SOLVED!-04 Trailblazer LT 4.2 L no crank - no start

[coolasice]

@budwich - ok...so I just had my wife turn the key to crank while I probed relay socket 85 with meter set to Ohms and I got a reading of 9.39 - 9.40 with probe in socket and a good ground. This held true while I continued to maintain contact at both ends even after she turned the key of until I released contact with the probes. When I re-contacted the same points with the key off, I had no reading at all until I had her push the key to the crank position again which resulted in another 9.30 - 9.40 Ohms reading. I believe that I should have had a reading of 0.00.

Those numbers actually sound OK.. Given the pcm controls the start relay by cranking the engine for a predefined time or until it sees the engine start. Most likely the circuit is transistor based which would account for the 9ohm load... Likely you would see the resistance disappear 15-20sec after key was initially turned....As a side note... Do you get voltage to the + side of the relay through the park/ neutral switch when the key is in crank position?

 

[budwich]

I wouldn't go there too fast...
Your issue is probably more with a grounding problem with the pcm.

AS you have just found out, you can't test grounds by doing a voltage test. You have to do a resistance test.

I suspect that your ground to your PCM isn't so good. Of course, you tested those much earlier... BUT I suspect you tested them the same way... ie. a voltage test.

You need to double check the ground going to what ever pin that you had before. The test need to be done to a KNOWN GROUND.

 

[budwich]

Those numbers actually sound OK.. Given the pcm controls the start relay by cranking the engine for a predefined time or until it sees the engine start. Most likely the circuit is transistor based which would account for the 9ohm load... Likely you would see the resistance disappear 15-20sec after key was initially turned....As a side note... Do you get voltage to the + side of the relay through the park/ neutral switch when the key is in crank position?

You could be right but then I would check the resistance of the relay coil. Add that to the 9 ohms and you will see how much "oomph" you are getting into the relay. Maybe not enough.

Of course, the "corollary" of all this, was the relay every changed?

PS. I see that it was.

 

[budwich]

OP... now that you are getting good at testing and the fuse box. I would get some wire, solder one wire to one side of the coil contact and solder another wire to the other side (note you can just use crimp connectors if you want). Use your "flatten wire" technique (there are other ways) and insert those into the appropriate spaces in the fuse block. Turn the key to start and listen / feel if the relay operates. Of course, you could also measure if it operates with a check across the other two contacts.

 

[coolasice]

You could be right but then I would check the resistance of the relay coil. Add that to the 9 ohms and you will see how much "oomph" you are getting into the relay. Maybe not enough.

Of course, the "corollary" of all this, was the relay every changed?

PS. I see that it was.

The 9ohms is nothing to worry about when the coil resistance is usually 80-150ohms...

 

[budwich]

??? are you sure?

 

[mrrsm]

The 80-150 Ohms is defining the CORRECT RANGE of resistance... anything below...and anything wildly above the range would suggest replacing the Relay IF this test is done on the bench ... and the resistance banged on the meter at only 9 Ohms.

EDIT - I see that it was a typo ....so...Not 9.... But 90 Ohms

 

[budwich]

90... ok. No problem then in terms of "path". So the bottom line, is that all the conditions may appear right for operation in terms of test measurements, but ultimately, is the relay operating? only our OP knows for sure... :-(

 

[coolasice]

The 80-150 Ohms is defining the CORRECT RANGE of resistance... anything below...and anything wildly above the range would suggest replacing the Relay IF this test is done on the bench ... and the resistance banged on the meter at only 9 Ohms.

EDIT - I see that it was a typo ....so...Not 9.... But 90 Ohms

You are getting confused, the 9ohms was through the pcm to ground... NO relay... The 9ohms would be the transistor "switch" ON resistance... If the relay is 80-150 the 9ohms is negligible...

 

[Rad]

I think a picture may be worth a 1000 words. Please note the safety pins inserted on each end of the externally mounted connector. Also, please note the wire stuck in the starter relay socket. With the battery connected, and the key in the off position, my test light (connected to at the positive Battery terminal) illuminates when I contact either of the safety pins indicating that I have ground. With the key in the run position, the test light goes out (not illuminated) at the safety pins. With the key in the crank position, the test light illuminates again.

