3.1 Intermittent spark

I have a 1995 Chevy Beretta that is experiencing intermittent spark on 3 cylinders. I have found this by putting a timing light on each plug wire and
looking at the strobe that I get from the timing light. 3 of the cylinders give very intermittent spark, and the engine runs rough. The other 3 cyls are fine. Plugs, wires, etc are new. Does this seem to be an ignition module problem? Or what else could it be? I am assuming it isn't the crank position sensor as I would think that would spread the problem across all 6 cylinders and the car isn't experiencing any typical CPS related problems (hard to start, dying, etc). Starts and runs ok except for the intermittent spark on those 3 cyls.
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Well, it's not the ignition module OR the coils. Replaced them with known good ones and the problem remains. Are we talking about a computer problem? The crank position sensor, as far as I know, would effect firing on each of the 6 cylinders. 3 of them are fine, all the time.
Any help would be really appreciated.
"Joe W." <v> wrote in message

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I have the factory service manual in front of me for the 1995 3100 (L82, VIN M, SFI) engine.
I'm curious as to which three cylinders are the problem. It may be something simple like a bad ground. Have the plug been gapped properly? Is the MIL on? Does the MIL work? Are there any codes? Do you have the wires in the correct order? Looking at the front of the engine it goes 1-4-6-3-2-5 on the coils.
I think I would focus on the wires and plugs for now, even if they are new. Recheck plug gaps and condition, check for good spark with a spark tester, check resistance of wires.
That's about as much help as I can offer for now.
Steve
"Joe W." <a> wrote in message

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Thanks for the reply, Steve. Cyls 1, 6, and 5 are are firing correctly. 4 gives a very slow, intermittant pulse with a timing light, 3 gives a medium speed intermittant, and 2 gives a very fast intermittant, like it is missing only a few sparks per few seconds.
Double checked the plug wires, the plugs and the gaps. The gaps on the plugs were a little wider than suggested (0.055" instead of 0.045") but after setting them straight, it didn't make a difference.
I don't believe that it is a ground issue, as there is only one positive and one ground going into the ignition module. If there was a wire grounding issue, I would expect the erratic spark to be across all cyls, not just 3 specific ones.
I am coming up empty trying to find the cylinder numbers for this motor. I believe that all the plugs are on correctly, but regardless of whether they are or not doesn't really go hand in hand with the problem of erratic spark.
Like I said before, I have moved wires around and the problems do not follow the wires. It looks to be something controlling the spark, and I know it isn't the ignition module or any of the coils.
I am up against a wall here, I am tempted to try to locate an ecu for this car but I want to be pretty sure before I dive into something more costly like that.

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I made a mistake earlier, I was thinking of the 3.1 and not the 3100. The coil packs are behind the intake manifold so looking at the front of the vehicle, the wires go 5-2-3-6-4-1. The cylinders are: front bank left to right 2-4-6 and the rear bank 1-3-5. It's odd that you say 1 6 and 5 are the problem ones. When you say that, are you refering to the cylinder position or the coil pack position? If refering to the ignition position, here's what I'm thinking:
You have the 5 and 2 backwards and it's actually 1, 6 and 2 that are giving you the problems, not 1, 6 and 5. The "waste spark" DIS system on these motors (and most of that era, 2.8, 3.1, 3100, 3400, 3.4, 3.5, 3.8) is not like a distirbutor system. Each wire from the ignition is not an "output" of spark. Terminal 1, 6 and 2 are where the "spark" comes out, goes through the wire, across the gap, through the engine block, across the gap of the "companion" cylinder plug, up the wire and back to the coil. In essence, when the ignition fires, it fire two cylinders at the same time, 2 and 5, 3 and 6 or 4 and 1. Both cylinders are at TDC at the same time, it's just that one is on the compression stroke and the other is not. Next time it fires, it's the opposite.
Anyway, what I'm getting at is that maybe, the "signal" is too weak on the "return" wire for your timing light to strobe properly. Try another timing light or test this theory on another 3100 motor. Either way, I would test the spark properly rather than using a timing light as a guide before you go replacing expensive components like PCMs.
If there was a problem with one of the sensors, a DTC would most likely illuminate the MIL.
Steve
"Joe W." <a> wrote in message

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Oh, and the check engine light is NOT on. It comes on like the rest of the lights when the ignition is on, but after the engine is started, it turns off. If the check engine light is off, I wouldn't think there would be any codes, but I can't do the "jump two pins" trick with the diagnostic port like with the other OBD1 vehicles because mine has a different plug than expected. Something about it being between OBD1 and OBD2 and chevy had a sort of hybrid port for some vehicles of my year.

