Ignition ECU operation, HEI

[nkemp]

Here is the schematic as modified for the current design.
GM HEI DeLorean mod.jpg

[dustybarn]

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.

Yeah, I can't say for sure that my problem was caused by the coil primary impedance, only that switching to a lower one fixed (or masked?) the problem. Adding a cap to clean up the module power supply honestly didn't occur to me.

[nkemp]

GM included a capacitor in their HEI distributors. I was trying to find the value but failed. Does anyone know or can measure the GM capacitor? From what I read, GM used it for radio noise suppression but maybe it provided other benefits.

As I was Googling to find the value when I found this "The two capacitors are optional but recommended. The 1.0 µF capacitor on the +12V (to ground) is helpful, it is similar to the one used on EDIS. What it does is provide brief energy storage for the discharge. The other capacitor will help eliminate back-fed noise to your MicroSquirt® controller. Use a 100 pF to 0.001 µF cap on the TTL trigger input wire to ground. What this does is shunt extremely fast noise spikes to ground and not let them feed back to the MicroSquirt® processor. The added capacitance is minimal - with the series resistance of 1,000 ohms (in the controller) and a 100 pF cap the RC 3dB time constant is 2 * π * R * C = 0.6 microseconds."

At MegaSquirt page.: http://www.useasydocs.com/details/coils.htm

[Bitsyncmaster]

I would guess my 1000 uf. cap is more than you really need. Maybe 100 uf or even 10 uf would have worked. The value depends on the wire resistance and inductance in the cars harness.

Normally cap filters use one large cap which is electrolytic and a smaller ceramic cap for higher frequency noise filtering. I used a 1000 uf. because I had one on hand. After I installed it my oscilloscope showed met that fixed the large voltage drops.

[Bitsyncmaster]

My car has been running fine but after doing a new engine harness, I've decided to try a higher resistance ignition coil just to reduce some heating of the ignition module. The module has current limiting of 6 to 7 amps. so when it limits that extra power is dissipated in the ignition module. I'm currently (no pun intended) running a 0.7 ohm coil so it will start current limiting above 5 volts on the coil. I have ordered a 1.5 ohm coil which will start current limiting above 10 volts on the coil.

So cranking a cold engine if the battery voltage drops to 10 volts, I still get full spark power. But when the engine is running I get 14.5 volts so it should keep the ignition module a lot cooler.

I have not done any testing of the heat of the ignition module, this is just my theory.

My new engine harness really shorted up the wiring to the ignition coil so the voltage is higher.

[Bitsyncmaster]

Got my new FLAME THROWER 1.5 ohm coil. I measured the primary and it is 1.23 ohms. Will install it tomorrow since it should be 65 deg. outside.

[nkemp]

Any recent update on this?

My car failed to start today and the ICU is in the suspect lineup. I smell gas and everything seems operational other than no ignition. Not sure how to test it other than to swap an ICU. Wiring up an HEI is relatively straight forward other than needing the connector. from the original in order to keep it PnP.

Anyway, I thought I'd reignite this thread (pun intended) to see what the latest info is.

[Bitsyncmaster]

I've not done any long distance driving but just the past few days have almost burnt a half tank of gas idling in my garage testing the temp gauge sensor. No problem with the ignition.

The GM module really runs a short duty cycle at low RPMs but it increases at higher RPM so I think it's best with the coil resistance of 1.5 ohms if your not using any resistors. Even though the GM module current limits (6 or 7 amps) I think the module would produce more heat when current limiting using a lower resistance coil.

[nkemp]

I believe that I have an original Bosch coil. Do you recall the resistance?

[nkemp]

another question ....

Does it make much difference which HEI module you get? It seems they should all be about the same.