I get no test light illumination at all at the wire in the relay socket no matter what the key position is in. However, this is where I measured the 9.3 - 9.4 Ohms resistance with the meter with the key in the crank position.

I must mention that I did check PIN 31 at PCM connector (C1) for positive battery voltage with the key in the crank position. Battery voltage is present.

I hope this helps a little!

PS - the pins are at the yellow/black wires through the connector.

 

[coolasice]

I think a picture may be worth a 1000 words. Please note the safety pins inserted on each end of the externally mounted connector. Also, please note the wire stuck in the starter relay socket. With the battery connected, and the key in the off position, my test light (connected to at the positive Battery terminal) illuminates when I contact either of the safety pins indicating that I have ground. With the key in the run position, the test light goes out (not illuminated) at the safety pins. With the key in the crank position, the test light illuminates again.

I get no test light illumination at all at the wire in the relay socket no matter what the key position is in. However, this is where I measured the 9.3 - 9.4 Ohms resistance with the meter with the key in the crank position.

I must mention that I did check PIN 31 at PCM connector (C1) for positive battery voltage with the key in the crank position. Battery voltage is present.

I hope this helps a little!

PS - the pins are at the yellow/black wires through the connector.

What harness is that that the pins are connected to?...
Ok.. those yellow and black wires are the vss wires.... nothing for the starting circuit goes through that connector...

 

[budwich]

I think a picture may be worth a 1000 words. Please note the safety pins inserted on each end of the externally mounted connector. Also, please note the wire stuck in the starter relay socket. With the battery connected, and the key in the off position, my test light (connected to at the positive Battery terminal) illuminates when I contact either of the safety pins indicating that I have ground. With the key in the run position, the test light goes out (not illuminated) at the safety pins. With the key in the crank position, the test light illuminates again.

I get no test light illumination at all at the wire in the relay socket no matter what the key position is in. However, this is where I measured the 9.3 - 9.4 Ohms resistance with the meter with the key in the crank position.

I must mention that I did check PIN 31 at PCM connector (C1) for positive battery voltage with the key in the crank position. Battery voltage is present.

I hope this helps a little!

PS - the pins are at the yellow/black wires through the connector.

you are certainly making this tough... )))

Your test light is only going to light at the "relay wire" when you hit the starter (which from your description isn't happening... :-( ). If it isn't, then do that resistance measurement again with a meter to the relay wire. It should read the same value that your read before (9.3-9.4).

I am get a funny feeling here. You are trying to read a RESISTANCE with the meter set on resistance / ohm AND NOT voltage / DC... right???

 

[Rad]

What harness is that that the pins are connected to?...
Ok.. those yellow and black wires are the vss wires.... nothing for the starting circuit goes through that connector...

Thanks! Didn't know that but good to know! I thought that since I was getting a response from different key positions from those wires that they were related to the starting system.
This is really beginning to confuse me. If when checking PIN 48 to the relay with C1 disconnected and I have 0.00 resistance, then why am I not getting a reading at the relay with C1 connected at the PCM with the key pushed to crank?
I am getting crank voltage at PIN 31 and PIN 29 is hot in run/start. That sort of leads me to believe that that the ignition switch is not a problem.
Could it be a theft prevention issue in the system preventing a signal to the relay even though I was able to measure 9.4 Ohms at that point with the key in crank?

 

[Rad]

you are certainly making this tough... )))

Your test light is only going to light at the "relay wire" when you hit the starter (which from your description isn't happening... :-( ). If it isn't, then do that resistance measurement again with a meter to the relay wire. It should read the same value that your read before (9.3-9.4).

I am get a funny feeling here. You are trying to read a RESISTANCE with the meter set on resistance / ohm AND NOT voltage / DC... right???

Correct! The meter is set to Ohms, not DC volts.

