Ignition Module
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Ignition Module
I have a 75 Stingray with the L82 and 4speed.
On a long drive the other day, car was running fine. Had some traffic, and cleared one long light, and hit another. I slowed to a stop and it died. Got it started again, but it didnt want to idle without gas. Cruise and acceleration was totally normal, but whenever i would come to a stop, the RPMs would degrade and it would die. If i continually blipped the throtte and kept the RPMs up, it would stay running. Carb has been rebuilt, and at first i thought fuel vapor lock, hot and in traffic, but i got to where i was going and stayed there for hours, and that night when i drove it, it did it all the way home; cruise and acceleration it ran really well, idle would die without gas. Drove it today and it was fine.
Sure felt like a fuel starvation problem or a vacuum leak. Didnt find any vac leaks, and seems weird that it would cruise and rev fine if there was a fuel problem.
Anyway, i was just curious about GM ignition modules. I know they are a weak point, but are they a either it works or it doesnt type thing? Do they ever degrade into working sometimes and not others, or do they just die altogether? If one was degraded, would keeping the RPMs up keep the car running, or would a bad module kill spark totally?
On a long drive the other day, car was running fine. Had some traffic, and cleared one long light, and hit another. I slowed to a stop and it died. Got it started again, but it didnt want to idle without gas. Cruise and acceleration was totally normal, but whenever i would come to a stop, the RPMs would degrade and it would die. If i continually blipped the throtte and kept the RPMs up, it would stay running. Carb has been rebuilt, and at first i thought fuel vapor lock, hot and in traffic, but i got to where i was going and stayed there for hours, and that night when i drove it, it did it all the way home; cruise and acceleration it ran really well, idle would die without gas. Drove it today and it was fine.
Sure felt like a fuel starvation problem or a vacuum leak. Didnt find any vac leaks, and seems weird that it would cruise and rev fine if there was a fuel problem.
Anyway, i was just curious about GM ignition modules. I know they are a weak point, but are they a either it works or it doesnt type thing? Do they ever degrade into working sometimes and not others, or do they just die altogether? If one was degraded, would keeping the RPMs up keep the car running, or would a bad module kill spark totally?
Team Owner
Joined: Jan 2007
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From: Southern Cal Ca
St. Jude Donor '22
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I found a module and a rotor in the storage bin in the back when i bought the car, so someone had some issues before, maybe. Kind of got me thinking about that module.
I will definitely check the advance while im in there. Didnt think of that, thanks for the tip.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
I would bet a chilidog, fries and a shake it's not the "modge". I have never heard of one degrading then coming back to life. Its either full on or full off.
I have had not only GMs fail but aftermarket too. In all cases they had the exact same scenario. Hot-parked for a several minutes then a "No Start" situation.
The heat builds up in the engine bay. That heat breaks circuits inside the module. No circuits = no spark, no idle, no nothing. Just like you turned off the key.
An old trick was to place a small bag of ice on the dizzy cap. Modge cools off, circuits complete again, starts right up. Until the next time its parked hot.
I have learned to keep a spare modge (pre-pasted*) and necessary tools to swap it behind the passenger seat.
*Heat Sink Compound.
The fact that you had to keep jazzing the throttle is proof you have a fuel problem, is hint number one. Hint number two is the carb has been rebuilt.
Leave some parts out or an adjustment is way off? First order of business would be to check the fuel filter. Lord knows what has been inside the gas tank. Then check float level.
I have had not only GMs fail but aftermarket too. In all cases they had the exact same scenario. Hot-parked for a several minutes then a "No Start" situation.
The heat builds up in the engine bay. That heat breaks circuits inside the module. No circuits = no spark, no idle, no nothing. Just like you turned off the key.
An old trick was to place a small bag of ice on the dizzy cap. Modge cools off, circuits complete again, starts right up. Until the next time its parked hot.
I have learned to keep a spare modge (pre-pasted*) and necessary tools to swap it behind the passenger seat.
*Heat Sink Compound.
The fact that you had to keep jazzing the throttle is proof you have a fuel problem, is hint number one. Hint number two is the carb has been rebuilt.
Leave some parts out or an adjustment is way off? First order of business would be to check the fuel filter. Lord knows what has been inside the gas tank. Then check float level.
