 454 engine. Last year did a complete engine rebuild among many other upgrades and restoration.  I used a new Accel HEI distributor, coil and all new ignition components including Bosch 'Ultra Premium Opti-Layer Mag-Core' plug wires.  Everything ran perfectly, passed Calif. smog, the engine (last big-block year w/carburetor) is completely stock.  One day, a month or so later the truck wouldn't start, no spark. Replaced coil, started right up, ran perfectly. A few weeks later, same problem, same fix. Added extra engine ground straps just in case.  A few weeks later same problem/fix. I has happened again for the fifth time now using various mfg. coils since. The engine has run just 1100 miles since the rebuild.  This is my designated go the river-tow my boat-fishing-camping, remote location-no cell phone access in the Sonoran Desert-kinda want it to start-truck! Any clue's would be greatly appreciated!   Getting to the point of considering going back to an old school points distributor if all else fails but not sure what would be involved in such a retro-conversion?  I greatly welcome any hints, ideas, or suggestions to diagnose the HEI or replace it with another type of ignition system that's more reliable...   Thanks!  

waynep712222 waynep712222
User | Posts: 209 | Joined: 03/13
Posted: 09/07/13
09:39 AM


lets start by checking out the electrical harness between the cap and the distributor housing.. these are available from several suppliers .. you might have one that was improperly assembled and it made its way into a unit..  i posted some part numbers at the lower left of this image.. you can ohm test this part.. easily... but if the wires are bad. or only partially connected out of sight..

Untitled4 Zpsec225251

Untitled19 Zpscaa09511

Untitled18 Zps37da024d

in this image of the top of the cap with the coil cover removed..

see the black wire coming out of the coil winding..  going over to the top of the lamiations and held down by the screw..


if your laminations are coated... painted.. as some are..  you will have issues with the spark return path...

this goes UNDER the coil.. into the cap before the coil is dropped in...

HEI Largecapgroundstrap

this connects to the center wire of the 3 up from the bottom...

if this is missing.. or there is a continuity issue.. please use an ohm meter.. ENGINE OFF.. to verify OO of resistance between the black wire and the ground strap and then also to the distributor housing...

the spark path.. coil output terminal, cap center button, rotor.. jumps to a cap termial, thru the spark plug wire .. spark plug.. jumps the gap.  thru the heads to the intake manifold.. to the hold down clamp bolt and clamp.. then the distributor housing .. thru the hold down screw.. the black wire to the cap.. the folded ground strap.. the coil laminations then to the black wire from the coil winding...

actually it goes the other direction but ..

why does the wiring explode on the coil.. because the laminations and coil windings build up so much voltage that it arcs over to the other wires to get to ground.. it can sound like a shot gun going off..

HEI Coil Missing Ground Connection Failure Zps5bad7687

this is what happened when the coil strap got left out..  the installer even went as far as grinding the paint off the top of the laminations look at the left side.

here are some more areas to test...  that are probably not the coil failure issue.. but learning them is important...

Hei 4Pinmodule Zps5e505aa8

Untitled3 Zpsb7fb0c42

Untitled5 Zps63341c9b

please verify that you have something like this between the back of the heads and the firewall...

Untitled22 Zps477f6001  

waynep712222 waynep712222
User | Posts: 209 | Joined: 03/13
Posted: 09/07/13
09:47 AM

now to continue the diagnosis...

is this distributor and engine installed in the 79 chevy truck????? yes or no.

since i am hoping that you have a digital volt meter... i would like you to run this test....

its printable.. i formatted this as a jpg image..  

VOLTAGE Drop Testing

please... post results by number...





i have found some loose ground wires on later chevy trucks.. that showed 8.5 volts of difference between the engine and the body on test 4..

if you get more than 0.04 or 0.02 volts.. you have an issue on the ground side..

this is only a start to the diagnostic tests..

but you have to PASS THIS ONE before you go on to the next steps or those results will be invalid...  

waynep712222 waynep712222
User | Posts: 209 | Joined: 03/13
Posted: 09/07/13
10:07 AM

80S Chevy Truck Battery Cables

additional tests... AFTER YOU HAVE completed and repaired any issues from test above...
and or terminals to clean..

engine running. headlights on... digital volt meter to 20 volts DC..

positive battery post to the alternator output terminal...    much less than 1.0 volts.. like 0.07 or 0.09.. would be great...

positive battery post .. to the RED wire in the alternator connector into the side.. this should be close to 0.02 volts...  this is the alternator sense wire.. it tells the voltage regulator how much the actual truck is using to keep up with demand..  where the wire on the back of the alternator is hooked thru a fusible link directly to the top post on the starter..

the side wire is hooked thru a smaller diameter fusible link that is also shared by the various electical devices inside the cab...

this is actually part of a 77 to 80 gm truck wiring diagram that i highlighted...