 

[budwich]

ok... so can you confirm that you are able to meter 9.3 ohms from the end of that relay wire to ground with key in start... this will confirm that you are indeed "seeing" the pcm and its attempt to "signal" the relay. IF you aren't, then that little wire has a problem... and it may be pointing to a problem in the relay socket area if its that hard to find a good connection.... maybe

 

[mrrsm]

@budwich suggested "Yes, playing with the fuse block might be encouraging trouble but it is a potential area for connectivity issues..." Would removing the Negative (-) cable and pulling off the Fuse Block and looking around underneath there and maybe lightly tugging on the wire connections be related? And removing the two nut/bolts holding down the 125 Amp Fuse on the left side of your image might show whether the contacts and other fuse-able links if any... need to be loosened, cleaned and re-tightened.

 

[budwich]

I wouldn't go there too fast.
OP, do the check that I suggested. Further, after you have had some "repeatable" results at that contact point, I would do one small simple thing... twist the tangs on the end of the relay, in particular, the two tangs associated with the coil. Just gently twist them enough so that they are no long perfect parallel / straight... more like a slight "cork screw idea" (BUT NOT quite that exaggerated). Plug it in... Go and start your vehicle...

 

[Rad]

@ budwich suggested "Yes, playing with the fuse block might be encouraging trouble but it is a potential area for connectivity issues..." Would removing the Negative (-) cable and pulling off the Fuse Block and looking around underneath there and maybe lightly tugging on the wire connections be related? And removing the two nut/bolts holding down the 125 Amp Fuse on the left side of your image might show whether the contacts and other fuse-able links if any... need to be loosened, cleaned and re-tightened.

I confirmed that I can repeat that test and get 9.4 ohms resistance each time. This time I also maintained contact between a good ground and the relay socket. As you mentioned earlier, 9.4 ohms remained displayed for 15-20 seconds after key was turned off and removed from the switch. But why will my test light not illuminate with it connected to the positive Battery terminal and probing the wire?
I agree that the PCM is sending the signal but maybe not strong enough to start the truck?
I am very hesitant to risk spreading the relay socket connections in the fuse block by putting a twist on the relay pins.

The sockets feel good and snug as I insert the relay as well as the wire.

 

[budwich]

well, it is what it is.... an impass...
so you say you are indeed measuring thru that added on wire to ground with the meter (key in start), and then when you do the same thing with the test light, you get no light.... right? not really questioning your work but its tough "seeing thru the internet"...

The guess here would be that the connection is very poor such that little or no current is finding its way thru to light the light BUT a meter cares little about current and actually uses very little to measure resistance. Having said that, your batteries in your meter are new right?

 

[budwich]

one more question, when you are doing your resistance measurements, what or where are you getting your ground point?

 

[Rad]

I make contact with one end of my meter probe at the ground point at the left side of the engine compartment which comes directly from the negative side of the battery while probing the wire stuck in the relay as my wife pushes the key to crank. I have also contacted the probe to the negative battery as well as another ground point at the firewall and get the same readings on the meter (9.4 ohms when contacting the wire end).

When performing this test with the test light clipped to the positive end of the battery terminal bolt and the light probe on the wire, I get no light when the key is pushed to crank. When I touch a vehicle ground or the negative battery terminal, I do get a light.

Going back once more to testing from pin 48 of C1 to the same relay socket, the resistance is 0.00 which tells me the wire is good. But that is with C1 disconnected from the PCM. It makes me wonder how the PCM itself is grounded to the engine, Maybe the lower left engine ground connection itself is "poor" (if that is where the PCM gets its ground from) resulting in a weak ground signal being sent from the PCM to the starter relay.

Your thoughts on this?

 

[budwich]

my comments... happy new year ....
thanks for the clarification.
"Your mission, should you accept it, is to get that light to light.... your car will explode in 30 seconds thereafter"...
I think its a mission impossible.

You certainly have some form of poor conductivity in the signal path which as you have seen is causing insufficient operation of the relay.

From your testing, it is clear that everything is working as it should so operationally, you need not look anywhere else but just focus on the relay activation link.

Since you have a second PCM, I would plug it in and do the same measurement test just see if things are different as perhaps its an internal pcm thing (doubtful) but you never know.

Further, another question arises from your first description of the problem. You indicated that your dash instruments were "wonky" when you try to start.... does this still occur?

