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Discussion Starter · #1 ·
My 92 is showing me diagnostic codes #27 and #36.

#27 is a quad-driver module #2 circuit - but I can't find any explanation as to what circuits/actuators/etc it controls.
#36: Opti-Spark Ignition Timing System. (High Resolution Pulse.)

My ECM was rebuilt about 5 years ago and recently my 92 began stalling out after running for about 20 minutes or so. It would start back up and stall again. After a few minutes off, I turned on car again with AC off and was able to keep it running long enough for a 6 mile drive home. I don't recall if the symptoms were the same as 5 years ago because I can't remember all the details - but in general the stalling out at intersections was makes me think it is the ECM again

I am going to clear the codes out and run/drive the car until it stalls again so I can get fresh codes. But if both return then I am at square one again.

I also had to replace my AC compressor recently and when then after the AC blower is stuck on full blast (10) even after the digital display shows it at the lowest blower setting - its still full blast. I read #28 quad-driver module #3 controls AC blower and temps on an LT1LT5 forum but have no idea if that applies to my 1992 C4.

Which of the above codes is more likely to be the cause my 92 stalling out after running for a good 20+ minutes? If it happens to be that I need a new/rebuilt ECM again, then who/where can that be taken care of? My local mechanic used a guy in Texas who died last year and now they too do not have a new reliable source to send them to.

Please be as descriptive as possible, I am not a mechanic so the more thorough the explanation is appreciated.
 

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My 92 is showing me diagnostic codes #27 and #36.

#27 is a quad-driver module #2 circuit - but I can't find any explanation as to what circuits/actuators/etc it controls.
#36: Opti-Spark Ignition Timing System. (High Resolution Pulse.)

My ECM was rebuilt about 5 years ago and recently my 92 began stalling out after running for about 20 minutes or so. It would start back up and stall again. After a few minutes off, I turned on car again with AC off and was able to keep it running long enough for a 6 mile drive home. I don't recall if the symptoms were the same as 5 years ago because I can't remember all the details - but in general the stalling out at intersections was makes me think it is the ECM again

I am going to clear the codes out and run/drive the car until it stalls again so I can get fresh codes. But if both return then I am at square one again.

I also had to replace my AC compressor recently and when then after the AC blower is stuck on full blast (10) even after the digital display shows it at the lowest blower setting - its still full blast. I read #28 quad-driver module #3 controls AC blower and temps on an LT1LT5 forum but have no idea if that applies to my 1992 C4.

Which of the above codes is more likely to be the cause my 92 stalling out after running for a good 20+ minutes? If it happens to be that I need a new/rebuilt ECM again, then who/where can that be taken care of? My local mechanic used a guy in Texas who died last year and now they too do not have a new reliable source to send them to.

Please be as descriptive as possible, I am not a mechanic so the more thorough the explanation is appreciated.
I also have a 92, and have similar problems. My dilemma at the moment is whether to replace the optispark, or get the ECM rebuilt again. Have you ever replaced the opti? Also, what was the car doing when you had your ECM rebuilt?
 

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Discussion Starter · #4 ·
I also have a 92, and have similar problems. My dilemma at the moment is whether to replace the optispark, or get the ECM rebuilt again. Have you ever replaced the opti? Also, what was the car doing when you had your ECM rebuilt?
@moodman - my issue ended up being the optispark. I had the ECM rebuilt like 5 years ago and from I remember the ECM symptoms were very similar to the optispark issues. I was afraid of the ECM going out again - both an expensive and not-so-timely repair. And worse, finding resources to rebuild the ECM is getting harder and harder these days. They replaced the optispark (as well as the water pump) and all was fine again.

My optispark was the original too, so it was 30 years old with 110,000 miles on it. But when it was failing it, at stop signs/lights, the car was idling very low and felt like it was going to stall out at any moment. Then when I pressed the gas to move through the interection the car did stall out. It symptoms were like a combination of the ECM failing and running out of gas at the same time. My 92 gas gauge has never been accurate, 1/10th of a tank of gas and the gauge is reading half a tank full. So I always set my tripometer to zero every time I fill up, then when it gets to 300 miles I fill up - regardless of what the gauge shows. I say this because I have run out of gas MANY times over the years so I know that chugging, stalling feeling very well.

So, it makes sense the sensation of the stall was familiar, the optispark wasn't getting gas to the engine properly - aka: running out of gas.

Clear the codes...wait for it stall again and then recheck the codes, hopefully you will only have the one code pointing to what the real problem is. Most likely, with an already rebuilt ECM, its your optispark.

Hope this helps.
 

