75-80(ish) HEI ignition modules

[TJ Hopland]

Several recent threads have come up regarding these distributors and modules.  At least one person has reported having intermittent issues that were apparently solved by replacing the module.  I have had many modules fail over the years and non of them have given warning or been intermittent.   I have had the Ford and Chrysler electronic ignitions from that era sometimes give you some hints before they failed but never had that happen with the GM style.  I was curious to hear if anyone else has had intermittent failures with these modules?   And if you did change a module and it cured your problem was that the only part you changed?   Another person mentioned replacing several parts at the same time so the module may not have been the issue. 

Since the module is electronic and electronics tend to be sensitive to heat its certainly possible to have the characteristics change based on temp from either operating or ambient conditions, I just never happened to experience that myself with these.

I have not bought any new modules recently for cars, I have bought cheap ones for retrofits on small engines.  Those applications are not critical if they quit so I just bought cheap ones, I think $14.   When I was buying them for cars back in the 90's it seemed like most of them were crap and didn't last.  The only ones that seemed to work were the AC delco ones.   Seems like those got into the $40-50 range last time I bought one from a dealer.   I started keeping an eye out for original GM ones in the U pull junkyards and didn't seem to have issues with those.   I don't know if I have any of those left, may have used them up.  I think I only have 1 car that uses em.

The 75-80(ish) 4 pin modules don't control the timing, they just convert the low voltage magnetic signal to a level that can run the coil.  They actually have a really basic Motorola designed microprocessor in em that controls the dwell time based on rpm.   78 Seville had a electronic timing control option, I don't remember the details on how those worked, I don't know how common of an option it was.   Some non EFI 80's had a 5 pin modules that I think were connected to a temp sensor that electronically retarded the timing based on temps.  Anything with computer controls which was most cars stating in 81 did go to full electronic timing.  These didn't have the vacuum (or mechanical) advance, there was a 4 wire harness out of the module that went to the computer.   I think the trucks did lag a bit behind as far as computers.       

When you replace them be sure to clean the mounting surfaces and then smear them up with the special white grease that should come with the new module.  Its heat transfer compound to help the module stay at a stable temp.   If it does not come with the new one you can still buy it in a little tube at a place like Radio Shack.   

[R Schroeder]

TJ. I remember posting about this problem a while back.
I remember you telling me about the wires that could be broken too.
I did check them out that day when I replaced the module. Could not find any breaks in them . So, I replaced the module.
I found 3 of them on Ebay, NOS, ones. I put one in and 10,000 miles or so later it is still running perfect.
Don't know if it was a fluke or it was the module. I sure would have put my money on the wires being broken.
I did buy the heat sink grease from Radio Shack though.
Maybe the other one was over heating. There wasn't much heat sink grease on it. Someone might have changed it in the past and forgot it.
I wouldn't buy any after market ones.
I replaced one in my sons car years ago, and it wouldn't hardly run. There wasn't any problem with the way the car was running in the first place. Kid just wanted a new one in there.
Had the same problem as the other poster had. Give it gas and it would quit or cut out.
I put the old one back in and the car ran good.
After market = junk.
That's about all I can say about my car.
Roy

Very good question though.

[bcroe]

TJ. I remember posting about this problem a while back.
I remember you telling me about the wires that could be broken too.  I did check them out that day
when I replaced the module. Could not find any breaks in them . So, I replaced the module. I found
3 of them on Ebay, NOS, ones. I put one in and 10,000 miles or so later it is still running perfect. 

Don't know if it was a fluke or it was the module. I sure would have put my money on the wires being broken.
I did buy the heat sink grease from Radio Shack though.
Maybe the other one was over heating. There wasn't much heat sink grease on it. Someone might have changed it in the past and forgot it. I wouldn't buy any after market ones.
I replaced one in my sons car years ago, and it wouldn't hardly run. There wasn't any problem with the way the car was running in the first place. Kid just wanted a new one in there.
Had the same problem as the other poster had. Give it gas and it would quit or cut out.
I put the old one back in and the car ran good.
After market = junk. That's about all I can say about my car. Roy  Very good question though. 

In a million miles, I have had some HEI module failures.  One just quit soon after I got the car; forgot
to note if it was aftermarket.  Another would cut out, but come back on.  I also found that the
pickup coil wires will wear out & break in 180,000 miles due to motion of the vacuum advance. 
Might have had another failure that was swapped but not diagnosed.  I don't change a lot of
parts, when only one is the issue.  I try to use 'yard' modules I have found; don't trust imports. 

Those modules must work very hard, handling a lot more power than points could.  Transistors do
fail under such conditions.  Microprocessors weren't available when the HEI came out, but the
module was smart enough to figure the required dwell (from the previous ign cycle).  The schematic
and chips doing that function are available and in my files.  Mapped advance curves are the job
of Engine Control Units staring in the 80s.  The 78 EFI Seville had a module that could be advanced
a discrete amount for cruising or retarded for starting; controlled by an ESS module.  But it still
used mechanical & vacuum advance.  Th3 77 Toro (403) is the first engine I noted using a completely
electronically mapped spark advance, also using a temp input.  80s CCC engines used a modulated
carb and an electronically mapped advance driving an HEI module, with a "limp home (dead ECU)"
mode.    Bruce Roe

[Walter Youshock]

I had a '77 sdv that I almost got killed in.  I was merging onto route 380 in PA, floored the car, it stumbled and a truck almost hit me.  After that, it was ok for awhile.

My dad took the car for a week and said it was fine.  One day, he was leaving work and the car wouldn't start.   The module was burned out and the pickup coil was actually broken.  The entire distributor needed a rebuild.

Needless to say, it was pretty scary.  Had it completely failed on the highway,  it could have been catastrophic.