Lastly, you didn't actually do the test that I suggested with the relay coil by attaching two wires to the coil and fuse box location and see if anything happens... of course, based on your test light, probably not but if you got the wire and things to do the connection, it costs little or nothing, just be careful in terms of wires hanging around (I have seen your pins... )

You might end up having to look into the fuse box structure at that area to see what's up.... but I would certainly try the "twist the relay pins" first before going there as I am more thinking that its poor connections at the fuse box. People in the past have had to twist fuse "legs" to get better contact on fuses locations so it wouldn't surprise me that your problem is right there.

 

[Rad]

Thank you, and happy new year to you and everyone else that spent their precious time with me on this. I just tried one more test which may be of interest. I wrapped a real thin telephone wire around the relay leg that is normally energized by the ground signal from the PCM, put the key in run, then made contact to vehicle ground with the other end of the wire.

The truck cranked, fired up, and shut off - security light flashing after shutting off. It might think it's being stolen since it won't stay running as it had before. Maybe too many battery disconnects and the fact that I replaced the ignition switch. To answer one of your other questions, since replacing the Ignition switch, the gauges no longer jump around.

I know that doesn't solve the issue, but nonetheless, it was a test that proved that a good ground signal going to that relay pin will fire up the truck. Will take a good look at the grounds on the engine next and take it from there.

It's a good thing that I have multiple vehicles to choose from so that we aren't stranded here, but this is my wife's Truck and she loves her TB.

 

[budwich]

good testing. hopefully your hangover won't be too bad and then you can work on this, on the other side...

I am not sure how the passlock system works but perhaps it comes into play in some form but from my general reads, the starter should still function but the system kills the injectors.

Perhaps the swap of pcm's and repeat test of resistance on the lead to confirm that it is indeed similar may help. Depending on the outcome, then possibly some re-learn is required thereafter... ???

 

[mrrsm]

@budwich... This place looks interesting, germane and informative... and has some Theft Deterrent Electrical Diagrams as well and covers a lot of new ground about things that can wrong with the BCM...(Who knew they might get soaking wet inside and corrode?)...and this guy explains TWO Security Re-Learn Procedures, as well... The 10 Minute AND The 30 Minute Re-Learn Steps.

http://www.bergerweb.net/PasslockFix/index.shtml

 

[Texan]

Try the security relearn in post #45 again.

 

[budwich]

its a new day and new year.... and actually now a different look at things based on the old year's last test ...

As I indicated a while back, when checking resistance especially on an unknown circuit, the first check is for voltage. This is "insurance" to prevent possible meter damage BUT also to prevent FALSE resistance readings... :-(

In this case, I think there is a false resistance reading of 9.3.

Basically, I think if Rad measure the "wired relay socket point" with a meter, he will find when attempting to crank, that the point is at 12.x volts (basically battery) and not ground. That is why the test light DOES NOT light because there is no voltage difference between the battery connection that the test light is hooked to and the test point. When there is no crank, there is nothing at the test point and hence the test light doesn't light then either.

So the mystery of why the vehicle doesn't start is because the relay coil does not get voltage (difference) provided to it by the PCM.

However, since the PCM is doing something during a crank, it appears to know that some stimulus (start voltage input) has happened causing it to change the output for the relay coil control from nothing to high voltage.

Further with the jumping using the "telephone wire connected to the relay control point and ground" causing a start, results in a quickly dying engine and theft light, which might be correct.

I am thinking that scenario happened like this.
Initial problem with "wife" was failed to crank / wonky instruments. Jump starting worked. Jumping at relay pins and such results in start, etc. Conclusion, replace switch. Switch doesn't get things working correctly, but appears to correct some things like wonky instruments. Now further testing get to this point with lots of checks of wiring and swap of cards. Results are basically the same, no start but the same for both pcm's.
That would point to either two failed pcm's, loss of "theft prevention understanding", or potentially improperly installed ignition switch.
I doubt there are two failed pcm's. Probably the "theft prevention" issue might be a cause or the ignition system is not installed correctly to allow the passlock system (what system is in this vehicle?) to function correctly.

As suggested by others, try the relearn first.

 

[coolasice]

just for a little added info... here is the diagnostic flow chart for no crank provided by GM..

 

[Rad]

good testing. hopefully your hangover won't be too bad and then you can work on this, on the other side...

