03 Tahoe no crank

[PreacherF15C]

Hi guys
I'm a 60 year old ex fighter pilot, retired airline pilot, but I've been building cars for over 40 years, and I owned a performance shop specializing in show quality restomods with 1000 HP for almost 15 years, even had some TV show spots. I know my way around a motor, but this has me pulling out what's left of my hair.
A friend gave me an 03 z71 Tahoe a few years ago, 320,000 miles on a 5.3. I decided to build an apocalypse truck, just for fun.
I rhinolined the entire exterior, upgraded the suspension, (bags) 6" lift on 35"s, custom leather interior, custom electronics,cameras, etc all shielded from emp.
Swapped in a fresh, tuned, LQ9 6.0 and tranny/transfer case from a Caddy, upgraded the cooling system, and added a root s/c
Turned out great, has a hotspot, wifi, inverters, even an onboard ham radio lol. Just purely for fun.

It had an intermittent crank problem with the 5.3, I ended up stripping the case off the starter relay so you can just squeeze it to start it, figuring it would be addressed as I upgraded.

But the damned thing persists. The only way it will start is to turn the key on, pop the hood, and squeeze (which of course jumps) the starter relay. Then it fires right up and runs like a scalded ass ape.
Everything is new, except the base wiring in the truck. The 5.3 was so tired I didn't save anything, it's all complete new LQ9 crate motor, which I bought accessories and all. New starter, solenoid, relay, etc.

WTF?? I'm hoping there's some secret that I am unaware of, because guys...I have gone through this damned thing from one end to the other, and it should be cranking like mad. There is no reason I can determine for this to be happening.

I'm too old for this *****.

Anyone have any ideas? I'm fresh out.

Thanks

 

[Fless]

Welcome to the forum from the Centennial State!

There are a few videos on YouTube on how to easily bypass the starter relay, so if none of that's been tried I'd recommend it. If nothing else it would confirm that the wiring from the relay to the starter is good.

Eric O (this is kind of an early one for him) shows how to test the relay pins for function.

Edit: here's the starting circuit diagram for an '04 Tahoe -- bet yours is the same, but YMMV.

 

[PreacherF15C]

Thank you! Right away something I haven't tried! You guys are awesome. I honestly don't know how we used to get by back in the pre-internet days. It seems impossible, lol
Again, thanks,!

 

[Fless]

Would love to know what you find! And make sure your engine and body grounds are great.

 

[Scottydoggs]

the under hood fuse boxes tend to melt up and or corrode. need to take it apart and inspect the plugs.

 

[retiredsparky]

With high mileage, the ignition switch tends to collect dirt in the internal lube grease, blocking the switch contacts from making a good connection. I have taken apart several bad ignition switches, and the contacts are usually quite dirty. Have you replaced the switch yet?

 

[lyncht1967]

as a test connect one end of a black jumper cable to batt negative and other end of black jumper cable to block somewhere and try cranking...if it starts you have a bad negative battery cable..common on these models. can do the same thing from batt pos to starter pos post and try cranking..if it starts you have a bad positive battery cable. also common issues on these models...i had the same issue a month ago and it was bad negative batt cable.

 

[Fless]

as a test connect one end of a black jumper cable to batt negative and other end of black jumper cable to block somewhere and try cranking...if it starts you have a bad negative battery cable..common on these models. can do the same thing from batt pos to starter pos post and try cranking..if it starts you have a bad positive battery cable. also common issues on these models...i had the same issue a month ago and it was bad negative batt cable.

A voltage drop test could be done, which is what you're describing.

 

[lyncht1967]

A voltage drop test could be done, which is what you're describing.

yes, but I don't know if he has a meter or test light, I try to suggest simple tests for joe everybody

 

[Fless]

yes, but I don't know if he has a meter or test light, I try to suggest simple tests for joe everybody

Based on the experience listed in the first post, I'd wager that he has one or both available to him. If he doesn't, there's not much cost involved with acquiring inexpensive tools for testing.

 

[rockola1971]

The engine crank electrical system kinda fools some people because many assume you are just looking for 12v+ and that is not the case. The starter relay needs 12v+ from Ignition fuse E AND the other side of the coil for the relay is looking for a 12v-(Ground Seeking) which is provided by the PCM once it decides to enable it. For the PCM to enable it there must be 12v+ going to the PCM terminal C2-59 which is provided by Crank Fuse 10a. Since the starter itself works manually, closing the solenoid proves this is a control power problem and not a load (starter) problem. I would goto the starter relay and put 2 wires on the bottom side so you get in circuit measurements while someone is cranking remembering one side is + and the other -. See which one is missing and work your way back from there to see where the open is.

 

[ExEl03X4]

When I need a quick and easy answer about a relay, I just switch the suspected relay with another from another position. In 50 years, I haven't seen very many bad relays but switching positions is easy and quick.