Ignition ECU operation, HEI

[Raphael]

I used a heatsink but it never gets really hot. I'm sure the module can be mounted directly on the stock ecu frame, this would offer enough surface to dissipate heat. after all, i had the same module inside a GM HEI distributor (on a 327 Chevy) and there is no heatsink, just the distributor itself..
I got rid of the ugly mounting plate too, with this setup the coil receives full voltage anytime so you don't need the ballast nor the hot start relay. it really cleans the firewall. engine compartment light got a manual switch.

[Bitsyncmaster]

Test drive felt good but when the cooling fans went on I think the engine had a bad skip. I put the blue GM unit on and it does not have that problem. I wired both the same.

I put the timing light on and I do see a big skip or crossfire. Now my test blue unit is not mounted on the metal plate and the "bad" Autozone unit is mounted.

Edit: Pulled the "bad" unit off the metal plate and it still has the problem.

Edit again:
Must be one unit has more sensitivity on the signal from the distributor coil.

[content22207_2]

That Duraspark ECU is looking better all the time, eh?

General Motors was all about pushing the performance envelope. Ford was all about simplicity and reliability.

(Can I hear an "Amen" from anybody who has struggled with a cantankerous Quadrajet carburetor...).

Bill Robertson
#5939

[Bitsyncmaster]

I put my oscilloscope on the 12 volt power going to the GM ignition ECU. As with all car circuits that 12 volt power is very noisy. I added a 1000 uf. 16 volt cap soldered onto the the ignition connector and it just about cleared all that noise.

Two screen saves from my oscilloscope showing the improvement. I did not get the problem with the "bad" ECU on my cold engine this morning. I will do another test drive later to see if that capacitor fixes the "crossfire" I had seen.

In any case I suggest everybody add a large capacitor onto the GM module. It may be some of the modules have a large cap inside them. I know the stock ECU has cap filters as probably the Ford ECU do.

[Bitsyncmaster]

It looks like that cap would fix the AutoZone GM module. But I tested standby current just powering the ECU with all other pins floating.

Stock ECU draws 400 ma.
AutoZone draws 140 ma.
The Blue GM HEI unit draws 40 ma.

I think I will buy another one of those blue modules (only $14 shipped) and will scrap the AutoZone module.

[content22207_2]

Isn't saving 100 milliamps a bit like eating a Whopper rather than a Big Mac because it only has 995 calories rather than a thousand?

Bill Robertson
#5939

[Bitsyncmaster]

Isn't saving 100 milliamps a bit like eating a Whopper rather than a Big Mac because it only has 995 calories rather than a thousand?

Bill Robertson
#5939

The blue one has already proved it worked better with that voltage noise. So it even produces less heat and less power.

[dustybarn]

INTERESTING. I may add a cap to mine, just for grins.

Hmmmm.... The "Lucas" ignition module in my XJS actually has a 4-pin GM module inside it..... along with a big 'ol capacitor.

[nkemp]

This might be worth reading: http://www.jalopyjournal.com/forum/t...estion.617596/

A couple key points:
- The coil must not exceed .5 Ohms. Check out the pictures a ways down as to what happens when higher resistance coils are used. The stock coil may be the best bet.
- Either here or elsewhere there is discussion about using heat sink compound. Some suggest using dielectric grease. Dielectric grease is not a good heat sink compound. Real heat sink compound is cheap and works well. Heat kills these modules.
- Another coil that we may want to consider is the S10 ignition coil. But it may not fit where the existing coil is located.

[Bitsyncmaster]

I'm now running my GM HEI ignition with the ACCEL 0.7 ohm coil and 0.040" plug gap. I have the 1000 uf cap added and have a spare ECU I carry just incase.

I did find the GM module does current limit at 6 amps so I would think the stock coil would work fine. But since one user said the ACCEL coil was better, I use it also. Maybe the added capacitor would have cured his problem with the stock coil.

The GM unit will produce less heat than the OEM ECU. I used thermal grease "heat sink compound" which you can buy online or at radio shack.

I also read the GM module makes a one ms. minimum off time just to make sure the spark time is long enough to finish.