I am not sure how the passlock system works but perhaps it comes into play in some form but from my general reads, the starter should still function but the system kills the injectors.

Perhaps the swap of pcm's and repeat test of resistance on the lead to confirm that it is indeed similar may help. Depending on the outcome, then possibly some re-learn is required thereafter... ???

I swapped the PCM today, went through the relearn procedure which worked just fine - car would not crank, also no ground signal at the starter relay. It behaves exactly like the original PCM. I re-installed the original PCM after that.

Try the security relearn in post #45 again.

This response is meant to follow the test I performed with the relay pin (85) wired and contacting to ground where engine cranked, fired up, then stalled. That's when the security light was flashing. I performed the 10 minute relearn procedure, turned the key off for a few seconds, repeated the same procedure and the truck cranked, started and stayed running. This was with the original PCM re-installed.

its a new day and new year.... and actually now a different look at things based on the old year's last test ...

As I indicated a while back, when checking resistance especially on an unknown circuit, the first check is for voltage. This is "insurance" to prevent possible meter damage BUT also to prevent FALSE resistance readings... :-(

In this case, I think there is a false resistance reading of 9.3.

Basically, I think if Rad measure the "wired relay socket point" with a meter, he will find when attempting to crank, that the point is at 12.x volts (basically battery) and not ground. That is why the test light DOES NOT light because there is no voltage difference between the battery connection that the test light is hooked to and the test point. When there is no crank, there is nothing at the test point and hence the test light doesn't light then either.

So the mystery of why the vehicle doesn't start is because the relay coil does not get voltage (difference) provided to it by the PCM.

However, since the PCM is doing something during a crank, it appears to know that some stimulus (start voltage input) has happened causing it to change the output for the relay coil control from nothing to high voltage.

Further with the jumping using the "telephone wire connected to the relay control point and ground" causing a start, results in a quickly dying engine and theft light, which might be correct.

I am thinking that scenario happened like this.
Initial problem with "wife" was failed to crank / wonky instruments. Jump starting worked. Jumping at relay pins and such results in start, etc. Conclusion, replace switch. Switch doesn't get things working correctly, but appears to correct some things like wonky instruments. Now further testing get to this point with lots of checks of wiring and swap of cards. Results are basically the same, no start but the same for both pcm's.
That would point to either two failed pcm's, loss of "theft prevention understanding", or potentially improperly installed ignition switch.
I doubt there are two failed pcm's. Probably the "theft prevention" issue might be a cause or the ignition system is not installed correctly to allow the passlock system (what system is in this vehicle?) to function correctly.

As suggested by others, try the relearn first.

I am definitely checking resistance and not voltage. But, for the heck of it, I performed a voltage check and no voltage measured on PIN 85 (which I did not expect). I am looking for a ground at that point. With the test light connected to POS Battery terminal, when contacting ground, my test light illuminates green. When connected to NEG Battery terminal and contacting 12V Battery source, my test light illuminates red.

I have done some reading on the passlock system and its relationship to the key lock cylinder and ignition switch assembly. The passlock sensor mounted on or near the ignition lock cylinder housing looks for proper rotation of the lock cylinder with he correct key installed. The following is a quote from cardone.com:


"The Passlock anti-theft system requires the presence of a key in the lock cylinder to enable starting. The lock cylinder contains a stationary Hall effect sensor and a rotating magnet. When the key is turned in the lock cylinder, the magnet creates a signal on the Hall effect sensor. The cylinder then sends a coded signal to the instrument panel cluster (IPC) or body control module BCM. If the IPC/BCM receives the expected coded signal, the IPC/BCM will send a message over serial data to inform the PCM that the vehicle may be started.

To perform a relearn on a Passlock system, make sure there is a fully charged battery in the vehicle. The battery will be used for 30 minutes without the alternator available for charging. Connect a battery charger to the battery to ensure power is available while programming. Both the IPC/BCM and PCM must be working and communicating with each other."

I think it's time to re-visit the ignition switch to make sure that the gears on the electrical portion of the ignition switch are in sync with the mating gears within the lock cylinder housing.

