79 looses spark when warm

[manoa matt]

Helping a friend diagnose an electrical problem. Stock 1979 with electronic
ignition. Coil was changed to an IAP dry coil about 6 years ago. (IAP no
longer sells this coil) Intesteringly the secondary resistance is not even
close to the OEM mirelli coil. Secondary resistance is around 6K, OEM is
around 10K-11K Regardless when the car is running and driving there are no
issues with lack of spark.

When the car is cold it will start up fine. You can start and shut it off
repededly and it will start every time when its cold. However if you start it
and leave it running, it will die sometimes after its warmed up. Either before
or after the fan comes on. No definitive time, sometimes its getting warmed
up and dies, sometimes you can let it idle for 30 min and then it will die.
If it sits overnight it will start right back up.

Control module is new, pickup is new, cap is good, rotor probably should be
changed, the dry coil is about 6 years old with less than 2000 miles on it.
I've cleaned the ground/mounting points for the heat sink and added an extra
ground wire from the heat sink to the ground pod. Also added a ground strap
from the back of the cam tower to the body, and another heavy gauge ground
from engine block to chassis.

Is the coil bad? Would the low secondary resistance indicate a problem with
the coil causing it to overheat? Rotor looks ok and tests within
specifictions for the resistance.

Anything else I might have overlooked?

Matt

[narfire]

Matt, have you pulled a plug and tested for spark right after it has died? Lots of fuel,? wondering if you have spark and the thing runs, perhaps the mechanical? fuel pump might have a bit of a leak or diaphram is pooched?
I dunno, to me it sounds more like a lack of fuel? but I'm far from an expert on carb'd cars.
Good connections at the back of the ignition,notice any browning of the wire(s)
Chris

[manoa matt]

Its getting fuel. If the pump or diaphragm was bad it would show symptoms at high rpms. Or it would sputter and cough out. It just dies all of the sudden with no warning.

I put a timing light on it and watch it strobe/flash while its running. After it dies, I crank it while activating the timing light and it will not strobe/flash.

I'm going to try putting the OEM coil back in.

[seabeelt]

How about the magnetic pick up unit. Should be 700-800 ohms between the two wires connected at the coil. Test cold and warm( when it dies ) and see if the resistance value changes
R/

[manoa matt]

"Coils rarely fail" is what I've heard alot. This was a "dry" coil sold as a performance upgrade from one of the popular vendors, however they no longer sell that particular item, possibly for good reason. The coil primary and secondary resistances were not within specifications, however the car would start on the first crank and run beautifully.

I replaced the control module with a new one and the coil with and OEM Marelli one, problem solved.

[majicwrench]

Coils rarely fail. Can't recall ever seeing a coil fail when hot. Modules fail a LOT, often heat related. I would bet the replacement module did the trick. Obviously is nice to change one thing at a time so you know what fixed it.
Keith

[manoa matt]

Keith, I still have the dry coil I removed from the car. The specifications for the OEM coils are: Primary: 0.75-0.81 ohms, Secondary: 10,000-11,000 ohms The readings I got from the dry coil were: Primary: 1.2 ohms, Secondary 5,5000 ohms

I don't know what the specifications for that coil were when new. I'll try it on my car and see what happens.

I've had a coil fail when hot. When I did my first resto in 2006 on my 78 I kept the points but installed an Accel "racing coil". It would start and run fine for a while, but shut off just like this car did. The coil would be too hot to touch and after cooling down a few hours it would start up fine. I played around with multiple ballast resistors and coils until I was blue in the face. I ended up switching over to the stock electronic breakerless system and never looked back.

[bobplyler]

I had a coil fail when it got hot. But it wasn't a fault of the coil. One of the radiator hoses had a pinhole in it. When it got hot, it sprayed a tiny amount of water on the coil.

[AriK]

I had the same problem as you (with my 79) with the only difference being that the electric fan would make it stall when warm. I started with the purchase of a generic coil from a 70s GM and it made things worse. With the advice of others (who said coils coils either work or dont work)i put back the original coil and changed all other ignition parts as well as installing another ignition switch & wiring and heavily checking the fan and its circuitry. Finally i got my hands on a used Marelli coil from the vendor who said "coils rarely fail" and bingo.