77 80 Chevytruck Fusible Links

there is a terminal block on the firewall .. please disconnect the battery and take that all the way apart.. clean the various layers and studs with a stainless steel wire tooth brush.. then reassemble..

i have found so many of those that the lower layers were loose..

on a chevy truck.. you will also want to verify between the body, the battery negative and the frame... yes.. the frame to the battery negative.. why.. where do the tail lights ground to.. the back of the frame...  there should be a ground strap around the passenger side forward cab mounting bushing..  once in a while these get left off..

and memories of the truck i once had...

a co worker sold me his 79 C20 454 camper special for cheep as the camper got totally trashed and somebody hit his drivers door when he opened it..  

i junked the camper..  had to beat it to parts to get it off in the trash dump as my friend got lost with the second truck to pull it out with..

i had it parked for a month as i was looking for a replacement door of the same color..  dark brown metallic. with a pale yellow insert.. but just as long as it was brown.. my sisters kid saw the truck and begged me for it..  as it was almost the same as what his dad had previously..   i had always wanted exactly that truck..

his begging overcame me..  i gave him the truck..  he got it home.. stripped the 454 out.. sold that to a friend of his and scrapped the rest of the low mileage 79 camper special..   i have thought about sending him lumps of coal in stocking for xmas.. but he would just set up a bbq with it..

do you still have the quadrajet on the 454... we have some talking to do after you get the electrical issues sorted out...   i am not going to sell you anything.. just try to get your truck to run better.. ..

there are a few things involved.. but.. we will talk about that when the time comes..  

waynep712222 waynep712222
User | Posts: 209 | Joined: 03/13
Posted: 09/07/13
10:12 AM

oh... why am i having you run all these charging system and voltage drop tests..

so we can make sure that you are not having voltage issues and blowing the coils up with excessive alternator output.. either on the positive side or the negative side..  

yellowcat yellowcat
New User | Posts: 14 | Joined: 09/13
Posted: 09/07/13
11:35 AM

Wow Wayne, thanks so much for your excellent reply!  So much to digest for the moment and I'm willing to go through the steps required to sort this problem out.  Yes I do have a new (rather complex) multimeter and still on the learning curve to figure out if I'm using the proper various settings (ohmmeter) and know I'm using it correctly. Thanks for including the truck wiring diagram (generator?) my manual doesn't have it. I can put in another coil and get it running to do the running tests for the charging system, (I've heard voltage spikes can fry coils.) Initially the truck ran missing the head to firewall ground strap. I did install one, actually 2 since, between recent coil changes, just to be sure but no change in coil life. I was very careful to remove paint on new coils where ground strap screw connects and clean most any other ground connections on the truck. I suppose the next step is to pull apart the distributor and do the checks you suggest.  To be continued on next post...  

yellowcat yellowcat
New User | Posts: 14 | Joined: 09/13
Posted: 09/07/13
12:10 PM

I miss my '64 Chevy suburban I sold a couple of years ago, when the ignition went off tune, ten minutes with an allen wrench and a dwell meter and I was good to go! I've put a lot of time. sweat and money into this '79 Dually, total engine rebuild, three angle valve job, Crane billet cam, Edelbrock intake manifold (still running the Quadrajet carb), A TCI TH400 tranny, after going through 3 trick 700R4's (bought it that way). I also spent for new exhaust system w/Flowmasters. This is still a Calif. emission control vehicle with a smog pump and miles of vacuum hoses, etc and it did pass smog. I've Googled 'HEI coil failure' and read many forums and such but no good leads until now, Thanks! I did replace the fuseable links too. Another clue I've seen is the possibility of incompatible plug wires that may feed back excessive voltage somehow to the coil, some say change the module but as I understand it bad coils ruin modules, not the other way around?  I'll again review your post and hopefully come up with the right questions to ask after it all sinks in...  