I mentioned in one of my earlier posts that it seemed odd that I used to have to push the key to the extreme end of the crank position for the truck to start. I just checked my neighbors 02 Trailblazer and her ignition energizes just a little past the run position and doesn't have to be pushed to the very end of switch travel like mine did (when it was still running).

ADDED: My thoughts on this - In the event that the ignition switch gears are not in correct sync with the lock cylinder housing gears, and the key is turned to crank - maybe the BCM is not receiving the correct signal and not sending the "OK to start signal" to the PCM resulting in no ground signal to the starter relay.
Being hopeful here.

just for a little added info... here is the diagnostic flow chart for no crank provided by GM..

Thanks for the info. Some of these checks I cannot do since I don't have a Tech2.

 

[mrrsm]

@Rad.... The second and third pages on this site might give you another way out of Purgatory and are in line with your latest thoughts... I won't be having a Happy New Year untol you can tell us that you are victorious!

http://www.bergerweb.net/PasslockFix/index.shtml

 

[Rad]

Thanks - will check it out! I just added my thoughts to my last comment. Let me know if you think this makes sense.

 

[mrrsm]

Thanks - will check it out! I just added my thoughts to my last comment. Let me know if you think this makes sense.

It makes perfect sense to me for a number of reasons:


First ... Because you had the Weird Key Rotation issues just before the electrical anomalies occurred.

Second... Because the "brilliant" anti-theft mechanism GM dreamed up is centered in that identical physical location... making the mistake of Mixing their Mechanical and Electrical Metaphors that invariably happens when simple Worm Gear or Rack and Pinion Drive Key-Ways have electronic devices added to them. It can't be a coincidence that so many complaints about NO STARTS and NO CRANK situations invariably begin when a KEY is inserted into a LOCK and everything goes to Hell after that as soon as it gets TURNED inside.

Third...Because that last information I described shows you a way to completely sidestep that crucial Ignition Switch Circuit and fool the BCM from then on with a fixed 2200 Ohm Resistor...and it also shows where the three wires coming out of the upper Ignition plug-in connector go when following them further back under the dashboard adjacent the radio box. You should be able to see what he suggests on that link I gave you and adapt the solution to your vehicle.

Fourth... Because if you take apart the "Ignition Switch" and the "Brake Switch"...their relay and contact layouts are harder to figure out than Chinese Math. What ever happened to a simple Turn-Key link between the Solenoid and the Battery and The Starter? The guys who design all of this stuff could **** up a Soup Sandwich!

 

[Rad]

I did note that the old switch I took out was already replaced once with a newer style switch (grey in color as opposed to black original) and it is highly possible that when it was replaced then by previous owner(s) (or whoever installed it) that it was aligned (barely) close enough to send a "correct" signal to the BCM back to the PCM to energize the starter relay when the key was pushed to MAX crank. I'll be on this first thing tomorrow. Would really hate to take it to the dealer (although they have Tech2 scanners with which they can "talk" to the PCM). Calling it the night for now. I will report back tomorrow.

 

[coolasice]

I did note that the old switch I took out was already replaced once with a newer style switch (grey in color as opposed to black original) and it is highly possible that when it was replaced then by previous owner(s) (or whoever installed it) that it was aligned (barely) close enough to send a "correct" signal to the BCM back to the PCM to energize the starter relay when the key was pushed to MAX crank. I'll be on this first thing tomorrow. Would really hate to take it to the dealer (although they have Tech2 scanners with which they can "talk" to the PCM). Calling it the night for now. I will report back tomorrow.

Can you do another test for us? Pull c2 on the pcm and test resistance of pin 65 (large ground wire) to the battery negative connection? I do in fact think this may all be ign switch related but checking for good ground can't hurt.

 

[Rad]

Well...today I dug into the ignition switch, lock cylinder and lock cylinder housing. I performed the steps as described at the following:
http://chevyavalanchefanclub.com/cafcna/index.php?topic=42089.0

I had high hopes that this was the problem but no dice! I even went out today and bought a new battery since the one in the vehicle seemed to run down quicker than I think it should. That also didn't make a difference.

