LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

ScannerDanner troubleshoots an LT1 ignition issue...

Old 05-14-2019, 07:09 PM
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ScannerDanner troubleshoots an LT1 ignition issue...

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Old 05-15-2019, 12:02 PM
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Re: ScannerDanner troubleshoots an LT1 ignition issue...

That was cool to see something with an older LT1 being diagnosed.
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Old 05-15-2019, 06:07 PM
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Re: ScannerDanner troubleshoots an LT1 ignition issue...

The 94 factory manual indicates DTC 41 sets when the voltage on circuit 423 (the white signal wire from the PCM to the ICM) exceeds 4.6 volts, and engine speed is less than 1,500 RPM.

I'd recommend trying Shoebox's checks first, if you don't have a scope:

4th Gen LT1 F-body Tech Articles

If that produces fuzzy results, then go to the scope;
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Old 05-15-2019, 08:14 PM
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Re: ScannerDanner troubleshoots an LT1 ignition issue...

In the Buick Know How video on the opti, the tech 1 points to DTC 41 as "IGN Control Open CKT" (14:55 in that video). So, his scanner tool does have the code description off....which is not unusual for non-OEM scanners. .

I think Danner's video is how a good tech worth his/her salt should approach solving this problem.

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Old 05-16-2019, 10:49 AM
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Re: ScannerDanner troubleshoots an LT1 ignition issue...

First..... I agree this is an excellent video on sound diagnostic procedures. I was impressed, and didn't mean to leave the impression that I was questioning his knowledge or skills. The tie in to possible ICM damage from high current draw on the control wire from the ICM to the coil was new to me.

He was starting from scratch, having to research the background of the Opti signals, how the PCM uses them, and the structure of the ICM. If everyone who comes here had his skills, or even the equipment to carefully follow his research, logic, procedures and conclusions, that would be great. Unfortunately, we often run into members who - a recent example - don't know the difference between an amp and an ohm. A serious fear/lack of knowledge of anything "electrical" seems to be a popular excuse for people needing a simple answer to a problem. Shoebox simplifies the process for those people.

You can probably help me understand why the code setting criteria is the voltage on the circuit (as opposed to extended cranking time). My thinking (always risky when a mechanical engineer dabbles in electrical stuff ) - If the criterion for setting the code is circuit voltage over 4.6 volts, and his scope was actually indicating a 0 - 5.0 volt signal, logic says the code would set, because of the open circuit. But that would require that in normal operation, the voltage range/reference is dampened by the circuitry of the ICM when the circuit is not open, so as not to exceed 4.6 volts. Appears from the diagnostic logic chart in the factory service manual that the range of the square wave is normally "between 1 and 4 volts" with the engine cranking. Am I headed in the right direction?
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Old 05-17-2019, 01:08 AM
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Re: ScannerDanner troubleshoots an LT1 ignition issue...

I didn't get that impression from your comment. I agree with you that for the general person trying to decipher LT1 ignition issues(or F-body issues in general), Shoebox's site is a good place to start. That's why I mentioned Danner's video being a good video for what a tech should be doing. Danner's video starts getting into the deeper end of the electrical troubleshooting pool.

I think you are headed in the right direction with your thinking.

The range of 1-4 volts is checked on the AC setting of a multimeter(DMM). They are doing this to get by without using a scope to see if square wave activity is on the control line. The AC scale reports the RMS value of a voltage. For a "true" RMS meter you can measure the RMS value of different voltage waves(sine, square, triangle, etc). If you don't have one of these "true" RMS meters, then you can only measure the RMS value of a sine wave....and your check for this value could read wrong when trying to measure a square or triangle waves. Just something to keep in mind.

Doing some quick estimations.....For a periodic 50% duty cycle square wave, Vrms = Vpeak * 0.707. So if the peak of this square wave was around 5 volts...Vrms shakes down to around 3.5V. The duty cycle of the wave on his scope is less than 50% so that voltage on a DMM will be lower than 3.5V(the formula changes if you tweak the duty cycle from 50%), but I would not expect it to be lower than 1V. So it should fall into the expected 1-4 VAC control line check outlined in the factory service manual.

I haven't seen a schematic of one of the control modules, but I think it will boil down to a high power transistor where the biasing of it under normal conditions would limit the control line peak voltage to around 4.5 volts.

Last edited by ACE1252; 05-17-2019 at 01:25 AM.
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