waynep712222 waynep712222
User | Posts: 209 | Joined: 03/13
Posted: 09/07/13
01:14 PM

to ohm the wires...

set the digital volt meter to 200K ohms... 200,000...

measure the resistance in the wire..  1,000 ohms per inch...

and at the 200K setting..  the meter should display less than the length of wire in inches...

what..   yea.. a 24" long wire should display 8 to 18 on the meter...

i am not familiar with that brand of wires.. i am hoping they are not solid core.. i forgot to check that fact..  spiral cores usually don't have issues.. s do carbon cores..

a hint... if you hate changing spark plugs..  you should be able to come up with a set of delco.. CR45TS Spark plugs for it...  putting the C infront of the R.. gets you a different spark plug.. one with a thicker center and side electrode.. they were designed for chevy astro and other chevy vans. that were really hard to get at the spark plugs.. so they increased the size of the electrodes and they go for years.. should be available in CR43TS, some stores may have CR44TS and most stores should have CR45TS..  now the only thing stopping you is the counter guys who have never stuck their hand in that bin.. so they don't know its there..

i can probably tell you why your truck ate 700r4 transmissions..

did you have the proper cable ratio adaptor on the throttle linkage lever.???

as TH350 and the sometimes link for the electric kick down on a TH400 are in the wrong place...  the distance from the center of the throttle shaft to the attaching pivot is too long. it should be 25MM or 1" exactly.. center to center

the TH350 carb and the TH400 carb have their kick down link mounted at about an inch and a quarter from the center of the shaft.. this gives the 700 throttle valve cable the WRONG pull ratio... and when you go full throttle.. you run out of cable core and yank the casing out of adjustment.. at that point the line pressure is too low and the transmission is smoked in a few blocks... sorry... thats the way it happens...  and way too often..    

not enough additional cooling will also smoke the 700R4..  as will kinks in the cooling lines... one had a blocked line that was not obvious.. it lasted 15 miles..

the replacement lasted 15 miles..  there was plenty of transmission oil cooler flow at the radiator.. but there was a check ball in the line for some reason that had slipped out of place???  check ball prevents converter drain back in some applications..  it plugged the link solid and the sun gears and sprags that are lubed only by cooler flow ate it big time..  

yellowcat yellowcat
New User | Posts: 14 | Joined: 09/13
Posted: 09/07/13
05:08 PM

Hey, thanks for mentioning the ohm meter settings, I also noticed you showed a setting on the diagram to test the pick-up coil, xlnt. The plug wires are likely carbon core, what I've always used in the past, maybe spiral core but I suppose I can look that up and admittedly don't know anything about spiral core characteristics.  I've used solid copper and steel core wires on race motors and past hot rods with no radios, kinda bulletproof for those applications but not for this street truck. I will check the resistance on the Bosch wires to eliminate that as an issue.  The spark plugs I used are Autolite Platinum AP145, no problems with them, stock replacement number if I recall. On 700R4 tranny's, had this truck for 14 years now, got it with that trans, previous owner wanted better mileage and it did get around 13MPG on the hwy.  I went through 3 of 'em, each lasting from 1 1/2 to 3 years. The last 2 were rebuilt with all the heavy duty trick racey bits, HD converters, extra capacity sump, massive oil cooler, 4 core desert radiator, etc.  The bottom line is they're a light duty trans, great for 'vettes, Camaro's and hot rods with small block motors, etc. but there'a reason they never put 'em in big heavy crew cab dually's with big block motors, they just can't take the strain over time, in my humble opinion. I put in a TCI heavy duty towing/RV TH400 and have had no issues since, other than 9 MPG, of course...  

waynep712222 waynep712222
User | Posts: 209 | Joined: 03/13
Posted: 09/07/13
06:56 PM

have had no issues other than 9 MPG, of course..

that's where  we may be heading next....

depending if you have an oxygen sensor bung installed already.. a narrow band oxygen sensor and a 35 buck air fuel ratio display.  

there is a need to look at idle transition air fuel ratio..

with a quadrajet.. the APT.. adjustable part throttle adjustment limits the downward motion of the primary metering rods into  the jets.. so the vacuum pulling down on the primary rod holder cannot pull them too far in so as it comes thru the idle transition .. the primaries do not go lean requiring more throttle to drop the manifold vacuum to get the power valve spring to allow the rods to lift slightly to get more fuel...

and you are going WHAT.. yea... coming off idle.. is it smooth... should be.. as you get farther into it.. it should still be smooth as quadrajets get into the primary mains at about 1000 RPM on the 454..  

