I believe I owe you a quick troubleshoot.
Can find this in most of the better shop manuals. many may not have that access though and have the access to this board.
First get some simple tools, won't need much...
1. 12v test light
2.simple volt/ohm meter
3.spark tester
Now this can be bought at the parts house or you can make one in a small amount of time. All it is , is a spark plug that has the ground electrode cut off so the spark has to jump approx .125 of an inch. it also has an alligator clip brazed to the hex of the plug shell. Cost: about 5.00 in the states or you can make one just as good out of an old spark plug for lots less.
Now let's get to work...
First Take the volt/ohm meter and verify the battery voltage across the bat terminals of the battery.
Then disconnect the "BAT" terminal on the dist cap (which is the 12v lead to the dist cap itself) and turn ign switch on and read voltage. Neg lead of meter to good ground and pos lead to the 12v wire terminal you just disconnected. Should be same as battery voltage or maybe a .1 volt less.
Reconnect the 12v wire to the cap and remove a plug wire. Connect the spark tester(alligator clipped plug)to the removed wire and put the clip on a good engine ground(bolt,alt case, etc.
Crank engine and if you get spark, look somewhere else as the HEI is good.
If not ... then here we go.
Remove coil cover on top of cap.
+ lead of voltmeter to "BAT" terminal
- lead to good ground
crank engine... less than 7 volts = problem in wiring from ign switch.
If more than 7 volts move the + lead to the "TACH" terminal of coil and turn ignition to "on" position. If reading now is less than 1 volt, then the ign coil is likely toast. If reading is 1-9 volts then the module is likely toast.If more than 10 volts, then attach spark tester to the carbon brush under the cap
(will need to lift cap off of housing and turn upside down for this, leave the little cap harness connected for all tests)
Spark tester will need a jumper wire from its shell to ground for this next test.
Crank the engine and spark should be jumping all over the tester. If that happens then look at cap for crack,water, mice, moth balls, old tools, small animals, etc.
If there is still no spark, then pull the pickup coil leads (green/white) from the module.
connect the - lead of voltmeter to good ground.
connect the + lead to "TACH" terminal.
Connect the 12v test light alligator clip to the "POS" battery post.
Turn ign switch to on position.
Watch the voltmeter as the test light is touched to the "G" terminal of the module FOR NO LONGER THAN 5 SECONDS AT A TIME, or you'll maybe smoke the module and be buying another.
What this does is put a trigger voltage to the module and causes the little transistor inside the module to close the coil primary ground circuit. Now when you remove the test light the voltage goes away and the transistor lets go and the coil primary collapses.
At this point the voltage reading on the meter should drop. And there should be a spark from the spark tester still connected to the carbon brush.
If voltage does not drop then look and be sure the module is properly grounded and all grounding connections are clean.
If voltage did drop and there was spark across tester, and you were tring to originally diagnose a "no spark/no start" condition....then replace the pickup coil. Why? because now you got spark when your supposed to have it, except the pickup coil is the only thing NOT still connected. Removing voltage from the module caused the transistor iside it to turn on/off. If you got spark at the tester then you know that that was happening.
You can also test the resistance of the pickup coil (green/white) wires and read the resistance between the 2. GM specs call for 500-1500 ohms.... I say 760-860 is good and anything else may work, but usually not in my experience, vey well.
If you got no spark when the test light was removed, then the module is likely bad, that is if you did the test right. It could also be because the ign coil ground was dirty/faulty etc. It is also possible that the ign coil itself is bad, so GM recommends a module testing unit to test the module seperately, to diffrentiate between the 2.
As confusing as this all may seem it is fairly starightforward.
Now let's take a look and see what we should do that's not really in the shop manual that happens in the real world.
Got one that has no spark and no start...
1.verify battery voltage.
2.check voltage at lead wire to cap with
key in "on" pos and in "cranking" pos.
Should be within batt voltage with key
on and around ideally 10 while cranking.
3.Remove the coil cover
4.Check primary resistance of ign coil
Red/White or Red/Yellow wires with plug
cap connector removed.
Good coil will read approx .5 to .9 of 1 ohm
Bad coil will be .2 or less or higher than
say 1.5 ohms.
5.Read secondary resistance of coil.
2 ways to do this with coil out of cap
and with it in the cap. Difference is ease
of testing and final reading due to carbon
brush.
Ohmmeter lead red to "ground" terminal
ohmeter lead black to carbon brush under cap
resistance will be 6000-30000ohms.
I like to read that with the coil out of
the cap, in that case test the round metal
disc at bottom of coil and the black ground
wire coming out of the coil epoxy.
Reading should be lower and usually in the
8000-9000 ohm range.
Reason it's lower is your not taking into
account the resistance of the carbon brush
itself. Most brushes read about the same
but do realize that hey can differ to some
extent.
If you get a good reading on the primary and
secondary then likely the coil is good and
at least you know it's not shorted and can
go on testing. I like to take the time to do
this before hanging the internals all over
the engine compartment. Usually if you find a
odd reading here you can save the detailed
testing as you will only find you already
have found the problem earlier.
6. Check the resistance of the pickup coil.
7. Check the grounding of the pickup coil
leads in relation to the housing. Reading
should be infinite here on both.
8. Then go into detailed testing after the
previous 7 steps have been looked at.
Might save you some time.
Hope that helps.
Got some distributors that have been here a long,long time. Can't get in touch with the owners for some reason... e-mail not working right or no answer at the phone number they listed and a couple have no return address, or phone number either. Am still trying to contact these individuals. Just to let everybody know.
See ya!
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