TB is DOA! PCM? BCM? OBD2 inop! 2003 TB LT I6 4WD 105k

Magyver

Original poster
Member
Apr 9, 2012
64
I promise my posts will be easier to read than my title.....but I have to get your attention first:thumbsup:

The story:
i was driving along as usual, in TB bliss, just going about my day. Right before parking I noticed that the windows didn't work from the master switch, but since they were up and I was parking it I didn't really think that much of it. When I tried to restart everything was dead, and no crank. Can't check for codes with scan tool because OBD2 port does not work.
Key in position 1 (acc)
Working-seats, headlights
Not working- Cluster warning lights, P/W, radio, dome (except when turned on by headlight switch), remote locking, 4WD switch lights.

Key in position 2 (ON)
Working- Seats, Headlights, Cluster warning lights, Gear position indicator (but no line under the letters)heater blower, rr defroster, turns/hazards, mirrors,
Not working- Radio, 4WD switch, remote locking, P/W, dome (except when turned on from Headlight switch)

Key in Position 3 (START)- Nothing happens except the dash gets dim.
There is 12.6 volts at the 1year2month old battery.
The starter spins and cranks the motor by jumping the pin locations where the relay goes. REPLACED RELAY just for insurance. REPLACED IGNITION SWITCH for insurance.
I've checked every single fuse under the hood and back seat, and verified continuity of each fuse with a fuse tester. I also pulled every circuit breaker just for good measure.
I pulled and left out for an hour the PCM fuses to attempt a reset. I've inspected the area wiring in the tailgate and pulled the main window switch, nothing out of the ordinary.

I finally had to give in and have it towed to my trusted mechanic, but I don't know if he has the right tool to interface with the PCM and BCM to analyze them. Neither he nor the dealer could look at it before Monday, so I brought it to him because he'll be cool with me under the hood in his lot over the weekend in case I gain a wee bit of miracle insight and can fix it. Plus, if I have to replace a module he will let me bring my own. Either way I have to pay for a flash at Chevy.

Any help troubleshooting this is more than appreciated. I have searched and searched, and looked for bad grounds, frayed wires, shorted this and burnt that. Most of the threads I've found here and around the interwebs don't describe quite this level of total deadness, so it must be something simple I just haven't found it yet.:confused:
 

Denali n DOO

Member
May 22, 2012
5,596
What about that 125A mega fuse, I'm not certain what happens when it blows but recall reading about it doing weird stuff...

Just a thought.
 

RayVoy

Member
Nov 20, 2011
939
Denali n DOO said:
What about that 125A mega fuse, I'm not certain what happens when it blows but recall reading about it doing weird stuff...

Just a thought.
It powers the rear seat fuse panel, nothing there that would stop the starter from spinning.
 

Magyver

Original poster
Member
Apr 9, 2012
64
Yup, forgot to mention that i get full voltage on both sides of that 125A fuse, and also at the POS post at the BCM.

I'll be standing by all evening in an effort to keep this moving along. Often I see threads die because the OP doesn't reply to questions, and doesn't post the result of his efforts, which helps no one, and the gurus can only do so much, so why waste keystrokes on threads that go nowhere? My job depends on my vehicle, and I have to get this resolved by Monday. Or at least a "back on road" date to plan car sharing with wifey.

Thanks in advance for sharing your brainpower.
 

MAY03LT

Member
Nov 18, 2011
3,425
Delmarva
Normally with the headlights working, the bcm is receiving inputs from the headlight switch and can control the output. But I read that you did a drl kill, and depending on how you did it, it could have taken the bcm out of the picture.

It sounds like a data communication fault. I don't know if aftermarket scan tools can run the class2 diagnostics like the tech2 can, but even if they can't, if you can't talk to any module then a communication problem is likely. There aren't that many modules in the truck, so a process of elimination won't cost you more then some of your time.

You got an aftermarket radio in it?
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
I see you tried reading codes. Does your cigarette lighter receptacle work?

You checked even the J-case fuses at the edge of the fuse block? Not just the regular blade-leg ones?

I'd start by checking voltages at the ignition switch. Assume you've seen the wire color chart I've posted a few times?

Any local buddies to swap PCMs with?
 

Magyver

Original poster
Member
Apr 9, 2012
64
MAY03LT said:
Normally with the headlights working, the bcm is receiving inputs from the headlight switch and can control the output. But I read that you did a drl kill, and depending on how you did it, it could have taken the bcm out of the picture.