Quadrajet Primary Circuits

Ctrp 1008 11 O2bquadrajet Racing Carburetor Tips2brod Shot

you can measure the primary rod holder lean height.. by pushing down with the dipstick tool...  scale starts 39 mm from the tip... the rod holder is usually 5mm down the vent when its seated and the screw is just starting to lift it..



its an 1/8" thick and 3/16" wide.. again.. 39 mm is zero..

the tool with the white knob is the proper tool to adjust the APT screw.. but its a borroughs special tool that no longer made.. and i have not had a quadrajet open lately to measure the double D size on the head..

many people tap the APT hole to 1/8NPT.. so they can just remove the plug carefully... and use the tool to give the APT a 1/8 turn..

Quadrajet Rebuild Tech

i normally have the owner do something like a dry hop after a burn out.. to see where the base line tuning is...

that ugly carb was on my friends 71 GMC.. its a 75 quadrajet that was just butchered.. along with everything on the truck.. after the rebuild i got him in the parking lot of his tiny tire shop doing dry hops.  after each one.. i gave it a quarter turn counter clockwise.. at the start.. the truck just fell on its face and popped thru the carb... but after 2.25 turns.. i could see a smile forming on his face.. i think we ended up at 2.75 turns above where the apt screw first started touching the primary rod holder.. i could feel it by pushing down on the thexton 370 dipstick. thru the bowl vent..   he had no idea a quadrajet could run that good... then he had me do a holley on his friends 62 galaxy .. then i did the holley on his other friends 70 torino 429SCJ.  that one had a flat spot coming off the primaries.. i installed the quick change secondary spring cover.. and went up one spring tension.. he came back with a huge smile on his face also..

this is all for later....  and i don't really want to mess up your ability to pass smog.. so this may or may not get done...

oh.. don't forget that EGR flow can also improve your fuel economy..

as it dilutes the incoming air charge oxygen content so you need less fuel to burn the reduced percentage  of o2..  and it pings less also as less less oxygen burns at a lower combustion temp..

what is not said... is that at cruise on the primaries.. at high vacuum levels.. the primary metering rods should be held fairly far down by the vacuum acting against the power valve spring..  if the system is just lean enough for take off.. you should get better fuel economy when the primaries are open.. but the secondaries are not..

there is also a secondary air door spring adjustment...

i don't know how far into the secondaries your engine runs, spring wind up is in 1/16 of a turn increments between 1/2 turn and 15/16 of a turn..  

yellowcat yellowcat
New User | Posts: 14 | Joined: 09/13
Posted: 09/07/13
07:20 PM

Just checked the Bosch site, turns out my 8mm "Ultra Premium Opti-Layer Mag Core" spark plug wires are Spiral Core, using low resistance stainless steel windings. O.K. very high tech wires, likely much more durable than carbon core. Not at all like old school racing solid wire core that I'm familiar with. Being touted as low resistance, stainless steel or not I would think they would work with most ignition systems out there, even old HEI's without issue. I would guess that resistance is resistance, regardless of the materials used and if it's within parameters it doesn't matter, yes?  Sure wouldn't hurt to check the resistance I suppose...  Wishin' now I'd payed more attention in electric shop class back in Junior High...  

waynep712222 waynep712222
User | Posts: 209 | Joined: 03/13
Posted: 09/07/13
08:46 PM

its ok... i payed attention... and have learned a LOT since...

i really don't think its the spark plug wires...

by the way... just what is failing on the coil???

is the primary opening up..  red to yellow or white..

or the secondary windings????

my thinking is you are getting too much voltage at some point either on the negative side because of a loose ground wire... building up too many electrons on the negative side in the engine block.. because they cannot get to the negative battery post..

or on the positive side.. because of perhaps a connection between the 2 ring terminals down on the starter motor.. or the firewall power distribution block..

