03 LB7 security light

lutzjk913

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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 wasn’t 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 can’t get that lucky. The tech calls me and says that the truck has lost all communication with the theft deterrent module and he can’t communicate with it. Very awkward imo.

I’ve did some in depth research over the last week and can’t really see where someone has had this issue. The truck starts and runs great. Doesn’t 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.
 

DAVe3283

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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.
 

lutzjk913

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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.
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.
 

DAVe3283

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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.
 

lutzjk913

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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.
 

dndj

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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.
 

lutzjk913

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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.
I’ll find out tomorrow what the tech used to look at it.
 

lutzjk913

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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.
 

DAVe3283

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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.
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.
 

lutzjk913

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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.
 

DAVe3283

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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.
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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dndj

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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.
 

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fl0w3n

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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.
I have nothing helpful to add, but I just want to say that I’m always blown away by the knowledge on here. I love reading through threads like this to learn

Dndj, what book are you posting pictures of?
 

lutzjk913

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I have nothing helpful to add, but I just want to say that I’m always blown away by the knowledge on here. I love reading through threads like this to learn

Dndj, what book are you posting pictures of?
Same here.

The knowledge base is insane with these guys.
 

lutzjk913

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groveport, ohio
Interesting fact

Since the paint job the truck does not have a ground strap going to the hood?

I don’t have any before pics under the hood to see though.
 

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