Like I stated before, I don't give up easy but I am really running out of options. I have spend 2 weeks trying to figure this out and don't know what else to check. If I can get it to stay running again by jumping it, I will put it back up on ramps and check the ground connection on the lower left side of the engine block (disassemble, clean and reconnect).

I will go ahead and check for resistance on pin 65 of C2 to negative battery terminal and report back. Thank you for suggesting that.

 

[mrrsm]

I'll give this man... Your Level of Determination on a Ten Point Scale is a RADical (11) ...!!!

It would be interesting to know when every single thing is turned off if you can detect any "drain' on the battery due to the Ignition Switch Not REALLY being disconnected by the removal of the Key. Did you try Starting the vehicle after pressing the shifter and working it back and forth through the PRND21 Sequence a half-dozen times... and then placing the Shift Handle in the NEUTRAL position and see if the changes you have made since the last time you tried this make any difference? Does a damaged or disabled Crankshaft Position Sensor play any part bin your schematic too?
I would also suggest that you get a can of CRC ELECTRICAL CONTACT SOLVENT AND AFTER REMOVING THE THREE PCM CONNECTORS... DOUCHE DOWN THE CONNECTORS AND THE PCM THREE INTERFACES AND AFTER THEY ARE BONE DRY... PUT THEM ALL BACK TOGETHER ...MAKING A SPECIAL EFFORT TO ENSURE THE RUBBER O-RINGS ARE ALIGNING TO THE PCM CORRECTLY.

 

[gmcman]

I don't believe I saw this mentioned, but is there an aftermarket stereo or alarm installed? Are you the original owner? If not, are you 100% sure there isn't a remnant of an aftermarket alarm tucked away under the panels above the footwells?

Maybe this scenario was just waiting to show it's ugly face the moment it lost power the first time....which could have been years.

Just a thought as I have dealt with alarms that were disarmed until there was an interruption in power then it reset.

 

[Rad]

Checked resistance on PIN 65 of C2 to NEG Battery as well as to left fender well ground point. Resistance measured 0.00. Checked resistance from PIN 65 mate on PCM and from PCM case itself to ground, resistance measured 0.75. Not sure if that is enough resistance to prevent a good strong ground signal from reaching the starter relay.

 

[mrrsm]

I swear to God Almighty... I am stupid... I should have thought about this much earlier... Please forgive me...

I've been thinking a bit more on this @Rad... and now I'm wondering if there is a problem inside your 4L60E with the Transmission Position Sensor or the entire Solenoid Harness. There is a Round Electrical Connector (the Wire harness goes back to the PCM) located on the Passenger Side of the Transmission that screws into a Socket like a Bottle Cap on the upper top portion of the right rear of the Transmission.

On occasion... the seal keeping the Automatic Transmission Fluid INSIDE will leak around the seal and invade the connector harness point and short it out! If you can jack up and support the vehicle on safe Jack Stands...crawl underneath the passenger side and feel around on top of that connector with a white paper towel. If you detect ANY leaking Blood Red Residue and the Connector Harness is soaked in Automatic Transmission Fluid... then The Fix for this is to drop the Transmission Fluid Pan and replace the Transmission Switch and Soleniod Harness and install a Brand New Connector to Plug Harness... with a New SEAL. This is a fairly straightforward repair procedure you can watch on Youtube and get the parts you need here:

http://www.carid.com/chevy-trailblazer-transmission-solenoids-sensors-switches-control-units/

I am guessing that the short you are trying to chase is there... and it is preventing the PCM from believing that the Transmission Position INSIDE is NOT dangerous and so the PCM will NOT allow the vehicle to start! It would very interesting to see an Electrical Diagram of the PCM to 4L60 "E" for Electronic Transmission and see how the TP Sensor behaves or affects the Start Up Sequence.

Here is the link to the Whole Story on the GM 4L60E harnesses and hardware:
http://static.summitracing.com/global/images/instructions/tci-377000.pdf
http://www.usshift.com/downloads/GMmanual.pdf

EDIT:

I just stumbled out to the garage and took some snaps of a 4L60E to better show you where to look...

 

[Rad]

I'll give this man... Your Level of Determination on a Ten Point Scale is a RADical (11) ...!!!

