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GM HEI ignition module testing

  
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GM HEI ignition module testing

 
John 283 John 283
User | Posts: 245 | Joined: 01/07
Posted: 10/19/05
03:10 PM

I know I read somewhere how to test and see if an HEI ignition module has gone bad with a multimeter.  Can someone help me out or guide me to this knowledge.  
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alant470 alant470
User | Posts: 96 | Joined: 01/05
Posted: 10/20/05
10:01 PM

The easiest way to test a module on the car is to ensure that the wiring & pickup coil are OK.  The module is essentially an amplifier with a built in analog to digital convertor.


 


Easy tests for HEI system:


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.

 

TRANSAM78 TRANSAM78
New User | Posts: 3 | Joined: 10/11
Posted: 10/12/11
03:19 PM

PLEASE CLARIFY YOUR STAEMENT......


2. Connect the ground side of your test lamp to the battery POSITIVE cable.??

THE GM SERVICE MANUAL STATES VOLTAGE PRESENT AT THE TACH TERMINAL WHEN STARTING....

ARE YOU LOOKING FOR A MOMENTARY GROUND CONNECTION??

OR IS STATEMENT 2 A TYPO....PLEASE RESPOND

THANX  

waynep7122 waynep7122
Enthusiast | Posts: 594 | Joined: 08/09
Posted: 10/12/11
11:52 PM

let me take you a little farther...

when you turn the key on..    positive power is sent through the red wire plugged into the BAT side of the HEI cap.. inside the cap you will notice the coil wires have a T shaped terminal....  the red wire goes into the coil.. but it also connects to the right side of the 3 wire harness from the cap to the housing from there to the B+ terminal on the module...


the TACH terminal in the cap.. also has a T terminal.. it also connects to the LEFT side of the 3 wire harness down to the housing and connects to the C connector on the module...





wait.. there is a third wire in the three wire harness...  this is where a LOT of problems crop up...
this is the ground strap... it goes into the cap before the coil does..  fits into the middle connection of the 3 wire harness..



the screw that goes through the coil laminations directly over the hooked  end needs to have the black wire from the coil..

why....  this is the ground for the coil...    without it .. the voltage can build up in the frame of the coil till it flashes over like a lightening bolt.. and can be as loud as a shot gun blast going off under your hood..  

the center wire goes down and is connected to the condenser hold down strap and the strain relief hold down..

this is really important.. the spark has to make it back up this wire and to the coil after it passes through the spark plugs..



back to the description...

there are 2 screw that hold this coil cover down..     you will notice the TACH and the BAT markings on it..

test with your test light clip hooked to ground to the BAT side.(red coil wire)... you should get voltage...

test with your test light clip hooked to POSITIVE.. the TACH side.(white or yellow coil wire) ..   while somebody cranks the engine...

the test light should flash..   this is because the module is making and breaking ground to the C connection inside the distributer. ....... this should make your test light flash..



i normally just remove the module and take it to the parts store as some of them have module testers... where they plug them in and an automated test is run...


i should probably continue on my HEI description...



the 4 pin GM HEI module...



you can see the pins on the right side of this module..    

B is the switched ignition power..  

c goes up to the Tach and to the yellow or white wire on the coil.

wait.. where is the ground connection..   see the 2 hold down screws...  the rivets going through the plastic when the screws go through them ground the module...

why is the ground important..  .. the module connects the C terminal to ground and then breaks that connection to discharge the ignition coil...


on the left end of this 4 pin module are the terminals that go to the pick up coil...

the pick up coil creates a small AC voltage.. usually just about 1 volt AC..  when the AC voltage comes to 0.3 volts positive. the power transistor in the module turns on and grounds the coil.. allowing the electrons to flow into the coil primary creating a magnetic field..

this AC voltage is created by the reluctor spinning above the pick up coil...

this is the second time i have typed this out tonight..  hmm...

when the tips of the  reluctor line up with the pick up coil tips.. the voltage will drop back to zero.  at this point the power transistor opens .. breaking the connection to ground..  this causes the magnetic field in the coil primary to collapse through the secondary windings and create a high voltage spark....

as the reluctor tips move away from the pick up coil tips.. the voltage continues its negative swing.  then it starts rising again as it approaches the tips again..


