#2

I want to make sure that we are talking about the same parts and nomenclature is not causing some confusion. Forgive me if you already know exactly what I’m talking about.



The first picture below is the Ignition Switch. You can see about a dozen inputs/output circuits. So you can have good voltage on one circuit and a drop on others.






The next picture is the Ignition Cylinder Lock. Nothing but the place the key goes in and rotates. From what you stated about having key problems occasionally, sounds like replacing that would be a good idea, especially if you find you have to replace the ignition switch (we don’t know that yet). IMPORTANT: Read up and watch videos on replacement of the Lock Cylinder. Again, I recommend a subscription to Alldata.com. Incorrect (and sometimes even correct?) replacement can lead to you having to do a PASSLOCK relearn (you can do this without special equipment) or your truck won’t run (anti-theft feature), or you could have other problems.





If you look at the first picture you can see the white gear looking thing in the black ‘box’. The black box is where the switch commonly fails. That’s the part the Lock Cylinder moves when the key is turned. Besides the fact that these trucks have a somewhat common problem with Ignition Switch failures, I’ve seen many cases where the weight of the keys makes the Ignition Lock sloppy and it messes up the Ignition Switch. A side note, personally if you need to replace either (or both) you may want to consider getting the OE GM parts (you should be able to get them from Rockauto.com). I’m finding more and more with electrical components that the generic parts store parts are just too much of a crap shoot for later failure to save a little money (note: they can also get OE parts) for you though, and some offer lifetime warranties on their generic parts…so there’s that.



I searched around and found a video from South Main Auto where he really goes into great and informative detail on Ignition Switch Replacement and how it operates. This guy is really good overall, and has great videos. Even though he is fixing a no start problem, it’s still the Ignition Switch replacement, if it is determined yours has failed.

 

Last one in response to the referred post. Let me know if I haven't cleared up anything for you.

#3

I completely understand why you would suspect the ECM. Been there. But I can't tell you how many times over the years (probably in the three digits) I've seen folks on forums, videos and yes, even professional auto shops, land on the ECM as being the problem and replace it only to find it wasn't the problem at all. Happens all day, every day. Off the top of my head I can't remember last time I saw an ECM change solve whatever issue. Albeit, it does happen. Like I had stated, these trucks have really robust ECMs. Once I even dropped a battery cable on the case and it sent up all kinds if sparks. Didn't do any damage at all.



Leaving aside there could very well be a wiring problem to or from the ECM, as far as the internals go, personally it would be about my last concern. Best thing to do is heading in the direction testing takes you.



An analogy: From your laptop you want to send a document to the printer to print out PRNDL. Unbeknownst to you there is a problem along the way. The page prints out nothing but lines. The computer sent the correct data, the computer is fine, the data just got corrupted along the way. You find it's a cable problem, you change it, and you get the document you wanted. OR you (you're the sensor) mis-spelled and typed RPNLD. The printer prints exactly that. Computer still sent the data it was given, computer itself is fine.



The ECM can be sending the correct data and any one of several things can be causing your issues. Bad I/P, bad ignition switch, broken wire, bad connector contact, short, etc, etc, etc.
Hope this all helps you out.

 

Have one more answer for you, went on to a professional tech site I belong too and looking in the archives I found from a GM Tech that in fact the the mileage for the odometer is stored in the I/P cluster, not the ECM after all. He states when you buy a new (to you) one or have yours serviced, only a shop that has been authorized by AC Delco repair center has the tool to reprogram the I/P cluster. He also stated that if the mileage is not known that they MAY take a receipt for an oil change, etc that has the mileage, but the burden for following Federal Law falls on the shop. Now this is answer was from 2002, so maybe things have changed as far as who can do it. But if you find you need to replace yours, you might want to insure a "refurbished" one is from a shop that is authorized to burn mileage in.

I went thru a dozen or so customer complaints and fixes for the I/P. Unfortunately, there was no one single thing that consistently stood out as the definitive fix. Confirmed fixes were all over the map, from I/P cluster replacement to broken wires to blown fuses and seemed most every component in between.

 

Thanks, that's good information to have. I think at this point you've convinced me to pass on the theory of the ECM as the major culprit here, and to do some testing before replacing anything.