Hey all, hoping someone can answer a question I have about the ignition module in a stock 1988 GTA 5.7L. We're still trying to solve a constant stumble that occurs while the car is in steady throttle, generally at higher speeds. It feels like it's not getting spark advance. We've unplugged the ECM advance, and weirdly enough there's no change in the idle note. There's no codes being thrown for this issue. I was wondering if the ignition module in the stock HEI setup sends/receives a signal to the ECM for spark advance? Just trying to rule out any issues and work down the list to solve the stumble. The car idles fine (that was another issue in another thread that's been solved), and starts up just fine on cold mornings. It doesn't die either while driving or while idling, but the stumble is persistent. And a pic for attention, because who doesn't like a black GTA by a glacier Thanks!

 

Datalog during the stumbling would be the best way to figure out what's happening.

 

Agreed, but that may not be possible at the moment unfortunately. But we figured the ignition module question would be worth clarifying, and ruling out if so.

 

Not exactly... the ICM sends a low-level signal to the ECM, which applies "advance" in the form of waiting somewhat less time than the next pulse is expected thereby triggering the next cyl to fire earlier than the "reference" output from the pickup coil amplifier as looked up in the "tuning" table, and that then sends a signal back to the high-power part of the ICM, which fires the coil. Unplugging the connector makes the module bypass the whole ECM trip, routing the "reference" straight to the output switch.

The timing should change SIGNIFICANTLY when put into "normal" mode compared to bypass, like, should advance by 25° or so. Does it? "Idle quality" isn't exactly definitive.

There used to be a separate ESC module that incorporated the input from the knock sensor, which makes the ECM DRASTICALLY delay the spark from where the table would ordinarily put it. Not sure if 88 TPI works that way or if it had been changed by then.

That looks like Alaska 1 near Anchorage?

 

Thanks for the explanation. I'll disconnect the ECM from the module, and check if there's any change. I'll update this thread tomorrow to report whether there was any difference in the timing in normal vs bypass mode. Yes to Alaska, but Juneau.

 

 

Well the stumbling is all solved. I feel like a complete retard, but oh well, at least it's fixed. One of the plug wires was on loose, but I hadn't noticed.. Yesterday I was poking around and I saw the boot was completely off the plug. Runs great now!

 

alot of the time it's the simple things and things like injectors and computers are running thru your mind