Duramax Diesels Forum Truck of the Week
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Old 01-07-2020, 07:23 AM
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Default 03 LB7 security light

Back story.

I bought a LB7 project truck to fix up and work on with my son. Picked it up last April.

It went to get painted the first week of September and the painter had the truck for roughly 8 weeks. We got it back at the end of October. The security light was on as soon as I picked the truck up and drove home. I let the painter know of this immediately since It wasnít on prior but is now.

I got the truck into get some power steering issues resolved and had the tech look at the security light. I was hoping it was just a clear the code and roll on but canít get that lucky. The tech calls me and says that the truck has lost all communication with the theft deterrent module and he canít communicate with it. Very awkward imo.

Iíve did some in depth research over the last week and canít really see where someone has had this issue. The truck starts and runs great. Doesnít miss a beat but the light is on steady.

Any help is appreciated guys.

The painter did have both doors and the whole front end taken apart to paint it. New passenger fender and whole new front bumper assembly.
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2008 CCLB LT2 DURAMAX LMM
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Old 01-07-2020, 08:06 AM
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Interesting that it still starts with the security light on. Does the truck have a tune with VATS disabled?

I doubt anything the painter did affected the security system. On the 2003-2007 Classic trucks, the Theft Deterrent Module is just software on the body computer (BCM), and the ignition switch itself. The doors and wiring on the front of the truck doesn't really have anything to do with it.

The only thing I can think of is if one of the fuses feeding the BCM blew, and the BCM isn't powered on at all. Do the interior lights still work (come on with the door, etc.)? Automatic headlights still work?

You might need to get someone with a high-end scan tool to diagnose this properly. Keep us posted, I'm curious what you find.
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Old 01-07-2020, 08:21 AM
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Quote:
Originally Posted by DAVe3283 View Post
Interesting that it still starts with the security light on. Does the truck have a tune with VATS disabled?

I doubt anything the painter did affected the security system. On the 2003-2007 Classic trucks, the Theft Deterrent Module is just software on the body computer (BCM), and the ignition switch itself. The doors and wiring on the front of the truck doesn't really have anything to do with it.

The only thing I can think of is if one of the fuses feeding the BCM blew, and the BCM isn't powered on at all. Do the interior lights still work (come on with the door, etc.)? Automatic headlights still work?

You might need to get someone with a high-end scan tool to diagnose this properly. Keep us posted, I'm curious what you find.
It is tuned. By a unknown guy from Tennessee. So not sure on the VATS.

When the painter had it the batteries were unplugged for a I know amount of time also. I can check on the fuses when within the next day or so. Where are they at? I assume under the hood fuse box.

Yes the the interior lights come on and auto lights still work as they should.
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2008 CCLB LT2 DURAMAX LMM
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456HP 925 TQ

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Old 01-07-2020, 08:27 AM
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Will also add

This is a base model truck
Crank windows
Manual locks
Manual 4x4
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2008 CCLB LT2 DURAMAX LMM
13.39 @ 99.44 mph W/ 1.76 60í
456HP 925 TQ

2003 Regular Cab LB7
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Old 01-07-2020, 08:46 AM
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Looks like the BCM is fed from 7 (!!) fuses. I'd attach the schematics, but I am out of my storage quota apparently.

Underhood fuse block:
  • TBC BATT (10A)
  • TBC IGN1 (10A)
Left interior fuse block (open driver's door to access):
  • TBC 2A (15A)
  • TBC 2B (15A)
  • TBC 2C (15A)
  • TBC IGN0 (10A)
  • TBC ACCY (10A)
Being a base model truck, it wouldn't surprise me if you don't have all the fuses listed above. But those are the ones to check to see if they're blown or loose.
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Old 01-07-2020, 08:47 AM
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Quote:
Originally Posted by DAVe3283 View Post
Looks like the BCM is fed from 7 (!!) fuses. See attached schematics.


Underhood fuse block:
  • TBC BATT (10A)
  • TBC IGN1 (10A)
Left interior fuse block (open driver's door to access):
  • TBC 2A (15A)
  • TBC 2B (15A)
  • TBC 2C (15A)
  • TBC IGN0 (10A)
  • TBC ACCY (10A)
Being a base model truck, it wouldn't surprise me if you don't have all the fuses listed above. But those are the ones to check to see if they're blown or loose.
Thanks Dave

I will check tonight or tomorrow and get back with ya.
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2008 CCLB LT2 DURAMAX LMM
13.39 @ 99.44 mph W/ 1.76 60í
456HP 925 TQ

2003 Regular Cab LB7
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Old 01-07-2020, 06:39 PM
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All fuses checked out ok

Nice and snug and not burnt.
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2008 CCLB LT2 DURAMAX LMM
13.39 @ 99.44 mph W/ 1.76 60í
456HP 925 TQ

2003 Regular Cab LB7
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Old 01-07-2020, 07:26 PM
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The LB7 vehicle theft deterrent system has what's called a "fail enable" mode with certain conditions which will cause the security light to remain on in the cluster but permit the vehicle to start and run. For example, this happens with DTC B2947, which is a failure when the BCM detects a short to ground on the passlock ignition key sensor power circuit while the vehicle is running. There's other failure modes and resulting codes which if detected while the vehicle is running, have similar effect.

The Tech2 can directly read whether the BCM has put the VTD (vehicle theft deterrent) into "fail enable" mode.

