Do you think you have enough of a heat sink on the module? It sounds like the old one could have been overheating. I do like the fact that the heat sink is above the HEI, though.

34 minutes ago, SteveJ said:

Do you think you have enough of a heat sink on the module? It sounds like the old one could have been overheating. I do like the fact that the heat sink is above the HEI, though.

Absolutely. The heat sink is a Bosch/Volvo unit, same mount footprint as the GM module. Heat Sink Compound was applied  

Edited November 3, 20231 yr by HusseinHolland

I'm not sure of the heat produced by the Volvo ignition vs the GM HEI. Thermal mass is what I'm concerned about. Is there enough mass to absorb the heat from the HEI and reject the heat into the environment. Most of the HEI heat sinks I have seen include cooling fins.

That makes me think that heat rejection might be an important consideration in design. If you have an IR thermometer, it might help you figure it out.

Thanks Steve.

Good point, however none of those sinks have even half the height of the Bosch heat sink though, so that mass would make a difference.

There is however no practical way to check the temp of the module while driving under load, which is when the condition exhibits itself.

If the symptom still exists with the GM module, I will get an OEM heat sink for it. My gut feeling is that is not the issue, especially comparing to the sink plate used by the OP, and then encased in the Datsun module casing.

I have seen posts cautioning against using generic / offbrand modules due to unreliability. I'm going to hope that's all it is. 

I'll report back.

What I'm thinking is that you carry an IR thermometer in the car and find a place to park safely when it acts up. If the problem is heat related, you should easily have a couple of minutes to measure the temp.

Also another trick is to keep a can of dusting air in the car. If you can hold it upside down in that tight space, spray the HEI with the dusting air. The contents should come out as a liquid when upside down and evaporate quickly when the liquid comes in contact with the HEI. That will cool the HEI rapidly. If full function returns, you most likely have confirmed an overheating problem.

7 hours ago, HusseinHolland said:

Thanks Steve.

Good point, however none of those sinks have even half the height of the Bosch heat sink though, so that mass would make a difference.

There is however no practical way to check the temp of the module while driving under load, which is when the condition exhibits itself.

If the symptom still exists with the GM module, I will get an OEM heat sink for it. My gut feeling is that is not the issue, especially comparing to the sink plate used by the OP, and then encased in the Datsun module casing.

I have seen posts cautioning against using generic / offbrand modules due to unreliability. I'm going to hope that's all it is. 

I'll report back.

It's not just about absorbing heat. You have to reject the heat, too. If the heat sink reaches equilibrium with the ignition module, it doesn't accept more heat. The cooling fins allow some air flow to release the heat. Mind you, the cooling fins should be mounted vertically to allow air to move past. If you mount the cooling fins horizontally like you. currently have, the heat cannot flow out as easily. On the other hand, you could mount the cooling fins pointing up, and that would allow heat flow, too.

Pin 1, injector control, is also on that blue wire.  I had found that with the stock ignition module my engine would not run when the tachometer was disconnected.  But, after the GM HEI modification it would run without the tach.  So, Pin 1 seems to be sensitive to whatever comes through that blue wire.  At least it was on my 76 and a 78 parts car that I had.  I had the 78 running well, and tried to start ito once with the tach out and it was dead.  And later, after messing with the resistor on the 76 it fell out after a drive and the engine wouldn't start.  Saw it laying on the passenger floor, put it back on, and back to normal.  No idea why it worked with the HEI module though.

You might try branching the circuit before the big resistor.  One to the tach, one to Pin 1.

Or, for an experiment, go back to the non-functional tach setup, low resistance, and see if it still breaks up.

 

Thank you @SteveJ @Zed Head 

With the heat sink, I found posts that said a fin had to be cut off the sink for the GM module to fit, and that the aluminum used was extremely soft. The other factor is that the Bosch one has a polished contact area, so no loss of surface area contact as with an unfinished surface. I'll consider replacing it if I seem to have a ongoing issue.

Today, there was nary a hiccup. However, traffic was heavier & I did not have an open road to push the car harder. I'll have to check it further. 

With my 75, the engine started with or without the resistor in line, with the stock ignition and with the HEI, so I can only assume the harness bridge is not after the resistor on mine.

 

On 11/2/2023 at 4:47 PM, HusseinHolland said:

breaking up   ...  kept clipping/dropping out

What was the tach needle doing?

4 hours ago, Zed Head said:

What was the tach needle doing?

I don't recall the tach dropping out, so it's not like the ignition system was altogether dead. I'm assuming it was the non-OEM component, it has not done it once since I swapped it out. I'll know for sure during the week, I'll be driving to work Mon-Wed. 

To conclude this for me at least, using the GM-branded module, the symptom has not returned. I've been driving and commuting in the car everyday (except a couple of rainy days) and have had no break up / cut out. Other issues, but those are fueling related 🤪

What brand was the bad one?