False Spark Knock?

[longbedder]

According to THIS link, you are correct (dyeager) that the split between pulse modulated operation and simple on/off operation was dictated by GVWR application (under 8500lbs uses on/off and a simple solenoid, 8500+lbs uses PWM and a EVRV).

So here's the catch: both applications (heavy and light) used a 1227747 ECM.

I am starting to think that the ECM in my Blazer may have been replaced somewhere along the way with a previous owner, and it may now have a "heavy" ECM...which has the exact same service number as the "light" ECM but a different EGR control method!

I do not know if the difference in EGR output is configured by the .bin file aboard the PROM and uses the same switching hardware aboard the ECM (don't see why it couldn't...). I suspect it is, and if so I could fix it by simply swapping the PROM out.

I don't own an oscilloscope, but I suspect Fordum may be correct. My LED rig burns dimly/flickers which is sorta consistent with a PWM signal.

 

[Fordum]

That would make more sense than a bad ECM. I make a living off failed electronic components, so I know more than most how often they fail, but the ECMs are incredibility rugged.
Unlike most situations, I never suspect the ECM until the very last thing. I have only ever seen one go bad.
That was on a friend's widow's Mazda, and it was a known defect. I was able to send it off and a company replaced some defective capacitors for a small amount of money, and gave a lifetime warranty.

But, I find it hard to believe that there is not a difference in a number somewhere. Unless there is a jumper or something in the harness that selects which system the module uses.
Nowadays, some modules have to be flashed at the dealer when they are replaced, but back then, if there was a difference in the programming, it came from the factory that way.
My '89 Ford had a pollution control number that you had to reference when replacing certain parts from the dealer so that they would match with the programming.

 

[dyeager535]

GM did it wiht the '7747, so the ECM must be capable of it, just a setting in the .bin (PROM) that determines which is used. Yet another reason there are a million PROM's from the factory.

The PROM itself will tell you what it's from, 3 or 4 digit letter code on it (like APYP), normally easy to find, and easy to google to find application. Of course, not all the codes have been referenced online, but many/most have.

 

[longbedder]

I popped the PROM cover on the ECM, and the broadcast code is ARJT. According to both gearhead-efi.com and an ebay vendor, that is the correct stock PROM chip for a '91 350 TBI with a 700R4 transmission...

 

[dyeager535]

Forgot to answer the schematic for you...no, GM didn't want their folks messing around with the ECM internals apparently.

Been here? http://www.diy-efi.org/gmecm/ Maybe they have schematics in there, not sure that I ever searched for that.

I see mention of a 1991 Sierra with ARJT, maybe GM used the same PROM in either trucks, but just based on how they did these things, it would seem somewhat unlikely. Unless I'm mistaken, a 1991 Sierra would be C/K, not R/V. All the TBI trucks that vintage seem to be pretty darn basic though, I can't imagine they would need a lot of functional changes between the two body lines.

 

[longbedder]

It was the ECM which caused the EGR not to open.

I found a 1227747 ECM (with an ANLX PROM) at a local junkyard on the way home, swapped it in and now I get a nice bright light when the EGR is supposed to be on. I then swapped my old ARJT PROM into the "new" ECM and life is still good. So the IC responsible for EGR ground switching had failed.

And for entertainment, here's a pic of my indicator light.

 

[longbedder]

Here's the culprit - in the second picture, the fourth pin up from the bottom is that which connects to the ground on the EGR solenoid. As you can see, it connects to the first leg of the Delco integrated circuit. The only thing immediately apparent is some minor heat-affected zones under the chip (some browned areas).

 

[dyeager535]

Interesting! Glad you got it though.

 

[longbedder]

Added and unexpected side benefit: my cruise control now works