one of the most important things you can remember.. there has to be dielectric tune up grease under the module or it will die...  if it starts again once it cools off does not matter .. once thermally damaged.. its toast.. it will fail eventually.. leaving you stranded... usually at the worst point..




and a treet for those who have read along..

a bunch of years ago.. my brother in law changed the engine in his suburban..  on the first long drive.. he had gone through 3 ignition modules and coils in 80 miles..   so he called me from an area by sea tac airport.. i ask him when he put the motor in if he had changed the distributer cap... he had.. i ask him not installed the ground strap under the coil when they swapped it over...   knowing that there are NAPA stores everywhere up there..  i told him to get to one and gave him the napa part numbers... after he hung up .. i thought about it.. and called the 3 local napa stores and described him and his helper..  had them put the parts on will call...   i did not hear back from him for months..  when i ask if my fix had worked.. he said it was really strange.. as soon as they walked into the napa store..  they were greeted by name.. the exact parts they needed were sitting on the counter and the receipt was already printed ..  this for some reason they could not understand.. thought something really out there happening..  how did the staff know them by name when they had never been there before.. how did the parts happen to be ready and the receipt printed...   i really spoiled it when i told him that i had called ahead and put it on will call..  

TRANSAM78 TRANSAM78
New User | Posts: 3 | Joined: 10/11
Posted: 10/13/11
08:54 AM

HOLD ON PLEASE............

I THINK YOU MADE A MISTAKE..........

DILECTRIC GREASE WILL GO AWAY UNDER HEATED CONDITIONS.............

AND ACTS LIKE AN INSULATOR TOO............

YOU ARE SUPPOSED TO USE SILICONE HEAT SINK COMPOUND WHICH IS WHITE..........

SINCE THE SCREWS GROUND THE UNIT THE WHITE GREASE WILL ALLOW BETTER HEAT TRANSFER...  

TRANSAM78 TRANSAM78
New User | Posts: 3 | Joined: 10/11
Posted: 10/13/11
11:18 AM

YOU ARE CORRECT

I AM WRONG

THOUGHT HEAT TRANSFER WAS IN OPPOSITE DIRECTION  

waynep7122 waynep7122
Enthusiast | Posts: 594 | Joined: 08/09
Posted: 10/13/11
01:11 PM

i recommend dielectric grease as thats what's called for...

white thermal compound is actually better... but its NOT available in every autoparts store... in fact i have never seen it in a parts store.. only serious electronics stores...


warning...    white thermal compound is NOT white lithium grease...

white lithium grease will NOT work for this thermally conductive application..

the HEI module and other modules like the ford TFI module have the power transistor directly applied to the backing plate..

there is no external heat sink to radiate heat to like a duraspark 2 or a mopar electronic module...

the heat built up by the transistor passing the massive amounts of current is incredible..   without the thermal grease.. the module cannot loose enough heat to the distributer housing to radiate it to the atmosphere...

what happens... the circuits get hot and expand.. at some point in this thermal runaway.. the traces/conductors will lift off the circuit board and then overheat and burn.. . sometimes they will just lift . when it cools.. they will make contact again..  opening when they get hot... this is why i mentioned that once it has failed.. its toast..   it can be used for testing only..

why if the circuit trace lifts would it get hot...

the circuit board or what ever the trace is attached to has more mass and will act like a heat sink to pull the heat from the conductor...   when your heat sink material gets too hot.. the inner material cannot dissipate more heat and the thermal  runaway happens again..

for those who want the white thermal compound..   computer components stores and radio shacks have it..

Prs1c 3312830W345

its used on top of most computer processors under the heat sink with the fan on it..  

sarum sarum
New User | Posts: 1 | Joined: 01/14
Posted: 01/26/14
03:10 AM

Thanks for putting up some great info.I would appreciate some help with a no spark at sparkplug problem on my 1965 Olds Cutlass Sports Coupe.The car has what must be a later model GM (ACDelco) HEI system installed. After reading  some perhaps less than accurate info on a number of sites I metered and came to the conclusion that the ignition coil had gone faulty.I then purchased and installed a new coil. Same result!Ithen found a YouTube video at https://www.youtube.com/watch?v=o8Ba1CLdvb which helped me decide nothing was wrong with original coil.I am going to look at the HEI module now but the first query I have is that I have not got the "mini coathanger" style earth strap you picture or any other strap apart from the black wired one attached to the coil.I wonder if this is because the HEI was not installed at the factory and something else was done to relay the "used" spark safely away.  