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@moodman - my issue ended up being the optispark. I had the ECM rebuilt like 5 years ago and from I remember the ECM symptoms were very similar to the optispark issues. I was afraid of the ECM going out again - both an expensive and not-so-timely repair. And worse, finding resources to rebuild the ECM is getting harder and harder these days. They replaced the optispark (as well as the water pump) and all was fine again.

My optispark was the original too, so it was 30 years old with 110,000 miles on it. But when it was failing it, at stop signs/lights, the car was idling very low and felt like it was going to stall out at any moment. Then when I pressed the gas to move through the interection the car did stall out. It symptoms were like a combination of the ECM failing and running out of gas at the same time. My 92 gas gauge has never been accurate, 1/10th of a tank of gas and the gauge is reading half a tank full. So I always set my tripometer to zero every time I fill up, then when it gets to 300 miles I fill up - regardless of what the gauge shows. I say this because I have run out of gas MANY times over the years so I know that chugging, stalling feeling very well.

So, it makes sense the sensation of the stall was familiar, the optispark wasn't getting gas to the engine properly - aka: running out of gas.

Clear the codes...wait for it stall again and then recheck the codes, hopefully you will only have the one code pointing to what the real problem is. Most likely, with an already rebuilt ECM, its your optispark.

Hope this helps.
Yes, that does help. But I don't understand why the opti can throw so many electrical codes.? If I keep this car, doing the opti would be the way to go. Do you know what brand opti you put in and how much it cost to have someone else do it?
 

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Discussion Starter · #6 ·
My understanding is that a #27 will easily trigger if another code has already triggered and/or it could have been a hiccup from months ago completely unrelated to the current problem. Once a code triggers it stays until its cleared. So in my case there was no real way of knowing what tripped the #27 - that's why you need to clear the codes and wait for the problem to happen again then recheck. Mostly likely you only see the one code related to the real problem. I don't have time to do the work anymore, especially when it comes to diagnosing codes. I sent it to my local corvette shop and it was like $1550 (before taxes) to replace the optispark and the water pump. They did not specify the optispark type - but I am certain it was either the OEM or the recommended replacement. I might call them to find out - add it to my records.

My big fear about the 1992 these days is the ECM - in the next 10 years who is going to be rebuilding them? Once there are no more sources to do it, and your 1992/1993 ECM dies, then your car is dead. That "special" one-off ECM they only made for the 1992/1993 corvette and no other car means there is virtually no after market replacements you can buy because there just aren't enough of them on the road for any 3rd party to invest in making them. I might actually go buy a used one and have it programmed for my car - just so it can sit on a shelf until my current one completely dies and there is no one else around to fix it.
 

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QDM or Quad Driver Module is an IC inbuilt to the ECM that is basically 4 transistors. The QDM is a go between for the voltage differentials between what the car runs on and what a logic circuit runs on. Computer circuits let out the magic smoke and catch fire at 12v. Moreso at 14.4v and everytime with more than 1 amp of current. So the quad drivers are like a solid state relay pack almost. They let the 5v running ECM connect to the 12v car's systems for sensations and controls.

A QDM fault is not a telltale of a failed ECM rather it is just an alert to the fact that there is an open or short with one of these circuits. And that could be the ECM, it could be the wiring, or it could be a sensor or control device. And sadly all are known culprits. OBD-1 was not friendly to diagnostics. And many hours into this diagnosis you might well learn of a failed controller you can not replace anymore and must try to have your or another already failed part band aided.

You should get away from the engine controller and harness and start looking for aftermarket solutions made today. Ask google, Cortana, or Siri which is a popular aftermarket fuel system for V8 and begin there. Anything with the name Holley, or Edlebrock, or FAST will bring the joy.

Run from FiTech. FiTech is a Chinese company that has no previous experience in engine control unlike Holley and Edlebrock whom profoundly understand driving our beloved v8 engines. Holley, Edlebrock, and FAST setup booths and big displays and have a presence at the SEMA shows. But nobody ever sees FiTech at a SEMA show with all of their competition. Don't buy the cheapest aftermarket EFI you can find. You will be punished for that choice.



And there is plenty more on that subject right here: From Fitech To Fast. Making Right The Wrongs | General Conversation (superchargerforums.com)
 

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@moodman - my issue ended up being the optispark. I had the ECM rebuilt like 5 years ago and from I remember the ECM symptoms were very similar to the optispark issues. I was afraid of the ECM going out again - both an expensive and not-so-timely repair. And worse, finding resources to rebuild the ECM is getting harder and harder these days. They replaced the optispark (as well as the water pump) and all was fine again.