where the sense wire on the side of the alternator is seeing reduced voltage coming back to it...  so it brings up the charging voltage in an attempt to make up for the reduced voltage feedback ..

you can put 18 to 22 volts out very easily with a delco alternator when the sense wire looses connection.. or like with the 98.. 8.5 volts between the engine block and the body on the ground side..  thats all output from the alternator case..  its just the restriction ...

the voltage drop tests actually take less than 5 minutes to complete..

i just end up typing and typing and typing away till somebody on the other end picks up a volt meter and a print of the diagram and run them..

but it could also be the first image up at the top...

or paint on the distributor housing or on the hold down clamp..

over in the nascar racing.. racers have started creating 4 ring terminal wiring harnesses..   one ring to each head.. the third to the distributor housing.. and the forth to a well connected bolt thru the firewall..  this is where the big black wires for the MSD box should also be grounded.. so it shortens the spark return path..

you can with the digital volt meter set at 20 volts DC.. touch the block and the distributor housing.. to check for voltage drop across that.. there should be 0.00 as there is a lot of contact there.. but man .. if its open.. its going to explode your volt meter circuit board.. as there could be significant voltage there.  

waynep712222 waynep712222
User | Posts: 209 | Joined: 03/13
Posted: 09/07/13
09:32 PM

the spiral core wires.. ,,


if the secondary windings are opening up...  as you have not said what is failing..

lets talk about the coil secondary...

when the magnetic field in the primary winding collapses. it starts building up voltage in the secondary winding as the lines of force pass thru..

the voltage builds until it can flash over both gaps and the thru the resistor in the spark plug and the resistance in the spark plug wires..  this raises the voltage that is needed to start the flash over..

with the reduced resistance of the spiral wound spark plug wires.. can it allow the secondary to discharge at a higher current than with carbon core wires????

HMMM... are the secondary windings burned out in your coil????

do you have a hand held automotive digital scope??? nope.. i do.. but i don't think you live anywhere near me in california..

thats an interesting thought..  lowered spark plug wire resistance increases amperage till the coil secondary windings burn out ....

i lost my older brother a few years ago.. he would know the answer to that..  

yellowcat yellowcat
New User | Posts: 14 | Joined: 09/13
Posted: 09/07/13
10:13 PM

Great tutorial on Quadrajet tuning! As with electronic ignitions, Quadrajet carbs and their function is something I've had little experience with, (obviously). My experience with tuning and rebuilding carb's has been mostly with Weber's and S.U.'s and a Holley or two, unfortunately. Right now my main concern lies beyond MPG, just want my truck to start when I'm a dozen miles from the nearest paved road. I don't know why the coils are failing. From any visual appearance, they look the same when I take them out as they did when I put them in. No visible burn spots on rotor, cap or anything else, all appears normal to me.  I've done the basic resistance test to the old coil according to my shop manual and they fail. (don't remember the exact's at the moment) I compare it to the same test I do to a new coil and get the right results with the ohm meter and put it in and all is well, for a while...  My distributor housing is shiny billet aluminum, (like that helps) and I did make sure to remove paint under the hold down clamp before installation. Perhaps another ground strap between the hold down clamp bolt and the firewall would be nice as well but I doubt it is my basic problem. Please bear with my ignorance on the subject but how would I determine if the problem is secondary windings or the primary opening up, You lost me there I confess.. Your advice and specifics on checking the charging system is very helpful and useful, well explained for those like me with a limited skill set in that dept.!  The alternator is a stock new replacement one I changed when the old one failed on the road 2 years ago before the rebuild and seems fine but I plan to check it as part of the elimination process..  Thanks again for your great effort, I'm sure others who are following this will profit from the education you are presenting here..  

waynep712222 waynep712222
User | Posts: 209 | Joined: 03/13
Posted: 09/08/13
08:05 AM

Minimum Primary Resistance Range: 200 ohm scale   .2 - .8
Minimum Secondary Resistance Range: 200K ohm scale 5.0k - 11.0k


primary test.. at 200 OHM scale.. red to white or yellow wire..

secondary test... at 200K ohm scale.. black ring terminal to coil output terminal. (where the carbon button fits against.. but direct without the carbon button. )  

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