Last edited by HeadsU.P.; Mar 27, 2023 at 06:35 PM.
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Maybe ill remove the mixture screws and do a quick and dirty blowout with the carb on in case some dirt got in there.
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Joined: Feb 2023
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I would bet a chilidog, fries and a shake it's not the "modge". I have never heard of one degrading then coming back to life. Its either full on or full off.
I have had not only GMs fail but aftermarket too. In all cases they had the exact same scenario. Hot-parked for a several minutes then a "No Start" situation.
The heat builds up in the engine bay. That heat breaks circuits inside the module. No circuits = no spark.
An old trick was to place a small bag of ice on the dizzy cap. Modge cools off, circuits complete again, starts right up. Until the next time its parked hot.
I have learned to keep a spare modge (pre-pasted*) and necessary tools to swap it behind the passenger seat.
*Heat Sink Compound.
The fact that you had to keep jazzing the throttle is proof you have a fuel problem, is hint number one. Hint number two is the carb has been rebuilt.
Leave some parts out or an adjustment is way off? First order of business would be to check the fuel filter. Lord knows what has been inside the gas tank. Then check float level.
I have had not only GMs fail but aftermarket too. In all cases they had the exact same scenario. Hot-parked for a several minutes then a "No Start" situation.
The heat builds up in the engine bay. That heat breaks circuits inside the module. No circuits = no spark.
An old trick was to place a small bag of ice on the dizzy cap. Modge cools off, circuits complete again, starts right up. Until the next time its parked hot.
I have learned to keep a spare modge (pre-pasted*) and necessary tools to swap it behind the passenger seat.
*Heat Sink Compound.
The fact that you had to keep jazzing the throttle is proof you have a fuel problem, is hint number one. Hint number two is the carb has been rebuilt.
Leave some parts out or an adjustment is way off? First order of business would be to check the fuel filter. Lord knows what has been inside the gas tank. Then check float level.
Like i said, Has been running fine for the last couple weeks since the rebuild. Last night was the longest and hottest drive so far, and it ran fine for the first 3/4 of the drive, then trouble. And runs fine today.
Really frustrating to figure out what changed.
Team Owner
Joined: Jan 2007
Posts: 53,648
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From: Southern Cal Ca
St. Jude Donor '22
for discloser
I had more than one module breakup under different conditions
one at 2100rpm broke up until car was warm.
had another break up 2500-3000k when hot.
in this case not a module.
I don't say things just to talk and I don't make absolute statements
I had more than one module breakup under different conditions
one at 2100rpm broke up until car was warm.
had another break up 2500-3000k when hot.
in this case not a module.
I don't say things just to talk and I don't make absolute statements
Le Mans Master
Joined: Aug 2017
Posts: 8,336
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From: Cool Northern Michigan
Another tip I forgot about. Find a Sparkplug Test Light. Either EBay or maybe Harbor Freight, about $5.
Next time your car quits, install the test light between the sparkplug and its cable. Crank and watch for spark.
Next time your car quits, install the test light between the sparkplug and its cable. Crank and watch for spark.
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I actually have one I can dig out and put in my car toolkit. Good idea, a handy thing to have available, thanks for the tip.
Dr. Detroit
Joined: Mar 2012
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From: New Braunfels Texas
I replaced and tested about 100 modules in the 90's.....and they most certainly can go bad at idle and have the rest of the range be ok.......this was a common and well known symptom of a failing module too.....I probably replaced an HEI module once a month for rough idle.
The tester for these modules back in the day was standard fair at most any Auto Parts outfit and some may still have them.....they test the dwell of the module in three different tests which simulated dwell at different RPM's because the dwell was variable.....if it is not dwelling well at low RPM....it will cause stalling and poor running.....usually black smoke coming from the exhaust as the dwell goes in and out and the ignition cuts in and out.
Furthermore.....it has been my experience on just about anything with a piston that coils are bad about 10% of the time.......a coil usually either works or it does not......90% of the time a bad coil is the leads going to it.
Replace the module.
Jebby
The tester for these modules back in the day was standard fair at most any Auto Parts outfit and some may still have them.....they test the dwell of the module in three different tests which simulated dwell at different RPM's because the dwell was variable.....if it is not dwelling well at low RPM....it will cause stalling and poor running.....usually black smoke coming from the exhaust as the dwell goes in and out and the ignition cuts in and out.
