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Discussion Starter #1
Help!! Were getting no spark at the plugs in our 1978 Corvette with HEI Ignition. We have changed the coil, had the module tested at Autozone (passed), tested the cap, rotor and pickup per manual; all with no luck.
It turns over strong, just NO spark. We did just change out the window switches (removed center console). The car did start a couple of times after this but now nothing. Please!! Any suggestions? The car is sitting in the middle of the drive and I need to get it going.

THANKS,
Brian and Richard
 

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Help!! Were getting no spark at the plugs in our 1978 Corvette with HEI Ignition. We have changed the coil, had the module tested at Autozone (passed), tested the cap, rotor and pickup per manual; all with no luck.
It turns over strong, just NO spark. We did just change out the window switches (removed center console). The car did start a couple of times after this but now nothing. Please!! Any suggestions? The car is sitting in the middle of the drive and I need to get it going.

THANKS,
Brian and Richard
Pull the large red wire form the dizzy marked BATT, and check for 12 volts with the key on.
 

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Pull the large red wire form the dizzy marked BATT, and check for 12 volts with the key on.

Very true, but better off to check for 12 at the tach terminal.....and for hell of it pull the wire off that terminal eliminating the tach and tach filter from the circuit...just in case...

you need find 12 at coil output with ignition in run position...make sure it's not disappearing when cranking....ignition switch failure....

IF the above is ok, you may well have a bad sending coil....it's the one with the two wire plug on it feeding the module input from the reluctor....check for coil continuity under there, I forget the resistance value, but it's a coil so it's not that high...maybe 200 0hms at most....

IF that coil is open the plastic sending module feeding to the spark coil up top will never get a signal to do so.....

that small coil down bottom is a bear to get to...maybe easier to just exchange the dizzy at a parts shop.....gotta pull the thing anyway....

I find it easy to use two clip leads across a battery to check HEI dizzy when outta the car....careful of those sparks, hook it up negative to case, +12 to battery post, and flip the dizzy over on the gear end....and you should hear the HV coil up top go clunk clunk for every spark made....
 

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Discussion Starter #4
check for 12 at the tach terminal.....and pull the wire off that terminal eliminating the tach and tach filter from the circuit.
you need find 12 at coil output with ignition in run position...
IF the above is ok, you may well have a bad sending coil....
...
THANKS,
-I do have 12V at the HEI (BAT Terminal), but I have not checked it while cracking the car. Will do it this afternoon when my son can help me.
-Sending Coil tested OK per book:
"each terminal to ground ohm test should indicate "Infinite" Resistance (Open)"
"Ohms between both terminals should be 500-1500 ohms"
-Tach Terminal, Tack Filter? what, were, huh?

Also, There is a large Resistor? type thing outside the dizzy, down by the base. It has a large white wire coming out of each end. Mine is not hooked up and I cannot locate any matching plugs that it may have hooked to. What is this? Do I need it hooked up?

ps This is our first vette, we have had it for only about 6 months now. It has been running great (but hard to start, with a lot of pumping and cranking needed). I know a good bit about cars but never had an HEI system.
I do have the Haynes manual and the GM manual. and smart enough to join this forum :thumbsup:
 

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Also, There is a large Resistor? type thing outside the dizzy, down by the base. It has a large white wire coming out of each end. Mine is not hooked up and I cannot locate any matching plugs that it may have hooked to. What is this? Do I need it hooked up?
That's the tach filter. It goes from the "Tach" terminal on the dizzy to a plug on the firewall. It filters the pulse signal to your electronic tach. If the dizzy has a wire plugged into the "tach" terminal, and your tach works, someone has bypassed the filter. (Prob bad)
 

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Discussion Starter #6
That's the tach filter. It goes from the "Tach" terminal on the dizzy to a plug on the firewall. It filters the pulse signal to your electronic tach. If the dizzy has a wire plugged into the "tach" terminal, and your tach works, someone has bypassed the filter. (Prob bad)
"dizzy" interesting term....
yes my tach had been working, but not now, now that my car won't start.
I just relooked at the connections for the Tach, saw how the filter had been bypassed. This makes me ask:
-How important is it to have the filter? ie should I spend the $ and replace it if bad? (once I get the car running I will try it with and without the filter)
-Can the car start/run with the Tach terminal unplugged?

THANKS and
ps Good Morning
 

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Discussion Starter #7
An Update
-Checked 12V at Bat connection at coil while cranking, tests good
-Checked for spark with and without Tach and filter, no spark

Are these the only components, what to do next?
 

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"dizzy" interesting term....
yes my tach had been working, but not now, now that my car won't start.
I just relooked at the connections for the Tach, saw how the filter had been bypassed. This makes me ask:
-How important is it to have the filter? ie should I spend the $ and replace it if bad? (once I get the car running I will try it with and without the filter)
-Can the car start/run with the Tach terminal unplugged?

THANKS and
ps Good Morning
The car will run fine without the tach plugged in.
The tach will work with the filter bypassed.
The tach filter is for radio noise suppresion, and protects the tach circuit board from spikes.
I would test it/replace it.
 