If you can pull the BCM codes, post them up and that will provide the next place to look.
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Old 01-07-2020, 08:20 PM
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Quote:
Originally Posted by dndj View Post
The LB7 vehicle theft deterrent system has what's called a "fail enable" mode with certain conditions which will cause the security light to remain on in the cluster but permit the vehicle to start and run. For example, this happens with DTC B2947, which is a failure when the BCM detects a short to ground on the passlock ignition key sensor power circuit while the vehicle is running. There's other failure modes and resulting codes which if detected while the vehicle is running, have similar effect.

The Tech2 can directly read whether the BCM has put the VTD (vehicle theft deterrent) into "fail enable" mode.

If you can pull the BCM codes, post them up and that will provide the next place to look.
Iíll find out tomorrow what the tech used to look at it.
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2008 CCLB LT2 DURAMAX LMM
13.39 @ 99.44 mph W/ 1.76 60í
456HP 925 TQ

2003 Regular Cab LB7
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Old 01-08-2020, 06:39 AM
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Called my customer this morning who looked at it.

He did use a tech 2 to look at it. Heís a old GM tech so not surprised he has one. I thought thatís what he used but wasnít sure.

Only code listed was a ďUĒ code.

He also said that it wouldnít let him even communicate with the module. Itís like it doesnít even have power to it.
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2008 CCLB LT2 DURAMAX LMM
13.39 @ 99.44 mph W/ 1.76 60í
456HP 925 TQ

2003 Regular Cab LB7
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Old 01-08-2020, 09:13 AM
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Quote:
Originally Posted by lutzjk913 View Post
He also said that it wouldnít let him even communicate with the module. Itís like it doesnít even have power to it.
Might as well poke your head under the dash and look at the BCM. Ensure everything is plugged in all the way. If it looks good, you can start checking continuity on the various power, ground, and data bus pins.

I'd upload the pinouts but it looks like they lowered the quota here, so I'm out of space.
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Old 01-08-2020, 09:41 AM
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Quote:
Originally Posted by DAVe3283 View Post
Might as well poke your head under the dash and look at the BCM. Ensure everything is plugged in all the way. If it looks good, you can start checking continuity on the various power, ground, and data bus pins.

I'd upload the pinouts but it looks like they lowered the quota here, so I'm out of space.
Damn.

Where could I find them?

I might take the truck and swing by his shop next week. It just seems awkward that after the batteries were unplugged for some time and then now this. I almost wander if there is a way to do a hard reset on them.
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2008 CCLB LT2 DURAMAX LMM
13.39 @ 99.44 mph W/ 1.76 60í
456HP 925 TQ

2003 Regular Cab LB7
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Old 01-08-2020, 09:47 AM
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Quote:
Originally Posted by lutzjk913 View Post
Where could I find them?
Something like AllDataDIY or a professional subscription like Mitchel OnDemand would have them. I started a thread asking about increased quota, so hopefully they bump mine up and I can post those documents.
Quote:
Originally Posted by lutzjk913 View Post
I might take the truck and swing by his shop next week. It just seems awkward that after the batteries were unplugged for some time and then now this. I almost wander if there is a way to do a hard reset on them.
Disconnecting the batteries for a few minutes is how you do a hard reset on all the modules.

Edit: sounds like Rob is going to be able to post the diagrams here for me later today.
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Last edited by DAVe3283; 01-08-2020 at 11:10 AM.
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Old 01-08-2020, 01:48 PM
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These are from DAVe3283.
Attached Files
File Type: pdf Body Control System Connector End Views.pdf (147.4 KB, 9 views)
File Type: pdf Body Control System Component Views.pdf (97.8 KB, 6 views)
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Old 01-08-2020, 07:02 PM
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Quote:
Originally Posted by lutzjk913 View Post
Called my customer this morning who looked at it.

He did use a tech 2 to look at it. He’s a old GM tech so not surprised he has one. I thought that’s what he used but wasn’t sure.

Only code listed was a “U” code.

He also said that it wouldn’t let him even communicate with the module. It’s like it doesn’t even have power to it.
U1xxx codes set means one of the modules on the class 2 data bus isn't communicating. Each module is required to send a keep alive message every 2 seconds on the class 2 data bus and when no message is detected for 5 seconds a U1xxx code sets in one or more modules which detect the condition. Each module has an 3 digit ID number for the xxx, eg. BCM would be U1064, ECM is U1016, and so on.

At key on, the BCM reads the passlock value from the key cylinder and if it matches, sends a password to the ECM which tells it to enable the injectors and allows the truck to start. FSM says "in the event of an open Class 2 serial data line between the BCM and ECM, the vehicle will become fail-enabled...." which is the condition where the engine will start but the security light remains on in the cluster.

Agreeing with others that this is pointing to something between the ECM and BCM. Note in the schematic below there's actually two different serial data busses connecting between the ECM and BCM. One looks like a dedicated point to point bus to me, the other looks like the main common class 2 serial data bus that connects between all the modules in the truck.
Attached Images
File Type: jpg IMG_2941.JPG (75.1 KB, 19 views)
File Type: jpg IMG_2942.JPG (149.4 KB, 12 views)
File Type: jpg IMG_2943.JPG (142.5 KB, 11 views)
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Last edited by dndj; 01-08-2020 at 08:44 PM. Reason: adding pics
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