It would be interesting to know when every single thing is turned off if you can detect any "drain' on the battery due to the Ignition Switch Not REALLY being disconnected by the removal of the Key. Did you try Starting the vehicle after pressing the shifter and working it back and forth through the PRND21 Sequence a half-dozen times... and then placing the Shift Handle in the NEUTRAL position and see if the changes you have made since the last time you tried this make any difference? Does a damaged or disabled Crankshaft Position Sensor play any part bin your schematic too?
I would also suggest that you get a can of CRC ELECTRICAL CONTACT SOLVENT AND AFTER REMOVING THE THREE PCM CONNECTORS... DOUCHE DOWN THE CONNECTORS AND THE PCM THREE INTERFACES AND AFTER THEY ARE BONE DRY... PUT THEM ALL BACK TOGETHER ...MAKING A SPECIAL EFFORT TO ENSURE THE RUBBER O-RINGS ARE ALIGNING TO THE PCM CORRECTLY.

@MRRSM - yes, tried the whole shifter thing, cranking in every position.

I don't believe I saw this mentioned, but is there an aftermarket stereo or alarm installed? Are you the original owner? If not, are you 100% sure there isn't a remnant of an aftermarket alarm tucked away under the panels above the footwells?

Maybe this scenario was just waiting to show it's ugly face the moment it lost power the first time....which could have been years.

Just a thought as I have dealt with alarms that were disarmed until there was an interruption in power then it reset.

Thanks for jumping in @gmcman - as far as I can tell, everything is original, right down to the radio w/CD player. No evidence whatsoever of an aftermarket alarm system ever having been installed. In fact, the wife pushed the red button on the key FOB and the alarm activated. I assume it was/is the factory alarm. The truck appears totally unmolested.

I swear to God Almighty... I am stupid... I should have thought about this much earlier... Please forgive me...

I've been thinking a bit more on this @Rad... and now I'm wondering if there is a problem inside your 4L60E with the Transmission Position Sensor or the entire Solenoid Harness. There is a Round Electrical Connector (the Wire harness goes back to the PCM) located on the Passenger Side of the Transmission that screws into a Socket like a Bottle Cap on the upper top portion of the right rear of the Transmission.

On occasion... the seal keeping the Automatic Transmission Fluid INSIDE will leak around the seal and invade the connector harness point and short it out! If you can jack up and support the vehicle on safe Jack Stands...crawl underneath the passenger side and feel around on top of that connector with a white paper towel. If you detect ANY leaking Blood Red Residue and the Connector Harness is soaked in Automatic Transmission Fluid... then The Fix for this is to drop the Transmission Fluid Pan and replace the Transmission Switch and Soleniod Harness and install a Brand New Connector to Plug Harness... with a New SEAL. This is a fairly straightforward repair procedure you can watch on Youtube and get the parts you need here:

http://www.carid.com/chevy-trailblazer-transmission-solenoids-sensors-switches-control-units/

I am guessing that the short you are trying to chase is there... and it is preventing the PCM from believing that the Transmission Position INSIDE is NOT dangerous and so the PCM will NOT allow the vehicle to start! It would very interesting to see an Electrical Diagram of the PCM to 4L60 "E" for Electronic Transmission and see how the TP Sensor behaves or affects the Start Up Sequence.

Here is the link to the Whole Story on the GM 4L60E harnesses and hardware:
http://static.summitracing.com/global/images/instructions/tci-377000.pdf

EDIT:

I just stumbled out to the garage and took some snaps of a 4L60E to better show you where to look...

Thanks for the tranny pics. I will definitely check that out!

PLEASE NOTE: This is a new discovery not tried before. I was thinking back to when I checked resistance at starter relay socket 85 and read 9.3 - 9.4 ohms with the multimeter. So, tonight, I had my wife crank the key while having my little test wire stuck in PIN 85 hole, and using my test light in total darkness, the test light illuminated a weak green light (which wasn't visible in daylight). So, as @budwich pointed out, there is a ground signal being sent from the PCM to the relay but not strong enough to energize the relay. Now to find out why it is so weak - but nonetheless present!

@coolasice is also still thinking that it is a ground issue of which I am now more convinced after this last test.