07 LT 5.3L Sudden no start/crank

[J-Herkel]

Im not quite sure what you jumpered in the ignition relay whether it be the control side or the controlled side. Im assuming you jumped the 2 contacts for the controlled side which is what feeds the 12v to the starter solenoid to initiate starter spinning over among other things that happen during that evolution. If not, correct me.
Now as far as the control side of the ignition relay, remember there are 3 things that you need here. (1) Good relay (its coil and contacts) (2) 12v going to the ign relay coil. (3) Ground to that same ignition relay coil. (just had this problem last week from another poster in 2000-2006 section where he was missing ground for the relay).
So here are the schematics involved and the chain of events that happen. (Little bit different from the 2000-2006 chassis).
And the first thing I would check is the main power trigger (Notice I did not say feed) for the control side of the ignition coil which is Fuse 34 2A marked as DLIS. This has to pass through your ignition SWITCH to Plug C1 Terminal 21 of your BCM. The BCM outputs 12v on Plug C4 Terminal 21 directly to the ignition coil relay which if it has a proper ground will energize the internal contacts to close and the starter will begin spinning over and the ignition system and fuel system will all be powered up along with the proper parts of the PCM.

So I would check the Fuse 34 2A DLIS first then if its ok, pull the ignition relay and check the socket for ground for the coil and then have some hold key forward to engine crank position and check the ignition relay socket for 12v at the coil positive terminal.

Let me know what you find or dont find.

So the Fuse you mentioned is good. I went as far as taking the entire Fuse box out and apart to inspect it all and there is no corrosion, broken pieces or blown fuses. I had my wife turn the key as I checked the voltage in the relay slot. There was a constant that read 12v and the signal that read 8v while the key is "on" then 12v when the key is being turned in the "crank" position. I took off grounds off that I could see/reach and cleaned them all. Also cleaned and checked the mega Fuse. All good. As stated before, every other electronic in the vehicle works and works well. No lack of power. When I turn the key to crank, it all shuts off as it's supposed to until you let go of the key and it all comes back on. I have an OBD2 code reader, nothing fancy, and it's not reading any codes. The anti theft dummy light is not illuminated either so I don't think it's locked out. I'm at a loss.

 

[mikez71]

You need to have power on the pink wire from ignition switch (J1-14 on the BCM connector)
J1-21 is just accessory power, J1-14 is the ignition starter signal.

Then I would check the yellow wire (J4-21 on BCM connector). If nothing there, I have to think it's the BCM...?

 

[mikez71]

Also that 8V with key on seems too low..
Just accessories on right?

EDIT: I thought this was starter supply voltage, if coil trigger voltage, should be 0V with key not in 'start' position!

 

[J-Herkel]

Also that 8V with key on seems too low..
Just accessories on right?

Yes, just accessories. It jumps to 12v when the key is being turned to the crank position.

 

[J-Herkel]

You need to have power on the pink wire from ignition switch (J1-14 on the BCM connector)
J1-21 is just accessory power, J1-14 is the ignition starter signal.

Then I would check the yellow wire (J4-21 on BCM connector). If nothing there, I have to think it's the BCM...?

Wouldn't it throw a code if it lost coms with the BCM?

 

[Doubeleive]

is the shift indicator showing it is in PARK?, try moving it to neutral that could be another reason for no start

 

[J-Herkel]

is the shift indicator showing it is in PARK?, try moving it to neutral that could be another reason for no start

I've tried N. Wiggling it while cranking, just took the ignition switch off and wiggled it while cranking. Stepping ******* the brake pedal. Nothing. It's like a wire or something somewhere just got cut or came off. Usually I'd see signs on failure before actual failure but this time it just stopped.

 

[mikez71]

It's not comms with the BCM, it's just the ignition/key start signal. 12V.
and the yellow wire is the starter relay trigger signal.. The BCM is in the middle.

Here's all I can come up with..

Ignition switch "START" -> 12V to BCM (PINK J1-14) [no power? bad ignition switch]
BCM -> 12V to Starter Motor Relay (YELLOW J4-21) [no power with GOOD ignition switch? maybe bad BCM]

Again, that 8V seems low to me.. It may not solve the problem, but I feel like you're skipping a step if you don't test those wires..
(Considering your lights all light up fine, no codes, jumping starter relay socket works, fuses good, new battery etc.)

Or find someone with a scanner that shows if the key is in the start position. (to rule out the ignition switch)

 

[Doubeleive]

I've tried N. Wiggling it while cranking, just took the ignition switch off and wiggled it while cranking. Stepping ******* the brake pedal. Nothing. It's like a wire or something somewhere just got cut or came off. Usually I'd see signs on failure before actual failure but this time it just stopped.

checked every fuse? with a meter....

 

[rockola1971]

So the Fuse you mentioned is good. I went as far as taking the entire Fuse box out and apart to inspect it all and there is no corrosion, broken pieces or blown fuses. I had my wife turn the key as I checked the voltage in the relay slot. There was a constant that read 12v and the signal that read 8v while the key is "on" then 12v when the key is being turned in the "crank" position. I took off grounds off that I could see/reach and cleaned them all. Also cleaned and checked the mega Fuse. All good. As stated before, every other electronic in the vehicle works and works well. No lack of power. When I turn the key to crank, it all shuts off as it's supposed to until you let go of the key and it all comes back on. I have an OBD2 code reader, nothing fancy, and it's not reading any codes. The anti theft dummy light is not illuminated either so I don't think it's locked out. I'm at a loss.

The Constant on 12v is the feed to the starter motor solenoid. This is part of the controlled part of the relay (Normally open Contacts). It seems your problem is in the control side of the relay (Coil). Not sure what that 8v is about but you have 12v when the key is put to the crank position which is what we are looking for. So assuming coil in relay is good, you have a bad ground to the relay coil or broken wire between that relay socket and chassis ground.
Terminal 86 is your coil 12v (Sounds like you got that when key in crank)
Terminal 85 is your coil ground
socket may be marked, your relay should have a mini schematic for terminals or the bottom near the stabs should be marked.

I am assuming you have had relay installed and had someone put key to crank and did NOT feel the relay slam shut.(click)