Truck Dies Seconds After Starting

[LoganA13]

Howdy y'all,
New hear and hoping someone has seen the issue i'm dealing with on this soon to be boat anchor of a truck..

Some background: The truck stopped communicating with the OBD2 connector several months ago. It was something weird but nothing i really had time to look at as the truck ran fine and had no issues. Just a MAF check engine light caused by the air intake hose clamp coming loose and the tube popping off.

A few days ago i was cruising down the road when the truck shifted down to 5th, Odometer bounced back and forth from 120-0 MPH a few times then pegged at zero, ABS and Brake light came on. Tried restarting the truck and it puttered out at idle and died a couple times before holding idle, it shifted hard into rev and drive, and stayed in 2nd gear. At this point i'm convinced the speed sensor pooped the bed. After a few restarts it starts to act right again and i'm able to at least get on the highway and head to the parts store. Cruise set at 80MPH i hit a decent size bump in the road and all the same problems start re-occurring. Restarting on the side of the road was futile at this point, speedometer pegged at 0 MPH and ABS and Brake lights were mocking me. I limped it down the side of the road (Stuck in 2nd gear) to the parts store. Changed the Speed sensor but no fix.

I start pulling the fuse box under the hood apart, no loose or damaged wires on the under side of the fuse block (no blown fuses), Every harness i could visually trace under the hood showed no evidence of damage. All the connections i could pull apart seemed fine and free of corrosion.

Tried starting the truck again but when i turned the key the only lights that came on the gauge cluster were the check engine and battery lights and transmission shift indicator. Truck will start and after about 2 seconds there is an audible click and the needles on the gauge cluster move to position, all lights come on. Within half a second of this happening the truck dies. Every. Damn. Time. I've inspected all chassis grounds, seemed fine. Resurfaced ECM chassis ground. Played with ignition connections, Tested OBD2 plug. I have 2.4-5 Volts from pins 5-6 and meter 62 ohms from pin 6-14 (sorry if i botched these pin numbers up, going off memory. but this is for the two terminating resistors in the Network circuit) and have verified at the ECM fuses that the computer is receiving 12 volts.

Another odd thing happening is the radio comes on with the key on, but the HVAC control unit will not turn until the truck has started and gauges attempt to turn on.

My question now is: Has any experienced something similar?? The fact that i've lost ECM communication awhile ago but just verified the network health last night makes me wonder if the ECM just finally crapped out on me, but could that cause the weird issue with the HVAC unit? I'm also wondering if it isn't a bad connection or short or grounding problem that has just slowly progressed (Kinda rooting for this option, kinda not wanting to track it down lol)

Any advice here would be greatly appreciated as this is my only vehicle at the moment and i would hate bringing it to a shop for an electrical issue.
Thank yall in advance!!

 

[2004LB7]

sounds like a chaffed wire to me. or some of the pins in the large bale connector have pushed back. have you tried cleaning the ground under the driver door?

 

[Chevy1925]

time to get it running and wiggling wires under the hood. check all the grounds at the bottom of the engine block as well. They are at the front of it just above the upper oil pan on both sides.

 

[Ron Nielson]

Cruise set at 80MPH i hit a decent size bump in the road and all the same problems start re-occurring. Restarting on the side of the road was futile at this point, speedometer pegged at 0 MPH and ABS and Brake lights were mocking me.

For me, this says wiring also, just as James and Jason posted. And it very well may be intermittent which makes it extremely difficult to find. A wire could be broken inside the insulation and appear whole to all appearances but the strands of the conductors touch and while they are touching, transmit power, but when hitting the bump, now the gap opens and the connection is broken and your truck dies or has other problems. And then sometime later, it acts as if nothing is wrong. It has gone back to its previous connection state.

Tested OBD2 plug. I have 2.4-5 Volts from pins 5-6 and meter 62 ohms from pin 6-14 (sorry if i botched these pin numbers up, going off memory. but this is for the two terminating resistors in the Network circuit)

When you tested the integrity on the GMLAN, that just means that there is a non-shorted connection - not that the info on the GMLAN is valid. It often happens that GMLAN devices pull the entire LAN down, sometimes preventing starting or other problems.

 

[DAVe3283]

The starting than immediately shutting off does sound like a security issue, which could easily be caused by a loss of communications between the ECU and body modules.

Since that started after handling the wiring near the fuse block, I would dig into that section again.

 

[TheBac]

Mouse chew-through or rub-through wiring under fuse block possible.

 

[LoganA13]

Hey y’all, appreciate the feedback. Here’s the skinny now: i must’ve wiggled something loose under the fuse box when i first pulled it apart. Found a bad connection under an ignition fuse, fixed that and got some life back in this ol girl. But I’m still losing comms with the TCM. Was intermittent and only dropped when i hit bumps, so obviously a poor connection somewhere. I combed through my ECM and TCM harnesses and found some chafed wires, cut of some corrosion & spliced them back together but still have no comms. Plugged a scanner in and no comms to TCM from OBD2 either. Can someone share the TCM to ECM CAN network pin outs? I’d like to ohm out that comm path and check for breaks, then attempt to jumper the TCM to the ECM if possible and see if it comes back. Let me know what y’all think, i’m just brainstorming over here and starting to get to my wits end

 

[LoganA13]

Oh and forgot to mention, I’ve totally lost comms with TCM now.. i plugged the scanner in (cheapish Innova 5410) and only see two modules available 7E8, 7EE. Not sure which modules they are and googles not being helpful, but i assume one is obviously ECM, and the other is not the TCM.. but shouldn’t i have quite a few modules displaying? BCM, GPM, ABS? Things like that?

 

[dndj]

Verify the Innova 5410 supports the LBZ TCM on a known good truck first. Or borrow a Tech2 from someone, which is guaranteed to support the TCM.