My optispark was the original too, so it was 30 years old with 110,000 miles on it. But when it was failing it, at stop signs/lights, the car was idling very low and felt like it was going to stall out at any moment. Then when I pressed the gas to move through the interection the car did stall out. It symptoms were like a combination of the ECM failing and running out of gas at the same time. My 92 gas gauge has never been accurate, 1/10th of a tank of gas and the gauge is reading half a tank full. So I always set my tripometer to zero every time I fill up, then when it gets to 300 miles I fill up - regardless of what the gauge shows. I say this because I have run out of gas MANY times over the years so I know that chugging, stalling feeling very well.

So, it makes sense the sensation of the stall was familiar, the optispark wasn't getting gas to the engine properly - aka: running out of gas.

Clear the codes...wait for it stall again and then recheck the codes, hopefully you will only have the one code pointing to what the real problem is. Most likely, with an already rebuilt ECM, its your optispark.

Hope this helps.
I appreciate your response. I did want to try to help you with the gas gauge. I can say with confidence that the problem you are having is the fuel pump assembly. I replaced mine (it's an easy job, BTW) and the fuel gauge is perfect now. The fuel pump has a float with it that sends a signal directly to the dash cluster. Also, the fuel pump assembly isn't that expensive and you can still get them. Highly recommended.
As far as rebuilding the ECM, I am wondering what exactly they do when they "repair" them. The company that did mine said the old style solder was replaced with silver solder. They said the old solder would get micro crack in the joints. But as far as the components on the board, I don't think they replace any components, just reflow the solder. But others might correct me on that. But since the board itself seems to be a simple circuit board, it doesn't seem that hard of a thing to repair by people who know circuits.
 

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As far as rebuilding the ECM, I am wondering what exactly they do when they "repair" them. The company that did mine said the old style solder was replaced with silver solder. They said the old solder would get micro crack in the joints. But as far as the components on the board, I don't think they replace any components, just reflow the solder. But others might correct me on that. But since the board itself seems to be a simple circuit board, it doesn't seem that hard of a thing to repair by people who know circuits.
When a 1980's or early 90's ECM gets "rebuilt" someone just like me with knowledge of electrical and access to schematics and specs and with a soldering station and hopefully a lab scope at least but sadly usually just a multimeter will use their eyes with a magnifying lense and some measurements to identify a failed circuit. Then we will replace that transistor, resistor, MOSFET, IC, or W/E. Then we will verify operation before resale or return to the customer. For burned traces on the board we will run a wire from solder point to solder point. ANYBODY can open one of these ECU's and very clearly see where the only work was done.

Solder joints can crack but we do not need to reflow all the solder joints on a board from that era. And in truth we couldn't without causing other problems. Because these boards didn't come from the days of SMD's and they were built a little differently. If you heated any of these old boards to the point of flowing the solder you would finish lifting the traces from the PCB and create a multitude of shorts from that.

Also as for ANY mainboard repair, when we burn an IC we affect other things on that board. We might spike voltage or ground or heat to a bad place and put great wear on other things up or downstream of that IC. But in our testing all we saw was the IC. And so we deliver it back correct and working but these other things fail soon. And so goes the problems with fixing hurt electronics. For anybody looking for a good time call A1 Cardone and order a ECM for your 1992. But don't install it. Take the screws out of the case and pull out the mainboard and just look at it. Then find your best ECM 'rebuilder' pay them their fee, and take that one apart and post up some side by side pics for us.

"Rebuilding" is such a misleading term in mainboard repair. Band-aide would be a much better word. I mean can you imagine lifting every single component off the mainboard to replace it? It still wouldn't change the traces that make up the circuits. So what do we think then? A new mainboard with new electronics inside the old case? Ubsurd on it's face, right? Just because of access to 1992 electronic components today and what we actually pay for one of these 'rebuilt' ECM's that cost less than 2hrs of skilled labor without parts? One would think if we just used a core and replaced all the internals that ECM would cost more than a new ECM because of all the time on top of the rare and expensive components. An ECM repair is a band-aide. How many band-aides are on your brain? And how many will you have before you realize at any moment you might just inadvertently bleed out?
 

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I also had to replace my AC compressor recently and when then after the AC blower is stuck on full blast (10) even after the digital display shows it at the lowest blower setting - its still full blast. I read #28 quad-driver module #3 controls AC blower and temps on an LT1LT5 forum but have no idea if that applies to my 1992 C4.
The blower fan is a body function and so it is controlled outside of the ECM. Specifically you should be looking at your blower high speed relay or your A/C control head or your A/C programmer to be the cause of a stuck on high blower speed.

Quad driver #3 in your C4 is for the A/C compressor circuit and the cooling fan relays.
 
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