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GM had this ALDL8192 on some of their vehicles before 1996 that doesn't use the flash codes, but the information I have on your vehicle states that it does have the flash code retrieval ability. Easy to tell though, if you connector has a terminal 'B' on the 12-pin connector (second from the right on the top) then you can get flash codes.
I think you can because the codes for your vehicle 1995 Chev Beretta 3.1 VIN M are ones like 16, 17 and the like. The ALDL8192 is like OBDII because of the P codes used, ie: P1640. However they are different than OBDII codes.
But if the MIL if not on, you needent worry about it right now.
Steve

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past
the
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... Each wire from the ignition is not an "output" of > spark. Terminal 1, 6 and 2 are where the "spark" comes out, goes through the > wire, across the gap, through the engine block, across the gap of the > "companion" cylinder plug, up the wire and back to the coil. In essence, > when the ignition fires, it fire two cylinders at the same time, 2 and 5, 3 > and 6 or 4 and 1. Both cylinders are at TDC at the same time, it's just that > one is on the compression stroke and the other is not. Next time it fires, > it's the opposite. > >
actually, those coil packs are wound as parallel windings around the same core. you can confirm this by pulling one wire & noticing that it only dies on that cylinder. you can double confirm it by taking that wire & shorting it to ground & noticing that the voltage on the companion plug does not double (as it would in a series circuit like you suggested). since both output terminals are connected to two seperate sets of windings that share the same input terminal, checking with an ohm meter indicates that they are connected together the way you suggest, but it's not the case.
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"Actually" I just regurgitated my answer from the GM factory service manual for my vehicle. So any errors in description of how it works you should take it up with GM engineers.
Here's a quote I found lingering on the 'net: "Since the polarity of the primary and the secondary windings are fixed, one plug always fires in a forward direction and the other in reverse. This is different than a conventional system firing all plugs the same direction each time."
Basically, I'm not saying you are wrong, because I don't know. For all I know you could have disected one while you were on retreat from your PhD studies in electrical engineering. But I do know that conventional ignition coils have one end of the secondary winding that goes to the distributor and the otehr end to ground. The GM DIS takes that ground connection of the secondary winding and connects it to the companion plug.
Remember, I'm not pulling anything out of my head, this is all infromation from books and websites. I can't find one that goes in depth very well.
Steve
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No need. You're right, he's wrong. Not to mention that he'd also have to take it up with the engineers at Ford, Chrysler and a few other manufacturers.

Absolutely 100% correct.

I'll say it then. He's wrong.

I can introduce you to at least a dozen electrical engineers at the electric motor plant down the street that don't understand it either. Also, I've seen the coils being manufactured in person which easily trumps anything he may have dissected.

100% true of GM HEI, other makes/brands sometimes connect the other end of the secondary to coil primary positive terminal.
I'll give uncle giggles 10 bonus points if he can explain why when one end of the coil secondary is connected to ground, the induced voltage isn't just shunted directly in that direction to ground and not across the spark plug gap. (electricity takes the path of least resistance and all that)

Indeed it does. Observing plug gap wear is sufficient proof that this is what occurs.

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reset snipped-for-privacy@hotmail-dot-com.no-spam.invalid (superchuckles) wrote:

Hogwash. You're suggesting that a DIS coil is or resembles a center tap transformer?

Secondary voltage is determined by the largest gap in the circuit. Your above 'test' is meaningless. It would also help you to remember that the secondary event is initially a capacitive event until current flow at which point, voltage drops quite a bit.

More hogwash. What "input terminal?" A DIS coil secondary is a simple series circuit, not unlike an extension cord in a bag hanging on a peg in a hardware store, i.e., a coiled wire with two ends.

Where do you get this stuff?
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