2003 H2, no aftermarket mods, owned truck since 2010ish?
Never any electric issues till now?

Issue was progressive...
At first, while driving, truck beeped and said unknown driver and flipped back to driver 1 in about 30 sec.

Didint happen again for a month, then twice in one day but always quick to resolve.

Past week has gone off a cliff, starts in driver 1, switches to unknown driver, whole dash warning lights come on all guages ex. tach and speedo stop working, stereo turns off, switched back to driver 1. Noticed it happened when I braked... Water intrusion?
checked battery. battery was 44% alt tested good, so replaced battery.

Well wasnt the battery as its still happening.
reset passkey, not that.

Pulled out the tech 2.

Tech2 and another scan tool both power up but "cant communicate" with anything? no body, no chassis, etc...
Ive checked the fuse for cig lgt and is fine, will check pins tomorrow. but tech2 worked fine when I replaced a height sensor a few weeks ago. a bent pin with prevent connection of course but I dont think a bent ob2 pin would cause a intermittent, progressive issue. Pretty sure issue lies somewhere else.

Now simply starts in driver 1 then switches to unknown driver within 30 sec, full warning lights, etc...and stays in this mode. turns off fine, doors lock fine, always use key to unlock doors, no issues.

Oddly there are zero starting or drivability issues whatsoever??? NOT even a hiccup?

Any idea where to start?

*didnt "need" a battery, old one still cranked fine but low voltage and 44%health.
Considered key cylinder but works fine.
Considering adding a ground in case 1 rotted away?
Dont want to throw # at it, point me in right direction.

Thoughts please!
Biggest pain is I have no idea how much gas I have, lol again runs great other than this new electrical gremlin.

 

WOW. That's the first I ever heard of that one. I have no clue.... The weird part is the tech II won;'t connect? Someone here might have an answer.

 

I would say start with the ground connections. Remove each one, inspect the ring terminal and replace if necessary, clean down to bare metal and reinstall. While you're at it, the battery cables.. they fail on the inside. If they're original I would go ahead and replace both.

Important ground connections are the one on the driver side of the engine block, halfway back, down low just above the oil pan lip. Then the ground strap from the back of the driver side cylinder head up to the firewall. Ground location above the driver side front tow hook, retained with a 10 mm bolt. Then get on your creeper and go around the outside of the vehicle, you'll find ground connections on some of the body support brackets that come off the frame. The worst offender is usually the one behind the driver side rear tire. There's also an important one behind your parking brake pedal. Simply pry upward your door sill plate, it snaps in and out. Then pull straight back on the piece around the parking brake pedal. It also snaps in and out. And you'll see the ground connection. If you've ever had water intrusion in the past, that one tends to get corroded.

Beyond that, I had similar symptoms that turned out to be the switch on the driver side of the transmission. Most parts suppliers refer to it as a neutral safety switch, but it's a lot more. It has two large plugs going into it, you'll want a heat gun to warm them up first in order to unplug them. It seems like everything goes through that thing. It tells the ECM and BCM what gear you're in, triggers your reverse lights, tells it when it is in park, etc.

Right before mine totally went out, it was switching between driver one and two.....

And this is one part you want to stick with genuine GM or AC Delco.

Here's what it looks like:

If you have trouble finding it under the Hummer H2, look for it under any 2003-2005 GM with the 4L65e automatic transmission.

The argument against it being this item is your communication issue with the Tech 2. That leans more towards having lost a ground connection. When that happens, electricity flows wherever it needs to to find a ground, traveling through other modules if it needs to, creating chaos in the system like you describe....

Search The forum.... Sometime ago someone posted a list of all the ground connections.

One final suggestion on where to look. Open your driver's door and look at the flexible piece that contains the harness that goes to your driver door control module with all the switches. That module with the switches is also a known issue, as well as the wires that go through that flexible boot. You pretty much have to pull the door panel off. It's not unusual for one of the wires inside that boot to break....

 

Here's the one I used on my 2003 H2

https://www.oregonperformancetransmission.com/

 

Or directly from GM....

 

not being able to communicate with the modules with the Tech II is an indication there is an issue with the data lines for the modules. If one of the modules shorts the data lines this will happen. You could unplug each of the modules one at a time then try the Tech II and see if it will communicate with the remaining modules. If it does start communicating then which ever module is disconnected has an issue.
I'm not positive on your truck but I believe the BCM contains the remote identification and all the driver memory settings for the 03 so you may start there.
I believe the truck identifies the driver based on the remote used to lock or unlock the truck or by the driver pushing the switch manually to identify what settings are wanted then the BCM restores the settings.
I don't know for sure how GM set it all up, I think I have the manual for the 03 somewhere.

do a google search with this "2003 GM truck how to locate a bad module that is shorting the data lines" a lot of good information

Neal

 

the Body Control Module (BCM) and the instrument cluster are grounded to the body structure under the driver's side dashboard. Key grounding areas include:
BCM Grounding (SP205): A critical ground splice (SP205) for the BCM is located under the driver's side dash, typically located near the BCM itself or along the wiring harness leading to the driver's side kick panel area.
Instrument Cluster/Lower Dash Grounds: The cluster and related dashboard electronics are often grounded at the lower left driver’s side kick panel, behind the trim near the parking brake assembly.
Main Grounds: Electrical gremlins in these areas are frequently caused by corrosion at these locations or water intrusion in the driver-side floor area.

 

Thanks for the replies!

Its -20 here and snowy so not a great time to work on truck but sure enough popped the hood and the first ground point I saw was terrible!
4 gauge wire crumbled in my fingers, lol, too far gone for cleaning so gave it a twist and voila! problem instantly resolved! for about a day, then back to fu*ked.


Waaay to cold to be digging around trying to replace the engine mount bolt, will do in spring.
Will also so all other ground in spring.

My 5min semi permanent fix?
4 gauge wire with terminals I made from parts lying around, attached to alternator ground and up to firewall (thick blue cable).


100% fixed!

Do I get bonus points as ground wire matched plug wires? lol

 

Just working on it outside at -20? Fahrenheit get you bonus points

Just cross your fingers and hope the rest of your grounds hold together till the weather warms up!

When you lose a ground, the electricity will find another route... Traveling through other modules creating havoc as you have experienced. I would address all your other grounds at the earliest possible break in the weather.

-20, where are you, Prudoe Bay, alaska? Basking in the sun down here in Oregon today

 

I'm with Mr Robert, you get bonus points for working on the H2 in -20 temps,

Neal