It sounds like a data communication fault. I don't know if aftermarket scan tools can run the class2 diagnostics like the tech2 can, but even if they can't, if you can't talk to any module then a communication problem is likely. There aren't that many modules in the truck, so a process of elimination won't cost you more then some of your time.

You got an aftermarket radio in it?

my drl kill was done as you laid it out on your YouTube video, which bypasses the BCM to give full power at all times to the low beams so HIDs would work properly, and also retain auto headlights. BTW, your video was easy to understand and the mod works great.

Yes I have an aftermarket radio, with a Schosche thingy to make the chime work.
 

MAY03LT

Member
Nov 18, 2011
3,425
Delmarva
Magyver said:
my drl kill was done as you laid it out on your YouTube video, which bypasses the BCM to give full power at all times to the low beams so HIDs would work properly, and also retain auto headlights. BTW, your video was easy to understand and the mod works great.

Yes I have an aftermarket radio, with a Schosche thingy to make the chime work.

Thanks man!:cool:

Next thing I'd do is unplug the Scosche and see what happens. The Scosches are known to pull down the data bus when they fail, not just on these but on most GMs from our era.
 

CaptainXL

Member
Dec 4, 2011
2,445
Crank is PCM controlled. if you bypassed the relay and it cranks then the PCM would be suspect as Bill mentioned.
 

Magyver

Original poster
Member
Apr 9, 2012
64
the roadie said:
I see you tried reading codes. Does your cigarette lighter receptacle work?

You checked even the J-case fuses at the edge of the fuse block? Not just the regular blade-leg ones?

I'd start by checking voltages at the ignition switch. Assume you've seen the wire color chart I've posted a few times?

Any local buddies to swap PCMs with?

The cig lighters all work - I read up on the tricky #13 fuse also running the OBD port
J-Case=high-amp square ones? yes I checked those
There's a ton of TBs around here but alas, I know none of them. The junkyards don't even have any last time I checked, although I will check again today. Can PCMs be swapped around? I thought they had to be flashed specific to the vehicle.

May03LT
Next thing I'd do is unplug the Scosche and see what happens. The Scosches are known to pull down the data bus when they fail, not just on these but on most GMs from our era.

I'll try this today

CaptainXL
Crank is PCM controlled. if you bypassed the relay and it cranks then the PCM would be suspect as Bill mentioned.​

Gotcha. Any insight to whats happening (or not happening) when the key is in ACC position? Maybe unplugging the Schosche will free up the BCM?

My plan for this morning so far:

Unplug the Scosche chime unit to see if communication to the BCM is restored

Get a new battery. My alternator is starting to show signs of diode irregularity and when I got it checked the slip said "excessive flutter". I posted once about it and snapped a pic of the readout i was given in case anyone cares to see it. I have been keeping tabs on it, checking the battery voltage from time to time, and it's been fine, but I imagine it's been living a hard life, and it's still under replacement warranty. What the heck, another variable eliminated at least.

Post up in Craigslist under the personals/casual encounters section for someone wanting to swap PCMs, no strings attached :biggrin:
 

Magyver

Original poster
Member
Apr 9, 2012
64
May03LT you are da man!

First thing i did was remove the Scosche adapter from the head unit and the ol girl started right up and is still running fine. With the engine running i plugged it back in and lost all gauges except the tach, 4wd switch, and windows. Just like you said, when it fails it takes the BCM with it. Fortunately the BCM survived. So its either replace the Scosche unit or decide on another solution to make the radio work. You have posted about the Metra adapter, but i would like to keep the chimes if possible. Has anyone figured out how to use the Scosche harness with the box?
 

DDonnie

Member
Mar 26, 2012
2,631
Magyver said:
May03LT you are da man!

First thing i did was remove the Scosche adapter from the head unit and the ol girl started right up and is still running fine. With the engine running i plugged it back in and lost all gauges except the tach, 4wd switch, and windows. Just like you said, when it fails it takes the BCM with it. Fortunately the BCM survived. So its either replace the Scosche unit or decide on another solution to make the radio work. You have posted about the Metra adapter, but i would like to keep the chimes if possible. Has anyone figured out how to use the Scosche harness with the box?

Personally, I say go with this. PAC C2RGM24 Class 2 Radio Replacemt G : Amazon.com : Automotive

I've heard PAC is more reliable. I have the onstar/bose version and my truck and no problems thus far.
 

