[sphynx88]

1978 Forebird Trans Am, 400. All original distributor.

So far I've tried:
-new battery
-12v at cranking to distributor
-new module
-cap and rotor are less than 6 months old, no burn marks.
-tested ohms on both sides to the coil
-tested ohms of the pickup
-direct ground from distirbutor housing to battery
-jumped ground from coil to distributor harness
-disconnected tach wire
-verified rotor is indeed turning

I'm about to just buy a new distributor assembly...

Car ran reliably for last 18+ months, died in the middle of drive. Cranks strong just won't fire.

[STEELCITYFIREBIRD]

Time to do some dynamic testing. Power at coil, ground side of coil being switched by module, module being switched by pick up coil.
Guessing...module or pickup coil ... follow the power, bench test the coil.
battery voltage at dizzy in run position too?

[george kujanski]

" No spark to HEI" Just to be clear...do you mean FROM the HEI to the plugs? If so, since you seem to have checked most of the things, the pickup coil is then the most likely. How did you check the pickup? Check the pickup coil for resistance while moving the wires. Should be a steady reading but not infinite (meaning open). i don't remember the value, but should not be open.

george

[Formulas]

Good negative ground back to battery from engine?

Not saying this is your problem but I also run a ground wire from dist body to engine

[sphynx88]

Will try some more dynamic testing and post back.

No spark meaning at spark plugs themselves.

I did jump a ground to battery so I don't think thats the issue but will retry

[pont3]

Originally Posted by Formulas

Good negative ground back to battery from engine?

Not saying this is your problem but I also run a ground wire from dist body to engine

Just curious, wouldn't the hold down clamp and bolt provide sufficient grounding of the distributor to the engine?


And to the OP, I would check the carbon contact in the distributor cap, (the little black ball that contacts the rotor). Also, make sure that the ground in the cap itself is properly routed.

[jamaca85]

check your timing chain

[Tom Vaught]

I replaced 5 HEI Modules on MY 1878 Trans Am.
NEVER AGAIN. Might have used the Mechanical stuff to run a MSD System but no more GM HEI modules for me ever again. I followed all of the installation procedures too (Grease, etc).

Best ignition I ever had was a Potted MSD ignition module sold by Ford Racing many years ago for the NASCAR boys.

Tom V.

[Formulas]

why do I ground the HEI body ? i usually paint the body and clamp then there is a gasket under the dist flange sooo i provide a positive path instead of a incidental one.. One of the module hold down screws provides a connection to the dist body for ground power... very cheap and easy to ensure a path for current thats all

[Bill Hanlon]

I don't remember where I found these, but they have helped me troubleshoot HEIs more than once.

[george kujanski]

Bill's post triggered my memory. A bad capacitor (condenser) may also kill the HEI output. The diagram is not quite correct that the capacitor is used only for radio suppression.

George

[242177P]

Had one of the OE coil wires in the cap break once, right at the connector. They're not very flexible, I'm guessing vibration killed it over the decades? Pop the coil cover, and physically check if one is loose (it will pass a casual visual inspection)

[bluebandit]

Check the wires from the pickup coil. The can break internally from being rotated back and forth by the vacuum advance unit.

[wh33lman]

Recently I had a rotor that tested fine on the multimeter, but would not pass spark when installed. Stumped a couple of us for a day... until I changed rotor just for the heck of it.

[Sirrotica]

The early rotors on HEIs were prone to burn through due to the density of the plastic used. Later GM went to a denser plastic to better insulate the spark from the coil of going to ground through the distributor shaft.

You would see a rainbow discoloration on the rotor surface where the spark was passing through the rotor material. The early rotors were black in color and when they changed the spec the new ones were off white. Doubtful that there are any black ones still out there, but you never know................

The pole pieces are subject to the green/white wires breaking internally as has been said previously. Tugging on them will show up if they are sound.

I once had a 75 Bonneville customers car that was aggravating as hell to pinpoint why it was running poorly and stalling, wires in the pole piece were just about broken entirely making it stall when the vacuum advance moved.

[Schurkey]

https://www.chevelles.com/techref/ftecref5.html
^^^ Step-by-step diagnostic procedure in second-half of article.

Keep in mind that an ohmmeter can tell you if an ignition coil is DEFECTIVE. It CANNOT tell you that the ignition coil is GOOD. Failed, or partially-failed coils can test "good" but won't work properly in service.

[Schurkey]

Originally Posted by Sirrotica

The early rotors were black in color and when they changed the spec the new ones were off white. Doubtful that there are any black ones still out there, but you never know..

GM issued a bulletin about this, they claimed that the rotor "voltage punch-through" on the black rotors was a myth. Wasn't long before they came out with the new 'n' improved white rotors. I guess they changed their minds.

I kept a black rotor for posterity. The guy I got it from says the black ones were "better" than the white ones. He prefers them.

Yeah, that's probably true IF (big IF) you're comparing a Genuine GM black rotor to a piece of crap off-brand Chinese white rotor.

Black rotors are all but gone in actual service now. Still some--like mine--on display.