Furthermore.....it has been my experience on just about anything with a piston that coils are bad about 10% of the time.......a coil usually either works or it does not......90% of the time a bad coil is the leads going to it.
Replace the module.
Jebby
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Joined: Feb 2023
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I replaced and tested about 100 modules in the 90's.....and they most certainly can go bad at idle and have the rest of the range be ok.......this was a common and well known symptom of a failing module too.....I probably replaced an HEI module once a month for rough idle.
The tester for these modules back in the day was standard fair at most any Auto Parts outfit and some may still have them.....they test the dwell of the module in three different tests which simulated dwell at different RPM's because the dwell was variable.....if it is not dwelling well at low RPM....it will cause stalling and poor running.....usually black smoke coming from the exhaust as the dwell goes in and out and the ignition cuts in and out.
Furthermore.....it has been my experience on just about anything with a piston that coils are bad about 10% of the time.......a coil usually either works or it does not......90% of the time a bad coil is the leads going to it.
Replace the module.
Jebby
The tester for these modules back in the day was standard fair at most any Auto Parts outfit and some may still have them.....they test the dwell of the module in three different tests which simulated dwell at different RPM's because the dwell was variable.....if it is not dwelling well at low RPM....it will cause stalling and poor running.....usually black smoke coming from the exhaust as the dwell goes in and out and the ignition cuts in and out.
Furthermore.....it has been my experience on just about anything with a piston that coils are bad about 10% of the time.......a coil usually either works or it does not......90% of the time a bad coil is the leads going to it.
Replace the module.
Jebby
I ordered another for troubleshooting. I think I'll replace it before I touch anything else just to kind of rule it out. If it still acts up, at least I've got a spare for when it does go bad. Then I'm jumping straight to fuel. It is odd that keeping the RPMs up keeps it running if it's an ignition problem, but I guess a module putting out a weak spark might manifest at idle more than at cruise.
Melting Slicks
Joined: Sep 2004
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From: Northeast
My ignition module experience happened last year. Driving at 45mph and the engine stopped running, popped the clutch still nothing and then turn the key off and on and still not starting but it did crank. Carb was getting fuel. Needed to flat bed home.
After testing determine it must be module or distributor. I called to a couple of places and they did not test them, so ordered one from Rock Auto. Installed and still crank but no start and remember someone mentioning on here that he went through 2 before one worked. I ordered another brand , installed, and car started right up & running fine since last year. Rock Auto refunded me. Might not be your problem but was mine. Good luck & remember the heat sink paste.
After testing determine it must be module or distributor. I called to a couple of places and they did not test them, so ordered one from Rock Auto. Installed and still crank but no start and remember someone mentioning on here that he went through 2 before one worked. I ordered another brand , installed, and car started right up & running fine since last year. Rock Auto refunded me. Might not be your problem but was mine. Good luck & remember the heat sink paste.
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My ignition module experience happened last year. Driving at 45mph and the engine stopped running, popped the clutch still nothing and then turn the key off and on and still not starting but it did crank. Carb was getting fuel. Needed to flat bed home.
After testing determine it must be module or distributor. I called to a couple of places and they did not test them, so ordered one from Rock Auto. Installed and still crank but no start and remember someone mentioning on here that he went through 2 before one worked. I ordered another brand , installed, and car started right up & running fine since last year. Rock Auto refunded me. Might not be your problem but was mine. Good luck & remember the heat sink paste.
After testing determine it must be module or distributor. I called to a couple of places and they did not test them, so ordered one from Rock Auto. Installed and still crank but no start and remember someone mentioning on here that he went through 2 before one worked. I ordered another brand , installed, and car started right up & running fine since last year. Rock Auto refunded me. Might not be your problem but was mine. Good luck & remember the heat sink paste.
Le Mans Master
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From: Omaha NE
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My factory module lasted 43 years. When it died it died....but it gave me a warning.
The year before I noticed a miss at higher RPM's....like around 4k. That's been gone since the new one was installed.
I like NAPA's because it's American made & it comes with heat sink paste...
The year before I noticed a miss at higher RPM's....like around 4k. That's been gone since the new one was installed.
I like NAPA's because it's American made & it comes with heat sink paste...