wayne712222 wayne712222
User | Posts: 92 | Joined: 10/13
Posted: 01/26/14
04:56 PM

lets start..

do you have a test light.. take the coil cover off the top of the cap.. so you expose the red, black and either white or yellow wires from the coil..
Heibigcap

Heicoilproblemat4k

stick the test light probe into the cap from the top to touch the red wire terminal..

ground the clip of the test lead..  slowly turn the ignition switch to the RUN position tehn to the cranking position.. then back to the run position.. then turn it off..

you should have power from the ON position thru cranking and back to the on position. not loosing power until you turn the key off..  


what do you have???  

wayne712222 wayne712222
User | Posts: 92 | Joined: 10/13
Posted: 01/26/14
05:10 PM

Hei 4Pinmodule Zps5e505aa8


Untitled6 Zpsd3bb2005

Untitled7 Zpsb4b38f49

Untitled3 Zpsb7fb0c42

Untitled5 Zps63341c9b

Untitled19 Zpscaa09511

Untitled4 Zpsec225251

HEI Coil Missing Ground Connection Failure Zps5bad7687


sometimes.. you have to change the ignition switch and modify the wiring to supply the HEI with enough current to operate properly..

the point type ignitions have an IGN1 and an IGN2..

ign1 has power in the ON position but NOT in the cranking position on the early ignitions.

IGN2 has power only in the cranking position..  this is for the resistor bypass.

on the later ignition switch.. the IGN 1 has power in RUN and CRANKING.. this is because the HEI needs full battery voltage to operate properly..


Gmkeyindash POINTS To HEI  

wayne712222 wayne712222
User | Posts: 92 | Joined: 10/13
Posted: 01/26/14
05:39 PM

want to see if its sparking.. take 8 spark plugs..   strip some copper wire.. wrap the spark plugs around the middle of the metal shell.   ground the end of the copper wire to the engine block.


 mark your spark plug wire locations..  move them one by one up to the wired spark plugs..  when you get them all there.. DO NOT HOLD them.. but position them where you can see the gaps... have somebody crank the engine..  are the spark plugs arcing..

shows its working...


a bench test..

this takes a jumper cable.. and a wire between the positive battery and the BAT terminal on the HEI CAP... and a metal plate of some kind..

this is different.. pay attention..

with the HEI fully assembled..  including the cap and the coil cover..

hook one jumper cable clamp to the metal plate..  turn the HEI over .. cap down into the metal plate.. hook the other jumper cable clamp on that end of the jumpers to the housing..

hook both clamps on the other end to the NEGATIVE POST on the battery..

now.. hook up the wire from the HEI BAT terminal to the positive battery..

since you are holding the HEI housing.. upside down on top of the now grounded metal plate.. the cap terminals should be an equal distance from the  metal plate.. and the coil cover touching..  spin the shaft.. the cap terminals will arc across to the metal plate if the HEI works..  

wayne712222 wayne712222
User | Posts: 92 | Joined: 10/13
Posted: 01/26/14
05:54 PM

want to have more fun...

if you set your harmonic damper to the timing mark you want..  and the #1 cylinder on the compression stroke..

with an HEI... or other distributors that use reluctors..

if you insert the distributor..  get the rotor pointing in the proper direction.. turn the housing to align the pick up coil teeth and the reluctor teeth.. you can lock the distributor down..

the HEI or other ignition module is going to fire the coil when the reluctor teeth align perfectly with the pick up coil teeth..


91 Jbody 20 Distributor Zpsc3e4178d

i just showed this 4 cylinder distributor as it was aligned perfectly...


This allows you to dead stick time a motor..  so when you turn the key.. it fires up on the first try.. no need to rotate the distributor to get it perfect..

guess what.. you can also take the distributor cap off..

turn the crank slowly in normal direction of rotation.. when the timing mark is visible. slow down.. watch the reluctor and pick up coil alignment.. when the pair line up perfectly.. look at the timing marks..  thats where your base timing is..

now you know where to set the crank to align the distributor housing, pickup coil and reluctor