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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
I have a L03 (305) TBI and I notice some miss fire at idle but difficult to figure it out where. Two cyl where down, very strange. I First checked the plugs and wires and all ok. So I hooked a spark plug to directly to the coil and I could notice the same missing spark (tan tan tan.(no spark)..tan tan tan. (no spark)..tan tan tan). With that test, the cap and rotor where eliminated as the source of the problem!. So it is either ignition module, pickup coil or the coil itself.
Check this out.
Red lines are the low that generates spark, the circle is the missing singal, so 3 cyl fires, one gets skipped then three more and so on!l
Signal from the pickup coil, there are no missing triggers!
So It seems to me that the problem is on the ignition module but it is a brand new one!., coil is out of the hook because it is doing what it is told!, So Where could be the problem? Maybe that oem replacement I got is not good, or who can tell me about the pickup coil signal, is 900mV p-p ok?
As far as I understand, for TBI, the distributor is semi autonomous. What I mean is that from the outside it only receives 12VDC, he commands the coil and sends back to ECU the same trigger signal for injector pulses to be generated. During startup, it works on bypass mode so the timing is not an issue. So I think that the safe way to fix this is to replace the distributor, but Im curious to know why is this happening.
What do you guys think?
Well, if you have verified the pickup coil, and everything else in the system checks out, I would replace the module again, and see what happens. Did it have any warranty on it??
Thanks Ploegi!,
Not sure if I can get a replacement as warranty but I bought an extra module just in case. If is not the module, they are so cheap that I can save it for a rainy day.
keep all posted on the results. I also have the the pickup coil, the problem is that i have no idea how to remove it, there are no screws!...
Here is the follow up!
It was the Pickup Coil!. When replacing the ignition module the pickup coil connector cracked in half so I was force to replace it.
I did a test again on the signal by twisting the distributor with my hand.
Twice the signal power and much more clean (less noise).
Also, I forgot to mention that on this sample It is three groups of sine waves because I twisted the shaft three times, the normal working (starting the car)would be a continuous sine wave.
So here is for the rest of the GM folks a example of a brand new pickup coil signal for future references
Any chance you are connecting the scope leads to the pickup coil backwards? The ICM should have a N & P molded into it (negative and positive).
I mention this as the coil signal is inverted. And I have seen pickup coils that have the leads reversed in the ICM connector. Causes spark timing issues when they are not correct.
Hi RBob!,
Thanks for asking. To be honest, I can't remember if I connected the scope inverted, I did not payed attention to dad..I had in mind that it was only a AC signal, so I though it did not matter, but you are right!! the icm has n and p and also there is no way to connect the pickup coil inverted to it, so the polarity should matter ... I was really more paying attention to the signal strength and shape. The Pickup coil when removed was very deteriorated, it was a good call to change it anyways in my case. I thought it was interesting to share the failure of missing sparks with such precision, making believe that it was the ICM and what really fixed the problem was the pickup coil. Do you think that the plot is not a good representation?. I would want to confuse anyone with the images if they are wrong.
Other then being inverted the signal is OK. The fast edge is supposed to be the falling edge. The slow rising edge arms the zero-crossing detector with the rapid falling edge triggering it.
Can I ask what you are using to test the spark pulses? I am having a similar issue after replacing distro, icm, coil, wires, and plugs and wanted to narrow it down like this. Great work!
Can I ask what you are using to test the spark pulses? I am having a similar issue after replacing distro, icm, coil, wires, and plugs and wanted to narrow it down like this. Great work!
Hi I know it has been a long time since this post, sorry that I did not reply on time. I was using a usb scope meter.
Im having ignition issues again but this time it is very erratic rpm signal at low rpm. Did you fix your problem?
01-26-2016, 08:53 AM