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An Update
-Checked 12V at Bat connection at coil while cranking, tests good
-Checked for spark with and without Tach and filter, no spark

Are these the only components, what to do next?
I have not read other thread there, if you have 12 at tach term with NO wire connected and ign ON, engine not running.....your spark coil primary is probably good....and so is the 12 feed off any other circuit....

beeleeme when I say I have caught HEI top coils being open circuit in the primary winding....twice....and I am convinced its not all THAT rare....I"m not a pro mechanic...so what are the odds...??...

then there is the sending coil....white/yel/ and green wires off it ....


but engine will run fine with/without anthig connected to the tach output terminal....my volt meter tests tell all....no trigger, and everything tests 12 or crank voltage all the time...period...

hense back to the trigger burried in the base of the HEI unit...under the cent/vac advance and shaft/plates/springs/etc....
 

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This may be old news, but the HEI was big time bad on rotors, I do not know how you checked the rotor, but the HEI will burn a whole thru the center of the rotor and you will never have any spark. If you have not replaced the rotor lately, get a new one, it may be that simple.

The other problem the HEI can have is there a metal ground strap that is attached to the coil, if strap is broken or missing, of if the wire in the wiring harness has failed going to the ground strap, no spark. I would try a rotor first, long time ago, I replace everything in a HEI, and the problem was a hole in the rotor you can not see.
 

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This may be old news, but the HEI was big time bad on rotors, I do not know how you checked the rotor, but the HEI will burn a whole thru the center of the rotor and you will never have any spark. If you have not replaced the rotor lately, get a new one, it may be that simple.

The other problem the HEI can have is there a metal ground strap that is attached to the coil, if strap is broken or missing, of if the wire in the wiring harness has failed going to the ground strap, no spark. I would try a rotor first, long time ago, I replace everything in a HEI, and the problem was a hole in the rotor you can not see.
Good point.:thumbsup:
 

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The rotor IS/WAS a really weak spot in the HEI's.

Another "quicky' test is to hook your 12v test light from the "TACH" terminal to ground. Crank, and if the test light flashes, the pickup coil/module is OK..
Of course the Tcah needs to be disconnected..


:cheers:
 

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Discussion Starter #14
This may be old news, but the HEI was big time bad on rotors, I do not know how you checked the rotor, but the HEI will burn a whole thru the center of the rotor and you will never have any spark. If you have not replaced the rotor lately, get a new one, it may be that simple.

The other problem the HEI can have is there a metal ground strap that is attached to the coil, if strap is broken or missing, of if the wire in the wiring harness has failed going to the ground strap, no spark. I would try a rotor first, long time ago, I replace everything in a HEI, and the problem was a hole in the rotor you can not see.
Thanks Belvin20, I will check the rotor again and consider swapping it with a new one. That is a cheap and easy check.
I checked the current one with an ohm meter; at center and end. It should 0 resistance. I will retest it with this new info and may go ahead and change it also. I probably out to go ahead and the the cap also. My auto parts store is pretty good about taking things back, if they do not help.
 

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Don't look for a large hole. It will be a tiny brown/black spot just off-center on the bottom of the rotor. It's also very common for the white wire on the magnetic pick-up to break inside yet still be attached.
Mike
 

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Undo the tack filter from the distributor. The tach filter will not let the car go through the circuit to the ignition module. I had the same problem. I undid the tach filter connector to the distributor and it started immediately. Think it is to the right facing the left bank. It is a radio filter and that is all it really is. I bought all kinda stuff for the distributor and even thought of buying a new distributor. It was that damned TACH FILTER ALL THE TIME. :thumbsup:
 

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Discussion Starter #17
This may be old news,... HEI will burn a whole thru the center of the rotor and you will never have any spark.

The other problem the HEI can have is there a metal ground strap that is attached to the coil, if strap is broken or missing,...I replace everything in a HEI, and the problem was a hole in the rotor you can not see.
There is a large silicon rubber disc between the coil and the cap, I assume that is there as an electrical insulator.
 

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Discussion Starter #18
FIXED!!! It was the module

I had someone suggest that the module may actually be bad, even if it passed the "bench test". I checked the price and at $15, what the heck, I bought one. 20 minutes latter we were back on the road.

THANKS for ALL the responses and suggestions...
:thumbsup: :thumbsup: :thumbsup: :thumbsup: :buhbye:
 

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Discussion Starter #19
OK, I thought I would take advantage of people reading this and ask another question. While under the hood I found a switch that is apparently activated when the throttle returns to idle....it is at the end of the linkage, kinda big solenoid looking thing....it is unplugged and I can't even locate the wire to plug it back up....what is it, what does it do and do I need it?
 

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Glad you found the problem. I hope you put the salve under the module or it will ground out to the engine block like the other one did. I took the module off the 75 to find my problem. I bought pickup coil (gotta pull the distributor out for that one) and I thought mabey it was the fusible link to the starter. Not. Just the tach filter. Glad you found the module bad. On the bench will not exactly tell you the problem. I brought mine to a parts house and they told me the truth. Mine was good. Like I said, no salve makes the damned thing ground out on the engine block and that puppy will sit there with its head down till you find him and bring him back to life. Good job! We got another C3 back on the road and ready. :thumbsup: :thumbsup:
 
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