Diesel Bombers - *** DTC... U1000? ***

Here's a $10 answer to go along with the $.50 cent I gave above:

Old Prelim Tech Bulletin-

Subject: All GM Vehicles Using Class 2 Communications Diagnostic Strategy for DTC U1000 or U1255 - PIT3154 - (09/10/2004)

Models: (All GM vehicles using Class 2 communications)

The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.

Condition/Concern:
DTC U1000 or U1255 set current or history, with or without other DTCs

Recommendation/Instructions:
DTC U1000 Class 2 Data Link
DTC U1255 Lost Communications

These DTCs set when the control module does not receive a message that it was expecting from another control module, and does not know which control module did not send that message.

1. If the DTC U1000 or U1255 is set in history with other DTCs set current or history, diagnose the other DTCs first.
2. If the DTC U1000 or U1255 set current, this usually indicates a module that is currently not communicating or a configuration issue. For example, if the control module is configured for an option (ie: Onstar, Keyless Entry, Memory Mirrors, etc.) that the vehicle does not have, it may expect to receive a message regarding this missing feature. This would usually occur due to the control module being recently replaced and incorrectly setup.
3. When the DTC U1000 or U1255 is current, the module that is not communicating may not even be listed on the Tech 2 on the Diagnostic Circuit Check/Class 2 Message Monitor list.
4. If the DTC U1000 or U1255 has set in history without other DTCs, replacing the control module setting the DTC is most likely NOT the solution. The module that has set the code is looking for an input from another module that is not communicating. Since the module did not receive an input that it is expecting to see, it sets a U1000 or U1255 which indicates there was a loss of communications. Look at the customer's complaint (ie: intermittent, erratic tachometer operation); this will probably be a better indication of the control module that is the source of the concern. Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition.

Description-

DTC U1000

CIRCUIT DESCRIPTION
Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message for State of Health monitoring. A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. When a module does not associate an identification number with at least one critical parameter within 5 seconds of beginning serial data communication, DTC U1000 is set. When more than one critical parameter does not have an identification number associated with it, the DTC will only be reported once.

The class 2 serial data line on this vehicle is a star configuration. The following modules communicate on the class 2 serial data line:

* The audio amplifier (AMP), w/UQ7 and Y91
* The body control module (BCM)
* The communication interface module (OnStar(R)), w/UE1
* The digital radio receiver (DRR), w/U2K
* The driver door module (DDM)
* The electronic brake control module (EBCM)
* The generator battery control module (GBCM)
* The HVAC control module
* The hybrid control module (HCM), w/HP2
* The inflatable restraint sensing and diagnostic module (SDM)
* The instrument panel cluster (IPC)
* The memory seat module (MSM), w/AN3
* The passenger door module (PDM)
* The powertrain/engine control module (PCM/ECM)
* The radio
* The rear seat audio (RSA) controller, w/UK6
* The rear seat entertainment (RSE) assembly w/U42
* The rear wheel steering control module (RWSCM), w/NYS
* The remote playback device - CD player (CDX), w/U1S
* The transfer case shift control module (TCSCM), w/NP1/NP8
* The transmission control module (TCM), w/M74

DTC DESCRIPTOR
This diagnostic procedure supports the following DTC:
DTC U1000 Class 2 Data Link Malfunction

CONDITIONS FOR RUNNING THE DTC

* Voltage supplied to the module is in the normal operating voltage range of 9-16 volts .
* DTCs U1300, U1301 or U1305 do not have a current status.
* The vehicle power mode requires serial data communication to occur.

CONDITIONS FOR SETTING THE DTC
A node alive message has not been received from an unidentified module within the last 5 seconds after establishing class 2 serial data communication.

ACTION TAKEN WHEN THE DTC SETS
The module uses a default value for the missing parameter.

CONDITIONS FOR CLEARING THE DTC

* A current DTC clears when the malfunction is no longer present.
* A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

DIAGNOSTIC AIDS
When a malfunction occurs while modules are communicating, a lost communication DTC is set as a current DTC. When the modules stop communicating the current lost communication DTC is cleared but the history DTC remains. When the modules begin to communicate again, the module with the open fuse will not be learned by the other modules so U1000 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only U1000 is set.