07 LT 5.3L Sudden no start/crank

[J-Herkel]

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)

Thank you. I have a friend coming over tomorrow with an advanced scanner to see if it comes up with anything. I'll keep you updated and highly appreciate your help!

 

[rockola1971]

Thank you. I have a friend coming over tomorrow with an advanced scanner to see if it comes up with anything. I'll keep you updated and highly appreciate your help!

Scanner will be practically useless for this failure other than to see input and output state of BCM and PCM. Need to ohm that ground terminal in the relay socket to chassis ground to see if it is electrically connected and rule out a busted wire.

 

[mikez71]

Good luck, fingers crossed! Scanner should be able to tell you if the ignition if ECM/BCM are receiving the start signal.
Should be able to rule something out I'm sure..

 

[rockola1971]

Good luck, fingers crossed! Scanner should be able to tell you if the ignition if ECM/BCM are receiving the start signal.
Should be able to rule something out I'm sure..

Dont we already know that is happening since OP is getting 12v at the relay coil when key is turned to engine crank? No way for that to get there without PCM and BCM being happy.

 

[mikez71]

Initially I thought he was measuring one terminal that went from 12V to 8V back to 12V.
Now I see he has one constant 12V, and the other goes from 8V to 12V.

bad ground doesn't explain the 8V, which should normally be 0V I thought?

Ohming the coil ground is solid advice. It looks like that ground isn't really shared with anything but the rear washer.
(even a functioning rear washer doesn't mean the starter relay coil ground is good, possible broken wire as you both mention)

Ground Zone #5, Not sure where it's located exactly, somewhere on left front side of motor..

 

[rockola1971]

Initially I thought he was measuring one terminal that went from 12V to 8V back to 12V.
Now I see he has one constant 12V, and the other goes from 8V to 12V.

bad ground doesn't explain the 8V, which should normally be 0V I thought?

Ohming the coil ground is solid advice. It looks like that ground isn't really shared with anything but the rear washer.
(even a functioning rear washer doesn't mean the starter relay coil ground is good, possible broken wire as you both mention)

Ground Zone #5, Not sure where it's located exactly, somewhere on left front side of motor..
View attachment 440867

Im not really concerned with the 8v right now since it didnt happen when the key was in crank position. The 12v according to OP was there in crank position so thats all the relay coil needs for the hot side. Now im concerned about the ground side not being there which would explain his symptom of no crank. Wire could have gotten bit by a field mouse, pinched by something, damaged during a past repair or ?????? OP Jumping the contact side of the relay and engine cranks proves that side of it is all good. So he has a control side problem and it appears the 12v side of that relay coil is all good. I bet if he sticks a meter lead in the ground side socket and measure resistance to chassis ground that he will see way more than the 2 ohms or less that I would expect to be there. Those fuse boxes are pretty famous for corrosion underneath right where the wires come up to the fuse and relay socket terminals from underneath. The green crusties have been spotted there many times.

 

[mikez71]

yep, we're on the same page.

I was just more focused on the + side of the circuit, being there are more connections. Then you show up and be all negative.

 

[rockola1971]

yep, we're on the same page.

I was just more focused on the + side of the circuit, being there are more connections. Then you show up and be all negative.

I see what you did there. Are you one of neutral kinda guys. Swiss maybe?