Distributor Module failure

I have a 1950 GMC truck that I restored with a 350 small block v8 with a stock set-up. I now have 30K on the motor and it runs great except for one problem: The module in the HEI distributor keeps failing. The mileage between failures is random: 3k, 5K, 8K. I have now put in 8! I pull the module out and test it: Yep, cooked. I have replaced the cap, coil, and alternator. I think the distributor is not grounded properly. There is no tach attached to it. What is the source of problem? Should I just replace the whole distributor and hope for the best? Your input would be greatly appreciated! -vJh

You are probably supposed to have a ballast resistor in there somewhere.

I’m very familiar with this problem. Are you using the silicone grease heatsink capsule that comes with the ignition control module? If so…throw it away next time and use a premium brand of silicon like Dow Corning or GE. If not…there’s your problem. It has to have that heatsink or it will fry over time.

@keith, GM HEI changed the game with old school ignition, and requires a full 12v source, no ballast. I think @missileman is dead on.

Full 12v source is correct. I have been using the heatsink grease that comes with the replacement module. One other thought I have had is: a possible broken ground line from the coil. This wire or cut metal lead located under the coil and is grounded on the plate. I will pull the coil and take a look. Of course if the ground was truly broken, the module would fry fast. Hm. I like @missileman’s advice. A quick and inexpensive solution. Definitely worth a try. -vjh

Take a close look at the under side of the rotor for tracks indicating spark passing through.

What year was the 350? What size was the original engine? It appears the voltage for the dist taps directly off a single port on the voltage regulator along with the gas guage, maybe a new voltage regulator?

I would guess that the current engine wiring has little in common with the original wiring, @Barkeydog.

@Rod_Knox My thought is if the voltage regulator was bad, it could be the cause of the problem.

The OE voltage regulator was for 6 volts. That HEI ignition won’t operate on 6 volts.

And take a look at the starter wiring @Barkeydog. Does there seem to be anything missing?

Make sure the coil itself is grounded by the “L” shaped bracket that one of the 4 coil screws goes through. And be sure there is a full 12+ volts going in to the distributor.

@Rod_knox I guess you have to tell me what is missing,

The starter has no solenoid, @Barkeydog. The starter was positioned adjacent to the floor near the accelerator. A plunger on the starter extended through the floor and pushing it moved the Bendix into engagement and closed the contacts between the field coils and the battery. Without a key, with no clutch or neutral safety switch, pushing that plunger would operate the starter and if in gear with the clutch released the truck would chug along.

@rod_knox you missed my point entirely, the +12 volt to the coil is regulated by a different supply than for the rest of the car, If that was faulting it could cause the problems as described.

With 8 failures already I’d be inclined to just replace the distributor with a new one. They aren’t that expensive are they? There are an assortment of aftermarket distributors for widely-sold trucks made in the 1950’s I think. I’d get one where you install a magnetic pick-up on the rotor shaft and there’s a electronic detector instead of points. I’d skip the capacitive discharge systems as that probably isn’t needed. Just a known good coil and condenser, with the electronic replacement for the points, you should be in fat city. If you aren’t sure about the coil, maybe replace that with a new one. It may be shorting out internally, creating too low of coil resistance, and damaging the ignition modules.

@GeorgeSanJose, the frame may be '50s, but the engine is a SBC 350 with HEI. Obviously not a '50s era engine. The truck has been upgraded to 12v, and uses a more modern starter with a built-in Bendix. Being a retrofit, I wonder if the engine and HEI are properly and sufficiently grounded.

Running extra grounds between the engine, chassis, and HEI may not be a bad idea. I prefer to run a heavy-duty ground wire from the battery to the engine block first, then to chassis ground. And run all ignition grounds to the engine block.

The 350 was a new crate motor that was set up with a Rochester 4 bbl and a stock HEI distributor. All the old wiring was stripped out and a new 12 volt harness installed. I have since changed out the alternator with a new one. The distributor cap and coil have been replaced. I put in new plug wires and checked connections.
The defined problem is: Perfect starts and perfect running, then total failure of the module after the engine is shut off. Replace the module, and she starts right up.
I am with @asemaster in his suggestion to check the grounding bracket. (see above) A prime suspect. I will take @Rod Knox advice and check the rotor for tracks and will check the voltage input. A Saturday at the Oak Tree Garage! I will report back. Thanks. -vjh

It sort of sounds to me like a thermal problem. With the engine running the fan is going, the coolant is flowing, and the block & head are kept relatively cool. When the engine is turned off the coolant flow stops, the fan stops, and the heat from the hotter parts internal to the engine and head migrate to the outer surface of the block, increasing the temperature there. At least it seems like that would happen. OP could do an experiment, put a thermometer or thermistor on the module – or this could be an excuse to buy one of those cool IR/Laser temperature pointers – then run the engine to normal temperature, turn it off, and see if the module gets hotter after the engine is turned off. If so, I’d suspect that as the most likely cause of the module failure.

I still think voltage regulator, I mean there are only two functions the post provides, gas gauge and distributor.