MAY03LT

Member
Nov 18, 2011
3,425
Delmarva
Magyver said:
May03LT you are da man!

First thing i did was remove the Scosche adapter from the head unit and the ol girl started right up and is still running fine. With the engine running i plugged it back in and lost all gauges except the tach, 4wd switch, and windows. Just like you said, when it fails it takes the BCM with it. Fortunately the BCM survived. So its either replace the Scosche unit or decide on another solution to make the radio work. You have posted about the Metra adapter, but i would like to keep the chimes if possible. Has anyone figured out how to use the Scosche harness with the box?

BOSS! Thanks for taking the time to update with the fix.:thumbsup:

The Metra also allows you to keep chimes and retained accessory power. I've been using the GMRC-01 (mine is non bose) since 07 and that's why I recommend them. Way back in the day, maybe 08/09, there was a batch of them that were prone to early failure. They wouldn't pull down the bus. The radio B+ driver, which powers the switched (red) wire of the radio, would fail and the radio wouldn't work. I can't prove this, but it seems that the ones that were from that batch were pulled from reputable stores and ended up on ebay where they were sold for less then half the cost of a new one and of course were not warrantied. Trust me, I didn't want to spend >$100 just to make the stupid radio work, but for long term reliability I'm glad I did.
 

Magyver

Original poster
Member
Apr 9, 2012
64
Just got home, everything seems to be in order. I found the hot lead in the Schosche adapter and jumped it to the power lead on the radio so it works, I just have to shut it off manually every time until the Metra arrives. The money i thought i would be spending is now slated for a new alternator, which was on my summer list anyway. it hasnt gone bad yet but i would rather be proactive and avoid any chance that it is the root of some other crazy symptom in the future. then I'll have a new one and an OK one, which i'll rebuild at some point.

I might have thought of the radio adapter, but that was probably hours from now. In my mind the radio is just a symptom of the problem, I'm still learning about how interconnected the BCM is to the operation of the truck. Now I know!



Thanks again for the help!
 

MAY03LT

Member
Nov 18, 2011
3,425
Delmarva
No prob man:cool:

I must say, your attention to detail when listing the symptoms was a big help.:yes:
 

Magyver

Original poster
Member
Apr 9, 2012
64
I was a nationwide field tech/installer for a few years building wifi networks, and that helped me hone my documentation skills. Plus, after reading what seems like a thousand threads online trying to learn about my problem, time after time the discussion reverted to terminology and eventually the responders gave up or the OP stopped posting. Thanks again for sticking with me.
 

Magyver

Original poster
Member
Apr 9, 2012
64
So I ordered a Metra GMRC-01 interface upon solving this issue, and that was great for a few months up to about 3 weeks ago when the same no start problem happened again. This time I knew where to look and which leads to jump, etc. so I ordered yet another GMRC-01, plugged it in, and this time routed the chime box into the glove box for easy access, after having the same issues AGAIN! Grrrrrrr, so instead of throwing it out and ordering yet another one, I decided to experiment and observe a little more closely, but I can't pinpoint exactly what's happening.
Scenario 1: driving along, then all the guages go dead, radio dies, windows don't work, but runs and drives normally. I can pull over, either shut it off or not, unplug the interface and jump the yellow (constant hot) to the pink (ign?), then gauges come back, radio works, etc.

Scenario 2: car has been parked for a while. Key fob no worky. Unlock with key, car sometimes starts but usually not. Unplug interface, jump yellow to pink, all is good, except I have to stop and restart the motor to get the radio to come on. Sometimes in this scenario the alarm toots briefly as I'm cranking but goes off immediately.

How would I test? I know everything goes through the master window switch and also the tailgate module. The remote entry works fine, I think this related to either BCM, or something attached that it depends on. If using the low budget harness will work I don't care if I have chimes at this point, but I do like having a stock feel to the vehicle.
 

Magyver

Original poster
Member
Apr 9, 2012
64
Scenario 3: starts fine, guages works, but no radio until I shut it off and restart. Not sure if the remote door locks work in this scenario.
 

gmcman

Member
Dec 12, 2011
4,658
Excuse me, sir. Seeing as how the VP is such a VIP, shouldn't we keep the PC on the QT? 'Cause if it leaks to the VC, he could end up an MIA, and then we'd all be put on KP.
 

Magyver

Original poster
Member
Apr 9, 2012
64
Adrian Cronauer didn't drive GMT360 I'm pretty sure. But good quote!
 

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